Interview Questions

1. What development tools have you used?
2. What languages have you programmed in?
3. What source control tools have you used?
4. What are your technical certifications?
5. What do you do to maintain your technical certifications?
6. How did your education help prepare you for this job?
7. How would you rate your key competencies for this job?
8. What are your IT strengths and weaknesses?
9. Tell me about the most recent project you worked on. What were your responsibilities?
10. From the description of this position what do you think you will be doing on a day-to-day basis?
11. What challenges do you think you might expect in this job if you were hired?
12. How important is it to work directly with your business users?
13. What elements are necessary for a successful team and why?
14. Tell me about the project you are most proud of, and what your contribution was.
15. Describe your production deployment process.
16. Give an example of where you have applied your technical knowledge in a practical way.
17. How did you manage source control?
18. What did you do to ensure quality in your deliverables?
19. What percentage of your time do you spend unit testing?
20. What do you expect in the solution documents you are provided?
21. Describe a time when you were able to improve upon the design that was originally suggested.
22. How much reuse do you get out of the code that you develop, and how?
23. Which do you prefer; service oriented or batch oriented solutions?
24. When is the last time you downloaded a utility from the internet to make your work more productive, and what was it?
25. What have you done to ensure consistency across unit, quality, and production environments?
26. Describe the elements of an in tier architecture and their appropriate use.
27. Compare and contrast REST and SOAP web services.
28. Define authentication and authorization and the tools that are used to support them in enterprise deployments.
29. What is ETL and when should it be used?
30. You have been asked to research a new business tool. You have come across two solutions. One is an on-premises solution, the other is cloud-based. Assuming they are functionally equivalent, would you recommend one over the other, and why?
31. What do you do to ensure you provide accurate project estimates?
32. What technical websites do you follow?
33. Have you used Visual Studio?
34. Have you used Eclipse?
35. What is a SAN, and how is it used?
36. What is clustering, and describe its use.
37. What is the role of the DMZ in network architecture?
38. How do you enforce relational integrity in database design?
39. When is it appropriate to denormalize database design?
40. What is the difference between OLAP and OLTP? When is each used?
41. You have learned that a business unit is managing a major component of the business using Excel spreadsheets and Access databases. What risks does this present, and what would you recommend be done to mitigate those risks?
42. What automated-build tools or processes have you used?
43. What is the role of continuous integration systems in the automated-build process?
44. Describe the difference between optimistic and pessimistic locking.
45. In databases, what is the difference between a delete statement and a truncate statement?
46. What are transaction logs, and how are they used?
47. What are the most important database performance metrics, and how do you monitor them?
48. What is the role of SNMP?
49. What is a cross site scripting attack, and how do you defend against it?
50. In network security, what is a honey pot, and why is it used?

Killer job interview questions

10 killer job interview questions and how to answer them

Dear Readers,
Are you ready for that all-important job interview? I’m sure that if you’re reading this you must be thinking about applying for a job abroad or in an international corporation. Most interviews are designed not only to check if you’re fit for the job, but also examine your command of spoken English. BEM Blog’s list of top 10 killer interview questions will help you prepare to get the job you’ve dreamed about.

image source: mensite.pl

1. First, tell me about yourself.

It may seem like an easy open-ended question anyone can answer correctly without much effort.
Actually, you can’t be more wrong. ‚Tell me about yourself’ does not mean ‚tell me everything’. What the hiring panel really wants to hear is a brief account of who you are and why you are the best candidate for this particular job.
Talk about what you’ve done to prepare yourself for the position and use a recent example to back it up. Ideally, you should go on for about 2-3 minutes and then ask if they would like to hear some more details. If they do, keep using examples to prove how your background and experience were useful in real-life business situations.

2. What is your long-term objective?

Be honest. Focus on your most achievable goal and how are you going to reach it. It is vital to have a clear vision of how your career should look like in the next 5 years or so and how to make this vision come true, for example:
Within the next five years, I would like to become the best team leader your company has ever hired. I want to work toward becoming the expert in managing others. I am confident that I’ll be fully prepared to take on any future responsibilities which might be presented  to me in the long run. For example, here is what I’m presently doing to prepare myself for a managing position… (and  go on to describe what you are already doing to reach your goals and objectives).

3. What is the most difficult situation you have had to face and how did you tackle it?

Have an example at hand. Select a difficult work situation (which was not caused by you and which can be quickly explained in a few sentences). When choosing the problem, focus on how YOU worked towards the solution. Focus on the skills required on the position you are applying for that helped you face the situation. Describe the results and tell them how the company benefitted from your actions.
Have in mind that the purpose of this question is to find out what your definition of ‚difficult’ is and to determine whether you can show a logical approach to problem solving. Explain how you defined the problem, what the other options were, why you selected the one you did and what the outcome was. Always end on a positive note.

4. What are your strengths?

Prepare a list of your proficiencies and choose three or four that are the most relevant to the job you are applying for. Concentrate on discussing your main strengths. Avoid popular clichés, such as:

1. I am a good team player…
2. I am a good negotiator…
3. I work very hard…

Focus on your more dynamic skills, such as:

1. I learn quickly.
2. I have a strong determination to succeed.
3. I have a positive attitude.
4. I can relate to people and achieve a common goal.
5. I make friends easily.

Remember that you may very likely be asked to give examples of the above, so be prepared.

5. What is your greatest weakness?

Do not say you have none - this will not sound very credible and might, in fact, make your interviewer  believe you are being over-confident. Another rookie mistake is trying to disguise one of your strengths as a weakness, for example:
Well, I’m such a hard worker. Sometimes I really work too hard. I should probably spend more time with my family, because all I do is work… Did I mention I work hard? hmmm… That’s because all I do is work, work, work…
You have two alternatives:

1. Use a professed weakness such as a lack of experience (not ability) on your part in an area that is not vital for the job.
2. Describe a personal or professional weakness that you’ve been working on improving. Focus on the steps you have taken to combat it, for example:
   I know the people from my team think I’m too demanding. I sometimes tend to drive them pretty hard but I’ve started reading on the subject some time ago and I’m getting much better at managing them by setting objectives.

6. Why do you want to leave your current employer?

Whatever your reasons for leaving were, do not think about them in negative terms. It is not appropriate to mention financial conditions as your primary reason for leaving. Focus on the working environment and state how you are looking for a new challenge, more responsibility, experience and a change of surroundings.

7. Why have you applied for this particular job?

By asking this question, your future employer is looking for evidence that the job suits you and involves doing things you enjoy. On the other hand, it gives him or her a chance to test your knowledge about the industry as well as the whole organisation. Make sure you have a good understanding of the role and the place you’re going to take in the company. Prove to them that your job goes in line with your character and passion.

8. How has your education prepared you for your career?

This is a broad question and you need to focus on the specific examples in your educational background which have given you the proficiency to do this particular job. If applying for a job in a technical field, be sure to mention any relevant achievements in that particular field and your passion for the subject.

9.What experience do you have in this field?

Never say ‚none’. If you’re applying for a job in a field totally unknown to you, think of any experience you’ve gained in learning new skills. Try to look for similarities between your previous jobs and the prospect one and come up with some examples of how you adjusted to the new situation, for example:
I’ve never worked in cosmetics before, but in my previous job I’ve learned a lot about sales and marketing and I’m confident I’ll be quick to learn the ropes of your industry very quickly.

10. What kind of salary do you need?

A question that can catch anyone off guard. Unless you have done some research and know precisely what your expectations are,try to avoid being the first to put the figure on the table. The best way to do this is by stating that your salary expectancy depends on the amount of responsibilities you’d have to take on. Do not have a specific amount in mind and do not be afraid to ask for more than the average industry wage. It is much better to provide your interviewer with a wide range rather than a specific sum, for example:
I’ve asked around and I know that a marketing manager doing a similar job in Berlin earns about EUR 3700 a month. Given that my job would also require organising three training seminars a month, I’d expect my salary to range between EUR 4000 and EUR 4500.

BZ

What are your experiences with tough job interviews? What was the hardest question you’ve been asked? Feel free to comment below.
VOCABULARY:

1. open-ended question – otwarte pytanie
2. panel – grupa, komitet
3. brief – krótki
4. achievable – osiągalny
5. approach – podejście
6. on a positive note – w pozytywny sposób
7. proficiency – zdolność
8. cliché – banał
9. credible – wiarygodny
10. rookie mistake – błąd początkującego
11. to read on the subject – czytać na jakiś temat
12. to to learn the ropes – poznać się na rzeczy
13. to catch sb off guard – zaskoczyć kogoś

PHP

1. What does a special set of tags <?= and ?> do in PHP? - The output is displayed directly to the browser.
2. What’s the difference between include and require? - It’s how they handle failures. If the file is not found by require(), it will cause a fatal error and halt the execution of the script. If the file is not found by include(), a warning will be issued, but execution will continue.
3. I am trying to assign a variable the value of 0123, but it keeps coming up with a different number, what’s the problem? - PHP Interpreter treats numbers beginning with 0 as octal. Look at the similar PHP interview questions for more numeric problems.


4. Would I use print "$a dollars" or "{$a} dollars" to print out the amount of dollars in this example? - In this example it wouldn’t matter, since the variable is all by itself, but if you were to print something like "{$a},000,000 mln dollars", then you definitely need to use the braces.
5. How do you define a constant? - Via define() directive, like define ("MYCONSTANT", 100);
6. How do you pass a variable by value? - Just like in C++, put an ampersand in front of it, like $a = &$b
7. Will comparison of string "10" and integer 11 work in PHP? - Yes, internally PHP will cast everything to the integer type, so numbers 10 and 11 will be compared.
8. When are you supposed to use endif to end the conditional statement? - When the original if was followed by : and then the code block without braces.
9. Explain the ternary conditional operator in PHP? - Expression preceding the ? is evaluated, if it’s true, then the expression preceding the : is executed, otherwise, the expression following : is executed.
10. How do I find out the number of parameters passed into function? - func_num_args() function returns the number of parameters passed in.
11. If the variable $a is equal to 5 and variable $b is equal to character a, what’s the value of $$b? - 100, it’s a reference to existing variable.
12. What’s the difference between accessing a class method via -> and via ::? - :: is allowed to access methods that can perform static operations, i.e. those, which do not require object initialization.
13. Are objects passed by value or by reference? - Everything is passed by value.
14. How do you call a constructor for a parent class? - parent::constructor($value)
15. What’s the special meaning of __sleep and __wakeup? - __sleep returns the array of all the variables than need to be saved, while __wakeup retrieves them.
16. Why doesn’t the following code print the newline properly?    <?php
                $str = ‘Hello, there.nHow are you?nThanks for visiting TechInterviews’;
                print $str;
        ?>
    Because inside the single quotes the n character is not interpreted as newline, just as a sequence of two characters - and n.
17. Would you initialize your strings with single quotes or double quotes? - Since the data inside the single-quoted string is not parsed for variable substitution, it’s always a better idea speed-wise to initialize a string with single quotes, unless you specifically need variable substitution.
18. How come the code <?php print "Contents: $arr[1]"; ?> works, but <?php print "Contents: $arr[1][2]"; ?> doesn’t for two-dimensional array of mine? - Any time you have an array with more than one dimension, complex parsing syntax is required. print "Contents: {$arr[1][2]}" would’ve worked.
19. What is the difference between characters 23 and x23? - The first one is octal 23, the second is hex 23.
20. With a heredoc syntax, do I get variable substitution inside the heredoc contents? - Yes.
21. I want to combine two variables together:

     $var1 = 'Welcome to ';
     $var2 = 'TechInterviews.com';
    

    What will work faster? Code sample 1:
    

    $var 3 = $var1.$var2;
    

    Or code sample 2:
    
    

    $var3 = "$var1$var2";
    

    Both examples would provide the same result - $var3 equal to "Welcome to TechInterviews.com". However, Code Sample 1 will work significantly faster. Try it out with large sets of data (or via concatenating small sets a million times or so), and you will see that concatenation works significantly faster than variable substitution.
22. For printing out strings, there are echo, print and printf. Explain the differences. - echo is the most primitive of them, and just outputs the contents following the construct to the screen. print is also a construct (so parentheses are optional when calling it), but it returns TRUE on successful output and FALSE if it was unable to print out the string. However, you can pass multiple parameters to echo, like:

     <?php echo 'Welcome ', 'to', ' ', 'TechInterviews!'; ?>

    and it will output the string "Welcome to TechInterviews!" print does not take multiple parameters. It is also generally argued that echo is faster, but usually the speed advantage is negligible, and might not be there for future versions of PHP. printf  is a function, not a construct, and allows such advantages as formatted output, but it’s the slowest way to print out data out of echo, print and printf.
23. I am writing an application in PHP that outputs a printable version of driving directions. It contains some long sentences, and I am a neat freak, and would like to make sure that no line exceeds 50 characters. How do I accomplish that with PHP? - On large strings that need to be formatted according to some length specifications, use wordwrap() or chunk_split().
24. What’s the output of the ucwords function in this example?

     $formatted = ucwords("TECHINTERVIEWS IS COLLECTION OF INTERVIEW QUESTIONS");
     print $formatted;

    What will be printed is TECHINTERVIEWS IS COLLECTION OF INTERVIEW QUESTIONS.
    ucwords() makes every first letter of every word capital, but it does not lower-case anything else. To avoid this, and get a properly formatted string, it’s worth using strtolower() first.
25. What’s the difference between htmlentities() and htmlspecialchars()? - htmlspecialchars only takes care of <, >, single quote ‘, double quote " and ampersand. htmlentities translates all occurrences of character sequences that have different meaning in HTML.
26. What’s the difference between md5(), crc32() and sha1() crypto on PHP? - The major difference is the length of the hash generated. CRC32 is, evidently, 32 bits, while sha1() returns a 128 bit value, and md5() returns a 160 bit value. This is important when avoiding collisions.
27. So if md5() generates the most secure hash, why would you ever use the less secure crc32() and sha1()? - Crypto usage in PHP is simple, but that doesn’t mean it’s free. First off, depending on the data that you’re encrypting, you might have reasons to store a 32-bit value in the database instead of the 160-bit value to save on space. Second, the more secure the crypto is, the longer is the computation time to deliver the hash value. A high volume site might be significantly slowed down, if frequent md5() generation is required.
28. How do you match the character ^ at the beginning of the string? - ^^

JAVA APPLET AND AJAX

When should I use an Java applet instead of AJAX?

Applets provide a rich experience on the client side and there are many things they can do that an AJAX application cannot do, such as custom data streaming, graphic manipulation, threading, and advanced GUIs. While DHTML with the use of AJAX has been able to push the boundaries on what you can do on the client, there are some things that it just cannot do. The reason AJAX is so popular is that it only requires functionality built into the browser (namely DHTML and AJAX capabilities). The user does not need to download and/or configure plugins. It is easy to incrementally update functionality and know that that functionality will readily available, and there are not any complicated deployment issues. That said, AJAX-based functionality does need to take browser differences into consideration. This is why we recommend using a JavaScript library such as Dojo which abstracts browser differences. So the "bottom line" is: If you are creating advanced UIs where you need more advanced features on the client where you want UI accuracy down to the pixel, to do complex computations on the client, use specialized networking techniques, and where you know that the applet plugin is available for your target audience, applets are the way to go. AJAX/DHTML works well for applications where you know the users are using the latest generation of browsers, where DHTML/AJAX "good enough" for you, and where your developers have JavaScript/DHTML/AJAX skills. Many amazing things can be done with AJAX/DHTML but there are limitations. AJAX and applets can be used together in the same UIs with AJAX providing the basic structure and applets providing more advanced functionality. The Java can communicate to JavaScript using the Live-Connect APIs. The question should not be should framed as do I use AJAX or applets, but rather which technology makes the best sense for what you are doing. AJAX and applets do not have to be mutually exclusive.

What kinds of applications is Ajax best suited for? 

We don’t know yet. Because this is a relatively new approach, our understanding of where Ajax can best be applied is still in its infancy. Sometimes the traditional web application model is the most appropriate solution to a problem.

Does this mean Adaptive Path is anti-Flash? 

Not at all. Macromedia is an Adaptive Path client, and we’ve long been supporters of Flash technology. As Ajax matures, we expect that sometimes Ajax will be the better solution to a particular problem, and sometimes Flash will be the better solution. We’re also interested in exploring ways the technologies can be mixed (as in the case of Flickr, which uses both).

Where can I find examples of AJAX? 

While components of AJAX have been around for some time (for instance, 1999 for XMLHttpRequest), it really didn't become that popular until Google took...

What is the XMLHttpRequest object? 

It offers a non-blocking way for JavaScript to communicate back to the web server to update only part of the web page.

Does Ajax have significant accessibility or browser compatibility limitations? Do Ajax applications break the back button? Is Ajax compatible with REST? Are there security considerations with Ajax development? Can Ajax applications be made to work for users who have JavaScript turned off? 

The answer to all of these questions is “maybe”. Many developers are already working on ways to address these concerns. We think there’s more work to be done to determine all the limitations of Ajax, and we expect the Ajax development community to uncover more issues like these along the way.

How do I access data from other domains to create a mashup with Java? 

From your JavaScript clients you can access data in other domains if the return data is provide in JSON format. In essence you can create a JavaScript client that runs operates using data from a different server. This technique is know as JSON with Padding or JSONP. There are questions as to whether this method is secure as you are retrieving data from outside your domain and allowing it to be excuted in the context of your domain. Not all data from third parties is accessible as JSON and in some cases you may want an extra level of protection. With Java you can provide a proxy to third party services using a web component such as a servlet. This proxy can manage the communication with a third party service and provide the data to your clients in a format of your choosing. You can also cache data at your proxy and reduce trips to service. For more on using a Java proxy to create mashups see The XmlHttpProxy Client for Java.

Does Java have support for Comet style server-side push? 

Current AJAX applications use polling to communicate changes data between the server and client. Some applications, such as chat applications, stock tickers, or score boards require more immediate notifications of updates to the client. Comet is an event based low latency server side push for AJAX applications. Comet communication keeps one of the two connections available to the browser open to continously communicate events from the server to the client. A Java based solution for Comet is being developed for Glassfish on top of the Grizzly HTTP connector. See Enabling Grizzly by Jean-Francois Arcand for more details.

How do I create a thread to do AJAX polling? 

JavaScript does not have threads. JavaScript functions are called when an event happens in a page such as the page is loaded, a mouse click, or a form element gains focus. You can create a timer using the setTimeout which takes a function name and time in milliseconds as arguments. You can then loop by calling the same function as can be seen in the JavaScript example below.

function checkForMessage() {
// start AJAX interaction with processCallback as the callback function
}
// callback for the request
function processCallback() {
// do post processing
setTimeout("checkForMessage()", 10000);
}

Notice that the checkForMessage will continue to loop indefinitely. You may want to vary the increment the interval based on activity in the page or your use cases. You may also choose to have logic that would break out of the loop based on some AJAX response processing condition.

Is the XMLHttpRequest object part of a W3C standard? 

No. Or not yet. It is part of the DOM Level 3 Load and Save Specification proposal.

AJAX BUTTONS

How do I handle the back and forward buttons?

While you could go out and create a custom solution that tracks the current state on your application I recommend you leave this to the experts. Dojo addresses the navigation in a browser neutral way as can be seen in the JavaScript example below.
function updateOnServer(oldId, oldValue,
itemId, itemValue) {
var bindArgs = {
url: "faces/ajax-dlabel-update",
method: "post",
content: {"component-id": itemId, "component-value":
itemValue},
mimetype: "text/xml",
load: function(type, data) {
processUpdateResponse(data);
},
backButton: function() {
alert("old itemid was " + oldId);
},
forwardButton: function(){
alert("forward we must go!");
}
};
dojo.io.bind(bindArgs);
}

The example above will update a value on the server using dojo.io.bind() with a function as a property that is responsible for dealing with the browser back button event. As a developer you are capable of restoring the value to the oldValue or taking any other action that you see fit. The underlying details of how the how the browser button event are detected are hidden from the developer by Dojo.
AJAX: How to Handle Bookmarks and Back Buttons details this problem and provides a JavaScript library Really Simple History framework (RSH) that focuses just on the back and forward issue.
How does HTML_AJAX compare with the XAJAX project at Sourceforge?
XAJAX uses XML as a transport for data between the webpage and server, and you don't write your own javascript data handlers to manipulate the data received from the server. Instead you use a php class and built in javascript methods, a combination that works very similiar to the HTML_AJAX_Action class and haSerializer combo. XAJAX is designed for simplicity and ease of use.
HTML_AJAX allows for multiple transmission types for your ajax data - such as urlencoding, json, phpserialized, plain text, with others planned, and has a system you can use to write your own serializers to meet your specific needs. HTML_AJAX has a class to help generate javascript (HTML_AJAX_Helper) similiar to ruby on rail's javascript helper (although it isn't complete), and an action system similiar to XAJAX's "action pump" that allows you to avoid writing javascript data handlers if you desire.
But it also has the ability to write your own data handling routines, automatically register classes and methods using a server "proxy" script, do different types of callbacks including grabbing remote urls, choose between sync and async requests, has iframe xmlhttprequest emulation fallback capabilities for users with old browsers or disabled activeX, and is in active development with more features planned (see the Road Map for details)
HTML_AJAX has additional features such as client pooling and priority queues for more advanced users, and even a javascript utility class. Although you can use HTML_AJAX the same way you use XAJAX, the additional features make it more robust, extensible and flexible. And it is a pear package, you can use the pear installer to both install and keep it up to date.
If you're asking which is "better" - as with most php scripts it's a matter of taste and need. Do you need a quick, simple ajax solution? Or do you want something that's flexible, extensible, and looking to incorporate even more great features? It depends on the project, you as a writer, and your future plans.
What browsers support AJAX?
Internet Explorer 5.0 and up, Opera 7.6 and up, Netscape 7.1 and up, Firefox 1.0 and up, Safari 1.2 and up, among others.
How do I send an image using AJAX?
While it may appear that images are being sent when using AJAX with an application like Google Maps what is really happening is that the URLs of images are being send as the response of an AJAX request and those URLs are being set using DHTML.
In this example an XML document is returned from an AJAX interaction and the category bar is populated.

<categories>
<category>
<cat-id>1</cat-id>
<name>Books</name>
<description>Fun to read</description>
<image-url>books_icon.gif</image-url>
</category>
<category>
<cat-id>2</cat-id>
<name>Electronics</name>
<description>Must have gadgets</description>
<image-url>electronics.gif</image-url>
</category>
</categories>


Notice that the image-url element contains the location of the URL for the image representing a category. The callback method of an AJAX interaction will parse the response XML document and call the addCategory function for each category included in the response XML document. The addCategory function looks up a table row element "categoryTable" in body of the page and adds a row to the element which contains the image.

<scrip type="text/javascript" >

...

function addCategory(id, name, imageSrc) {

var categoryTable = document.getElementById("categoryTable");
var row = document.createElement("tr");
var catCell = document.createElement("td");
var img = document.createElement("img");
img.src = ("images\\" + imageSrc);
var link = document.createElement("a");
link.className ="category";
link.appendChild(document.createTextNode(name));
link.setAttribute("onclick", "catalog?command=category&catid=" + id);
catCell.appendChild(img);
catCell.appendChild(link);
row.appendChild(catCell);
categoryTable.appendChild(row);
}
</script>

...

<table>
<tr>
<td width="300" bgoclor="lightGray">
<table id="categoryTable" border="0" cellpadding="0"></table>
</td>
<td id="body" width="100%">Body Here</td>
</tr>
</table>


Note that the source of the image is set to the image source. The image is loaded by a subsequent HTTP request for the image at the URL "images/books_icon.gif" or "images/electronic_icon.gif" that occurs when the img element is added to the categoryTable.

Will HTML_AJAX integrate with other Javascript AJAX libraries such as scriptaculous ? How would this integration look like? 

HTML_AJAX doesn't have specific plans to integrate with other JavaScript libraries. Part of this is because external dependencies make for a more complicated installation process. It might make sense to offer some optional dependencies on a library like scriptaculous automatically using its visual effects for the loading box or something, but there isn't a lot to gain from making default visuals like that flashier since they are designed to be easily replaceable.

Most integration would take place in higher level components. Its unclear whether higher level components like that should be part of HTML_AJAX delivered through PEAR or if they should just be supported by HTML_AJAX and made available from http://htmlajax.org or some other site. If your interested in building widgets or components based on HTML_AJAX please let me know.

HTML_AJAX does however offer the ability to use its library loading mechanism with any JavaScript library. I use scriptaculous in conjunction with HTML_AJAX and I load both libraries through the server.

To do this you just need to register the library with your server and load add its flag to your include line.

<?php
$this->server->registerJSLibrary('scriptaculous',
array('prototype.js','scriptaculous.js','builder.js','effects.js','dragdrop.js','controls.js','slider.js'), '/pathto/scriptaculous/');?>
<script type="text/javascrpt" src="server.php?client=scriptaculous"></script>

AJAX BENEFITS

Is the server or the client in control?

It depends. With AJAX the answer is more in between. Control can be more centralized in a server-side component or as a mix of client-side and server-side controllers.

* Centralized server-side controller - When having a more centralized controller the key is to make sure the data in client-side page is in sync with that of the server. Some applications may keep all the state on the server and push all updates to client DOM via a simple JavaScript controller.
* Client and server-side controllers - This architecture would use JavaScript to do all presentation related control, event processing, page manipulation, and rendering of model data on the client. The server-side would be responsible for things such as business logic and pushing updated model data to the client. In this case the server would not have intimate knowledge of the presentation short of the initial page that would be sent to the client page request.

There are some use cases where an entire AJAX application can be written in a single page. Keep in mind if you choose this type of architecture that navigation and bookmarking should be considered.
Both methods are viable depending on what you are trying to accomplish. I tend to prefer spreading the control across the client and server.

Is Ajax just another name for XMLHttpRequest? 

No. XMLHttpRequest is only part of the Ajax equation. XMLHttpRequest is the technical component that makes the asynchronous server communication possible; Ajax is our name for the overall approach described in the article, which relies not only on XMLHttpRequest, but on CSS, DOM, and other technologies.

How do I abort the current XMLHttpRequest? 

Just call the abort() method on the request.

What is the minimum version of PHP that needs to be running in order to use HTML_AJAX? 

The oldest PHP version i've fully tested HTML_AJAX is 4.3.11, but it should run on 4.2.0 without any problems. (Testing reports from PHP versions older then 4.3.11 would be appreciated.)

Why does HTML_AJAX hang on some server installs 

If you run into an HTML_AJAX problem only on some servers, chances are your running into a problem with output compression. If the output compression is handled in the PHP config we detect that and do the right thing, but if its done from an apache extension we have no way of knowing its going to compress the body. Some times setting HTML_AJAX::sendContentLength to false fixes the problem, but in other cases you'll need to disabled the extension for the AJAX pages. 

I've also seen problems caused by debugging extensions like XDebug, disabling the extension on the server page usually fixes that. Questions dealing with Using HTML_AJAX, and general JavaScript development

How do I get the XMLHttpRequest object? 

Depending upon the browser... if (window.ActiveXObject) { // Internet Explorer http_request = new ActiveXObject("Microsoft.XMLHTTP"); } else if...

Are there any security issues with AJAX? 

JavaScript is in plain view to the user with by selecting view source of the page. JavaScript can not access the local filesystem without the user's permission. An AJAX interaction can only be made with the servers-side component from which the page was loaded. A proxy pattern could be used for AJAX interactions with external services. 

You need to be careful not to expose your application model in such as way that your server-side components are at risk if a nefarious user to reverse engineer your application. As with any other web application, consider using HTTPS to secure the connection when confidential information is being exchanged.

What about applets and plugins ? 

Don't be too quick to dump your plugin or applet based portions of your application. While AJAX and DHTML can do drag and drop and other advanced user interfaces there still limitations especially when it comes to browser support. Plugins and applets have been around for a while and have been able to make AJAX like requests for years. Applets provide a great set of UI components and APIs that provide developers literally anything. 

Many people disregard applets or plugins because there is a startup time to initialize the plugin and there is no guarantee that the needed version of a plugin of JVM is installed. Plugins and applets may not be as capable of manipulating the page DOM. If you are in a uniform environment or can depend on a specific JVM or plugin version being available (such as in a corporate environment) a plugin or applet solution is great. 

One thing to consider is a mix of AJAX and applets or plugins. Flickr uses a combination of AJAX interactions/DHTML for labeling pictures and user interaction and a plugin for manipulating photos and photo sets to provide a great user experience. If you design your server-side components well they can talk to both types of clients.

Why did you feel the need to give this a name? 

I needed something shorter than “Asynchronous JavaScript+CSS+DOM+XMLHttpRequest” to use when discussing this approach with clients.

Is AJAX code cross browser compatible? 

Not totally. Most browsers offer a native XMLHttpRequest JavaScript object, while another one (Internet Explorer) require you to get it as an ActiveX object....

Techniques for asynchronous server communication have been around for years. What makes Ajax a “new” approach?

What’s new is the prominent use of these techniques in real-world applications to change the fundamental interaction model of the Web. Ajax is taking hold now because these technologies and the industry’s understanding of how to deploy them most effectively have taken time to develop.

Is Ajax a technology platform or is it an architectural style? 

It’s both. Ajax is a set of technologies being used together in a particular way.

Concurrent Request IN AJAX

How do I handle concurrent AJAX requests?

With JavaScript you can have more than one AJAX request processing at a single time. In order to insure the proper post processing of code it is recommended that you use JavaScript Closures. The example below shows an XMLHttpRequest object abstracted by a JavaScript object called AJAXInteraction. As arguments you pass in the URL to call and the function to call when the processing is done.
function AJAXInteraction(url, callback) {

var req = init();
req.onreadystatechange = processRequest;

function init() {
if (window.XMLHttpRequest) {
return new XMLHttpRequest();
} else if (window.ActiveXObject) {
return new ActiveXObject("Microsoft.XMLHTTP");
}
}

function processRequest () {
if (req.readyState == 4) {
if (req.status == 200) {
if (callback) callback(req.responseXML);
}
}
}

this.doGet = function() {
req.open("GET", url, true);
req.send(null);
}

this.doPost = function(body) {
req.open("POST", url, true);
req.setRequestHeader("Content-Type", "
application/x-www-form-urlencoded");
req.send(body);
}
}

function makeRequest() {
var ai = new AJAXInteraction("processme",
function() { alert("Doing Post Process");});
ai.doGet();
}

The function makeRequest() in the example above creates an AJAXInteraction with a URL to of "processme" and an inline function that will show an alert dialog with the message "Doing Post Process". When ai.doGet() is called the AJAX interaction is initiated and when server-side component mapped to the URL "processme" returns a document which is passed to the callback function that was specified when the AJAXInteraction was created.
Using this closures insures that the proper callback function associated with a specific AJAX interaction is called. Caution should still be taken when creating multiple closure objects in that make XmlHttpRequests as to there is a limited number of sockets that are used to make requests at any given time. Because there are limited number of requests that can be made concurrently. Internet Explorer for example only allows for two concurrent AJAX requests at any given time. Other browsers may allow more but it is generally between three and five requests. You may choose to use pool of AJAXInteraction objects.
One thing to note when making multiple AJAX calls from the client is that the calls are not guaranteed to return in any given order. Having closures within the callback of a closure object can be used to ensure dependencies are processed correctly.
There is a discussion titled Ajaxian Fire and Forget Pattern that is helpful.

What do I do on the server to interact with an AJAX client? 

The "Content-Type" header needs to be set to"text/xml". In servlets this may be done using the HttpServletResponse.setContentType()should be set to "text/xml" when the return type is XML. Many XMLHttpRequest implementations will result in an error if the "Content-Type" header is set The code below shows how to set the "Content-Type".

response.setContentType("text/xml");
response.getWriter().write("<response>invalid</response>");

You may also want to set whether or not to set the caches header for cases such as autocomplete where you may want to notify proxy servers/and browsers not to cache the results.

response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
response.getWriter().write("<response>invalid</response>");

Note to the developer: Internet Explorer will automatically use a cached result of any AJAX response from a HTTP GET if this header is not set which can make things difficult for a developer. During development mode you may want set this header. Where do I store state with an AJAX client

As with other browser based web applications you have a few options which include:
* On the client in cookies - The size is limited (generally around 4KB X 20 cookies per domain so a total of 80KB) and the content may not be secure unless encrypted which is difficult but not impossible using JavaScript.
* On the client in the page - This can be done securely but can be problematic and difficult to work with. See my blog entry on Storing State on the Client for more details on this topic.
* On the client file system - This can be done if the client grants access to the browser to write to the local file system. Depending on your uses cases this may be necessary but caution is advised.
* On the Server - This is closer to the traditional model where the client view is of the state on the server. Keeping the data in sync can be a bit problematic and thus we have a solution Refreshing Data on this. As more information processing and control moves to the client where state is stored will need to be re-evaluated.

Whats with the -alpha in the install instructions? 

HTML_AJAX hasn't had a stable release yet and the pear installer doesn't install non stable packages by default unless you specify a version.

How do I submit a form or a part of a form without a page refresh? 

When creating a form make sure that the "form" element "onSubmit" attribute is set to a JavaScript function that returns false. 

<form onSubmit="doAJAXSubmit();return false;" >
<input type="text" id="tf1" />
<input type="submit" id="submit1" value="Update"/>
</>

You can also submit data by associating a function with a form button in a similar way.

<form onSubmit="doAJAXSubmit();return false;" >
<input type="text" id="tf1" />
<input type="button" id="button1" onClick="doAJAXSubmit()" value="Update"/>
</>

Note that the form "onSubmit" attribute is still set. If the user hits the enter key in the text field the form will be submitted so you still need to handle that case.

When updating the page it is recommend you wait to make sure that the AJAX update of the form data was successful before updating the data in the page. Otherwise, the data may not properly update and the user may not know. I like to provide an informative message when doing a partial update and upon a successful AJAX interaction I will then update the page.

How do I test my AJAX code? 

There is a port of JUnit for client-side JavaScript called JsUnit

What exactly is the W3C DOM? 

The W3C Document Object Model (DOM) is defined by the W3C as the following: The Document Object Model is a platform- and language-neutral interface...

When will HTML_AJAX have a stable release? 

Once all the major features are complete and the API has been tested, the roadmap gives an idea of whats left to be done.

What parts of the HTML_AJAX API are stable?

We don't have a list right now, but most of the API is stable as of 0.3.0. There should be no major changes at this point, though there will be lots of new additions.

What Browsers does HTML_AJAX work with? 

As of 0.3.0, all the examples that ship with HTML_AJAX have been verified to work with
* Firefox 1.0+
* Internet Explorer 5.5+ (5.0 should work but it hasn't been tested)
Most things work with
* Safari 2+
* Opera 8.5+

AJAX ISSUES

Are there Usability Issues with AJAX?

The nature of updating a page dynamically using data retrieved via AJAX interactions and DHTML may result in drastically changing the appearance and state of a page. A user might choose to use the browser's back or forward buttons, bookmark a page, copy the URL from the URL bar and share it with a friend via an email or chat client, or print a page at any given time. When designing an AJAX based application you need to consider what the expected behavior would be in the case of navigation, bookmarking, printing, and browser support as described below.

* Navigation - What would be the expected behavior of the back, forward, refresh, and bookmark browser buttons in your application design. While you could implement history manipulation manually it may be easer to use a JavaScript frameworks such as Dojo that provides API's history manipulation and navigation control.
* Bookmarking and URL sharing - Many users want to bookmark or cut and paste the URL from the browser bar. Dojo provides client-side for bookmarking and URL manipulation.
* Printing - In some cases printing dynamically rendered pages can be problematic.
Other considerations as a developer when using AJAX are:
* Browser Support - Not all AJAX/DHTML features are supported on all browsers or all versions of a browser. See quirksmode.org for a list of browser support and possible workarounds.
* JavaScript disabled - You should also consider what happens if the user disables JavaScript. Additionally, there are several legitimate reasons why JavaScript and CSS support may be unavailable on a user's web browser.
* Latency - Keep in mind latency in your design. A running application will be much more responsive than when it is deployed.
Latency problems: myth or reality?
* Accessibility - Guaranteeing your site is accessible to people with disabilities is not only a noble goal, it is also requited by law in many markets. Some marvelous enabling technology is available to help people use the Web in spite of disabilities including visual, auditory, physical, speech, cognitive, and neurological disabilities. With a little forethought, and comprehension of some well documented best practices, you can assure that your application is compatible with that enabling technology.
Degradability is the term used to describe techniques used by web applications to adapt to the wide range of web browser capabilities. Many AJAX libraries have automatic degradability built in. But if you are coding your own custom AJAX functionality, simply taking some care to follow the best practices promoted by standards bodies like the World Wide Web Consortium (W3C), and grass root movements like the Web Standards community and many others, your application can run usefully on browsers that are incapable of AJAX behaviors. Granted, your application may loose some of the "wow factor" on these less capable browsers, but your application will still be usable.
Remember to not design with AJAX just for the sake of coolness. The reason you built your application is so people will use it. And people will not use your application if your application is not compatible with their web browser.

Are there any frameworks available to help speedup development with AJAX? 

There are several browser-side frameworks available, each with their own uniqueness...

Is Adaptive Path selling Ajax components or trademarking the name? Where can I download it? 

Ajax isn’t something you can download. It’s an approach — a way of thinking about the architecture of web applications using certain technologies. Neither the Ajax name nor the approach are proprietary to Adaptive Path.

Should I use an HTTP GET or POST for my AJAX calls? 

AJAX requests should use an HTTP GET request when retrieving data where the data will not change for a given request URL. An HTTP POST should be used when state is updated on the server. This is in line with HTTP idempotency recommendations and is highly recommended for a consistent web application architecture.

How do I debug JavaScript? 

There are not that many tools out there that will support both client-side and server-side debugging. I am certain this will change as AJAX applications proliferate. I currently do my client-side and server-side debugging separately. Below is some information on the client-side debuggers on some of the commonly used browsers.

* Firefox/Mozilla/Netscape - Have a built in debugger Venkman which can be helpful but there is a Firefox add on known as FireBug which provides all the information and AJAX developer would ever need including the ability to inspect the browser DOM, console access to the JavaScript runtime in the browser, and the ability to see the HTTP requests and responses (including those made by an XMLHttpRequest). I tend to develop my applications initially on Firefox using Firebug then venture out to the other browsers.
* Safari - Has a debugger which needs to be enabled. See the Safari FAQ for details.
* Internet Explorer - There is MSDN Documentation on debugging JavaScript. A developer toolbar for Internet Explorer may also be helpful.

While debuggers help a common technique knowing as "Alert Debugging" may be used. In this case you place "alert()" function calls inline much like you would a System.out.println. While a little primitive it works for most basic cases. Some frameworks such as Dojo provide APIs for tracking debug statements.

How do I provide internationalized AJAX interactions? 

Just because you are using XML does not mean you can properly send and receive localized content using AJAX requests. To provide internationalized AJAX components you need to do the following:

* Set the charset of the page to an encoding that is supported by your target languages. I tend to use UTF-8 because it covers the most languages. The following meta declaration in a HTML/JSP page will set the content type:

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
* In the page JavaScript make sure to encode any parameters sent to the server. JavaScript provides the escape() function which returns Unicode escape strings in which localized text will appear in hexadecimal format. For more details on JavaScript encoding see Comparing escape(), encodeURI(), and encodeURIComponent().
* On the server-side component set the character encoding using the HttpServletRequest.setCharacterEncoding() method. Before you access the localized parameter using the HttpServletRequest.getParameter() call. In the case of UTF this would be request.setCharactherEncoding("UTF-8");.
A server-side component returning AJAX responses needs to set the encoding of the response to the same encoding used in the page.
response.setContentType("text/xml;charset=;UTF-8");
response.getWriter().write(" <response>invalid </response>");

For more information on using AJAX with Java Enterprise Edition technologies see AJAX and Internationalization and for developing multi-lingual applications see Developing Multilingual Web Applications Using JavaServer Pages Technology.

Some of the Google examples you cite don’t use XML at all. Do I have to use XML and/or XSLT in an Ajax application? 

No. XML is the most fully-developed means of getting data in and out of an Ajax client, but there’s no reason you couldn’t accomplish the same effects using a technology like JavaScript Object Notation or any similar means of structuring data for interchange.
Are Ajax applications easier to develop than traditional web applications?

Not necessarily. Ajax applications inevitably involve running complex JavaScript code on the client. Making that complex code efficient and bug-free is not a task to be taken lightly, and better development tools and frameworks will be needed to help us meet that challenge.

When do I use a synchronous versus a asynchronous request? 

Good question. They don't call it AJAX for nothing! A synchronous request would block in page event processing and I don't see many use cases where a synchronous request is preferable.

AJAX ADVANTAGES

Did Adaptive Path invent Ajax? Did Google? Did Adaptive Path help build Google’s Ajax applications?

Neither Adaptive Path nor Google invented Ajax. Google’s recent products are simply the highest-profile examples of Ajax applications. Adaptive Path was not involved in the development of Google’s Ajax applications, but we have been doing Ajax work for some of our other clients.

Is it possible to set session variables from javascript? 

It's not possible to set any session variables directly from javascript as it is purely a client side technology. You can use AJAX though to asyncronously...

Cannot parse XML generated by JSP I am generating an XML using JSP, when i run the JSP in IE it shows the XML as per DOM, but when i try to parse it using Javascript , the command xmldoc.documentElement...

This is working code I am using, it might help you. if (!isIE) xmldoc = req.responseXML; else { //IE does not take the responseXML as...

What do I need to know to create my own AJAX functionality? 

If you plan not to reuse and existing AJAX component here are some of the things you will need to know.
Plan to learn Dynamic HTML (DHTML), the technology that is the foundation for AJAX. DHTML enables browser-base real time interaction between a user and a web page. DHTML is the combination of JavaScript, the Document Object Model (DOM) and Cascading Style Sheets (CSS).
* JavaScript - JavaScript is a loosely typed object based scripting language supported by all major browsers and essential for AJAX interactions. JavaScript in a page is called when an event in a page occurs such as a page load, a mouse click, or a key press in a form element.
* DOM - An API for accessing and manipulating structured documents. In most cases DOM represent the structure of XML and HTML documents.
* CSS - Allows you to define the presentation of a page such as fonts, colors, sizes, and positioning. CSS allow for a clear separation of the presentation from the content and may be changed programmatically by JavaScript.
Understanding the basic request/response nature of HTTP is also important. Many subtle bugs can result if you ignore the differences between the GET and OIst methods when configuring an XMLHttpRequest and HTTP response codes when processing callbacks.
JavaScript is the client-side glue, in a sense. JavaScript is used to create the XMLHttpRequest Object and trigger the asynchronous call. JavaScript is used to parse the returned content. JavaScript is used to analyze the returned data and process returned messages. JavaScript is used to inject the new content into the HTML using the DOM API and to modify the CSS.

Do I really need to learn JavaScript? 

Basically yes if you plan to develop new AJAX functionality for your web application.
On the other hand, JSF components and component libraries can abstract the details of JavaScript, DOM and CSS. These components can generate the necessary artifacts to make AJAX interactions possible. Visual tools such as Java Studio Creator may also use AJAX enabled JSF components to create applications, shielding the tool developer from many of the details of
 AJAX. If you plan to develop your own JSF components or wire the events of components together in a tool it is important that you have a basic understanding of JavaScript. There are client-side JavaScript libraries (discussed below) that you can call from your in page JavaScript that abstract browser differences. Object Hierarchy and Inheritance in JavaScript is a great resource for a Java developer to learn about JavaScript objects.


Do Ajax applications always deliver a better experience than traditional web applications? 

Not necessarily. Ajax gives interaction designers more flexibility. However, the more power we have, the more caution we must use in exercising it. We must be careful to use Ajax to enhance the user experience of our applications, not degrade it.

What JavaScript libraries and frameworks are available? 

There are many libraries/frameworks out there (and many more emerging) that will help abstract such things as all the nasty browser differences. Three good libraries are The Dojo Toolkit, Prototype, and DWR.
* The Dojo Toolkit contains APIs and widgets to support the development of rich web applications. Dojo contains an intelligent packaging system, UI effects, drag and drop APIs, widget APIs, event abstraction, client storage APIs, and AJAX interaction APIs. Dojo solves common usability issues such as support for dealing with the navigation such as the ability to detect the browser back button, the ability to support changes to the URL in the URL bar for bookmarking, and the ability to gracefully degrade when AJAX/JavaScript is not fully support on the client. Dojo is the Swiss Army Knife of JavaScript libraries. It provides the widest range of options in a single library and it does a very good job supporting new and older browsers.
* Prototype focuses on AJAX interactions including a JavaScript AJAX object that contains a few objects to do basic tasks such as make a request, update a portion of a document, insert content into a document, and update a portion of a document periodically. Prototype JavaScript library contains a set of JavaScript objects for representing AJAX requests and contains utility functions for accessing in page components and DOM manipulations. Script.aculo.us and Rico are built on top of Prototype and provide UI effects, support for drag and drop, and include common JavaScript centric widgets. If you are just looking to support AJAX interactions and a few basic tasks Prototype is great. If you are looking for UI effects Rico and Script.aculo.us are good options.
* Yahoo UI Library is a utility library and set of widgets using the APIs to support rich clients. The utility library includes support for cross-browser AJAX interactions, animation, DOM scriptging support, drag and drop, and cross browser event support. The Yahoo UI Library is well documnented and contains many examples.
* DWR (Dynamic Web Remoting) is a client-side and server-side framework that focuses on allowing a developer to do RPC calls from client-side JavaScript to plain old Java objects in a Java Enterprise Edition web container. On the server side DWR uses a Servlet to interact with the Java objects and returns object representations of the Java objects or XML documents. DWR will be easy to get up and running and plays well with other Java technologies. If you are looking for a client-side and server-side framework that integrates well use DWR.
* Google Web Toolkit (GWT) is client/server framework provided by Google that allows a developer to write an AJAX application in pure Java. The GWT takes care of the details of generating all the client-side code using a Java-to-JavaScript compiler. One of the key benefits of the GWT Software Developer Kit (SDK) is that it allows you to debug your applications in what is known as GWT hosted mode using an embedded browser (IE on Windows and Mozilla/Gecko on Linux) that is tied to the toolkit. In GWT hosted mode you setup through the code and debug it as it is running on both the client and server. The GWT contains a default set of widgets and widget containers. An application is built by coding a set of widgets and containers together much like would be done in a Swing application. The GWT Software Developer Kit (SDK) is limited to Linux and Windows XP/2000 though the web applications it generates are compatible with the latest generation of the mainstream browsers.

There are many new and emerging libraries for JavaScript and this list only reviews some of the more common libraries. When making a choice choose the library which suites your needs the best. While it might be better to choose one, there is nothing stopping you from using more than one framework. For a more extensive list of client-side frameworks see: Survey of AJAX/JavaScript Libraries.

What is the difference between proxied and proxyless calls? 

Proxied calls are made through stub objects that mimic your PHP classes on the JavaScript side. E.g., the helloworld class from the Hello World example.
Proxyless calls are made using utility javascript functions like HTML_AJAX.replace() and HTML_AJAX.append().

Should I use XML or text, JavaScript, or HTML as a return type? 

It depends. Clearly the 'X' in AJAX stands for XML, but several AJAX proponents are quick to point out that nothing in AJAX, per se, precludes using other types of payload, such as, JavaScript, HTML, or plain text.
* XML - Web Services and AJAX seem made for one another. You can use client-side API's for downloading and parsing the XML content from RESTful Web Services. (However be mindful with some SOAP based Web Services architectures the payloads can get quite large and complex, and therefore may be inappropriate with AJAX techniqes.)
* Plain Text - In this case server-generated text may be injected into a document or evaluated by client-side logic.
* JavaScript - This is an extension to the plain text case with the exception that a server-side component passes a fragment of JavaScript including JavaScript object declarations. Using the JavaScript eval() function you can then create the objects on the client. JavaScript Object Notation (JSON), which is a JavaScript object based data exchange specification, relies on this technique.
* HTML - Injecting server-generated HTML fragments directly into a document is generally a very effective AJAX technique. However, it can be complicated keeping the server-side component in sync with what is displayed on the client.
Mashup is a popular term for creating a completely new web application by combining the content from disparate Web Services and other online API's. A good example of a mashup is housingmaps.com which graphically combines housing want-ads from craiglist.org and maps from maps.google.com.

AJAX

What's AJAX ?

AJAX (Asynchronous JavaScript and XML) is a newly coined term for two powerful browser features that have been around for years, but were overlooked by many web developers until recently when applications such as Gmail, Google Suggest, and Google Maps hit the streets.

Asynchronous JavaScript and XML, or Ajax (pronounced "Aye-Jacks"), is a web development technique for creating interactive web applications using a combination of  XHTML (or HTML) and CSS for marking up and styling information. (XML is commonly used, although any format will work, including preformatted HTML, plain text, JSON and even EBML).

The Document Object Model manipulated through JavaScript to dynamically display and interact with the information presented

The XMLHttpRequest object to exchange data asynchronously with the web server. In some Ajax frameworks and in some situations, an IFrame object is used instead of the XMLHttpRequest object to exchange data with the web server.

Like DHTML, LAMP, or SPA, Ajax is not a technology in itself, but a term that refers to the use of a group of technologies together. In fact, derivative/composite technologies based substantially upon Ajax, such as AFLAX, are already appearing.

Ajax applications are mostly executed on the user's computer; they can perform a number of tasks without their performance being limited by the network. This permits the development of interactive applications, in particular reactive and rich graphic user interfaces.

Ajax applications target a well-documented platform, implemented by all major browsers on most existing platforms. While it is uncertain that this compatibility will resist the advent of the next generations of browsers (in particular, Firefox), at the moment, Ajax applications are effectively cross-platform.

While the Ajax platform is more restricted than the Java platform, current Ajax applications effectively fill part of the one-time niche of Java applets: extending the browser with portable, lightweight mini-applications.

Ajax isn’t a technology. It’s really several technologies, each flourishing in its own right, coming together in powerful new ways. Ajax incorporates:
* standards-based presentation using XHTML and CSS;
* dynamic display and interaction using the Document Object Model;
* data interchange and manipulation using XML and XSLT; * asynchronous data retrieval using XMLHttpRequest;
* and JavaScript binding everything together.

Who’s Using Ajax ?

Google is making a huge investment in developing the Ajax approach. All of the major products Google has introduced over the last year — Orkut, Gmail, the latest beta version of Google Groups, Google Suggest, and Google Maps — are Ajax applications. (For more on the technical nuts and bolts of these Ajax implementations, check out these excellent analyses of Gmail, Google Suggest, and Google Maps.) Others are following suit: many of the features that people love in Flickr depend on Ajax, and Amazon’s A9.com search engine applies similar techniques. 

These projects demonstrate that Ajax is not only technically sound, but also practical for real-world applications. This isn’t another technology that only works in a laboratory. And Ajax applications can be any size, from the very simple, single-function Google Suggest to the very complex and sophisticated Google Maps

At Adaptive Path, we’ve been doing our own work with Ajax over the last several months, and we’re realizing we’ve only scratched the surface of the rich interaction and responsiveness that Ajax applications can provide. Ajax is an important development for Web applications, and its importance is only going to grow. And because there are so many developers out there who already know how to use these technologies, we expect to see many more organizations following Google’s lead in reaping the competitive advantage Ajax provides.
Moving Forward

The biggest challenges in creating Ajax applications are not technical. The core Ajax technologies are mature, stable, and well understood. Instead, the challenges are for the designers of these applications: to forget what we think we know about the limitations of the Web, and begin to imagine a wider, richer range of possibilities

Should I consider AJAX? 

 
AJAX definitely has the buzz right now, but it might not be the right thing for you. AJAX is limited to the latest browsers, exposes browser compatibility issues, and requires new skill-sets for many. There is a good blog entry by Alex Bosworth on AJAX Mistakes which is a good read before you jump full force into AJAX. 

On the other hand you can achieve highly interactive rich web applications that are responsive and appear really fast. While it is debatable as to whether an AJAX based application is really faster, the user feels a sense of immediacy because they are given active feedback while data is exchanged in the background. If you are an early adopter and can handle the browser compatibility issues, and are willing to learn some more skills, then AJAX is for you. It may be prudent to start off AJAX-ifying a small portion or component of your application first. We all love technology, but just remember the purpose of AJAX is to enhance your user's experience and not hinder it.

Does AJAX work with Java?

Absolutely. Java is a great fit for AJAX! You can use Java Enterprise Edition servers to generate AJAX client pages and to serve incoming AJAX requests, manage server side state for AJAX clients, and connect AJAX clients to your enterprise resources. The JavaServer Faces component model is a great fit for defining and using AJAX components.

Won't my server-side framework provide me with AJAX?

You may be benefiting from AJAX already. Many existing Java based frameworks already have some level of AJAX interactions and new frameworks and component libraries are being developed to provide better AJAX support. I won't list all the Java frameworks that use AJAX here, out of fear of missing someone, but you can find a good list at www.ajaxpatterns.org/Java_Ajax_Frameworks.
If you have not chosen a framework yet it is recommended you consider using JavaServer Faces or a JavaServer Faces based framework. JavaServer Faces components can be created and used to abstract many of the details of generating JavaScript, AJAX interactions, and DHTML processing and thus enable simple AJAX used by JSF application developer and as plug-ins in JSF compatible IDE's, such as Sun Java Studio Creator.

Where should I start?

Assuming the framework you are using does not suffice your use cases and you would like to develop your own AJAX components or functionality I suggest you start with the article Asynchronous JavaScript Technology and XML (AJAX) With Java 2 Platform, Enterprise Edition.
If you would like to see a very basic example that includes source code you can check out the tech tip Using AJAX with Java Technology. For a more complete list of AJAX resources the Blueprints AJAX home page. 

Next, I would recommend spending some time investigating AJAX libraries and frameworks. If you choose to write your own AJAX clients-side script you are much better off not re-inventing the wheel. 

AJAX in Action by Dave Crane and Eric Pascarello with Darren James is good resource. This book is helpful for the Java developer in that in contains an appendix for learning JavaScript for the Java developer.

JAVA INTERVIEW QUESTIONS

1. What is the difference between procedural and object-oriented programs?- a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOP program, unit of program is object, which is nothing but combination of data and code. b) In procedural program, data is exposed to the whole program whereas in OOPs program, it is accessible with in the object and which in turn assures the security of the code.
2. What are Encapsulation, Inheritance and Polymorphism?- Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse. Inheritance is the process by which one object acquires the properties of another object. Polymorphism is the feature that allows one interface to be used for general class actions.
3. What is the difference between Assignment and Initialization?- Assignment can be done as many times as desired whereas initialization can be done only once.


4. What is OOPs?- Object oriented programming organizes a program around its data, i. e. , objects and a set of well defined interfaces to that data. An object-oriented program can be characterized as data controlling access to code.
5. What are Class, Constructor and Primitive data types?- Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a type of object according to the data the object can hold and the operations the object can perform. Constructor is a special kind of method that determines how an object is initialized when created. Primitive data types are 8 types and they are: byte, short, int, long, float, double, boolean, char.
6. What is an Object and how do you allocate memory to it?- Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.
7. What is the difference between constructor and method?- Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.
8. What are methods and how are they defined?- Methods are functions that operate on instances of classes in which they are defined. Objects can communicate with each other using methods and can call methods in other classes. Method definition has four parts. They are name of the method, type of object or primitive type the method returns, a list of parameters and the body of the method. A method’s signature is a combination of the first three parts mentioned above.
9. What is the use of bin and lib in JDK?- Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.
10. What is casting?- Casting is used to convert the value of one type to another.
11. How many ways can an argument be passed to a subroutine and explain them?- An argument can be passed in two ways. They are passing by value and passing by reference. Passing by value: This method copies the value of an argument into the formal parameter of the subroutine. Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.
12. What is the difference between an argument and a parameter?- While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.
13. What are different types of access modifiers?- public: Any thing declared as public can be accessed from anywhere. private: Any thing declared as private can’t be seen outside of its class. protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in the other packages. default modifier : Can be accessed only to classes in the same package.
14. What is final, finalize() and finally?- final : final keyword can be used for class, method and variables. A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods. A final method can’t be overridden. A final variable can’t change from its initialized value. finalize() : finalize() method is used just before an object is destroyed and can be called just prior to garbage collection. finally : finally, a key word used in exception handling, creates a block of code that will be executed after a try/catch block has completed and before the code following the try/catch block. The finally block will execute whether or not an exception is thrown. For example, if a method opens a file upon exit, then you will not want the code that closes the file to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this contingency.
15. What is UNICODE?- Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.
16. What is Garbage Collection and how to call it explicitly?- When an object is no longer referred to by any variable, java automatically reclaims memory used by that object. This is known as garbage collection. System. gc() method may be used to call it explicitly.
17. What is finalize() method?- finalize () method is used just before an object is destroyed and can be called just prior to garbage collection.
18. What are Transient and Volatile Modifiers?- Transient: The transient modifier applies to variables only and it is not stored as part of its object’s Persistent state. Transient variables are not serialized. Volatile: Volatile modifier applies to variables only and it tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of the program.
19. What is method overloading and method overriding?- Method overloading: When a method in a class having the same method name with different arguments is said to be method overloading. Method overriding : When a method in a class having the same method name with same arguments is said to be method overriding.
20. What is difference between overloading and overriding?- a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method. b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass. c) In overloading, separate methods share the same name whereas in overriding, subclass method replaces the superclass. d) Overloading must have different method signatures whereas overriding must have same signature.
21. What is meant by Inheritance and what are its advantages?- Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses.
22. What is the difference between this() and super()?- this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.
23. What is the difference between superclass and subclass?- A super class is a class that is inherited whereas sub class is a class that does the inheriting.
24. What modifiers may be used with top-level class?- public, abstract and final can be used for top-level class.
25. What are inner class and anonymous class?- Inner class : classes defined in other classes, including those defined in methods are called inner classes. An inner class can have any accessibility including private. Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.
26. What is a package?- A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management.
27. What is a reflection package?- java. lang. reflect package has the ability to analyze itself in runtime.
28. What is interface and its use?- Interface is similar to a class which may contain method’s signature only but not bodies and it is a formal set of method and constant declarations that must be defined by the class that implements it. Interfaces are useful for: a)Declaring methods that one or more classes are expected to implement b)Capturing similarities between unrelated classes without forcing a class relationship. c)Determining an object’s programming interface without revealing the actual body of the class.
29. What is an abstract class?- An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.
30. What is the difference between Integer and int?- a) Integer is a class defined in the java. lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other. b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.
31. What is a cloneable interface and how many methods does it contain?- It is not having any method because it is a TAGGED or MARKER interface.
32. What is the difference between abstract class and interface?- a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract. b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods. c) Abstract class must have subclasses whereas interface can’t have subclasses.
33. Can you have an inner class inside a method and what variables can you access?- Yes, we can have an inner class inside a method and final variables can be accessed.
34. What is the difference between String and String Buffer?- a) String objects are constants and immutable whereas StringBuffer objects are not. b) String class supports constant strings whereas StringBuffer class supports growable and modifiable strings.
35. What is the difference between Array and vector?- Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.
36. What is the difference between exception and error?- The exception class defines mild error conditions that your program encounters. Exceptions can occur when trying to open the file, which does not exist, the network connection is disrupted, operands being manipulated are out of prescribed ranges, the class file you are interested in loading is missing. The error class defines serious error conditions that you should not attempt to recover from. In most cases it is advisable to let the program terminate when such an error is encountered.
37. What is the difference between process and thread?- Process is a program in execution whereas thread is a separate path of execution in a program.
38. What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?- Multithreading is the mechanism in which more than one thread run independent of each other within the process. wait (), notify () and notifyAll() methods can be used for inter-thread communication and these methods are in Object class. wait() : When a thread executes a call to wait() method, it surrenders the object lock and enters into a waiting state. notify() or notifyAll() : To remove a thread from the waiting state, some other thread must make a call to notify() or notifyAll() method on the same object.
39. What is the class and interface in java to create thread and which is the most advantageous method?- Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here.
40. What are the states associated in the thread?- Thread contains ready, running, waiting and dead states.
41. What is synchronization?- Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.
42. When you will synchronize a piece of your code?- When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption.
43. What is deadlock?- When two threads are waiting each other and can’t precede the program is said to be deadlock.
44. What is daemon thread and which method is used to create the daemon thread?- Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.
45. Are there any global variables in Java, which can be accessed by other part of your program?- No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.
46. What is an applet?- Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable browser.
47. What is the difference between applications and applets?- a)Application must be run on local machine whereas applet needs no explicit installation on local machine. b)Application must be run explicitly within a java-compatible virtual machine whereas applet loads and runs itself automatically in a java-enabled browser. d)Application starts execution with its main method whereas applet starts execution with its init method. e)Application can run with or without graphical user interface whereas applet must run within a graphical user interface.
48. How does applet recognize the height and width?- Using getParameters() method.
49. When do you use codebase in applet?- When the applet class file is not in the same directory, codebase is used.
50. What is the lifecycle of an applet?- init() method - Can be called when an applet is first loaded start() method - Can be called each time an applet is started. paint() method - Can be called when the applet is minimized or maximized. stop() method - Can be used when the browser moves off the applet’s page. destroy() method - Can be called when the browser is finished with the applet.
51. How do you set security in applets?- using setSecurityManager() method
52. What is an event and what are the models available for event handling?- An event is an event object that describes a state of change in a source. In other words, event occurs when an action is generated, like pressing button, clicking mouse, selecting a list, etc. There are two types of models for handling events and they are: a) event-inheritance model and b) event-delegation model
53. What are the advantages of the model over the event-inheritance model?- The event-delegation model has two advantages over the event-inheritance model. They are: a)It enables event handling by objects other than the ones that generate the events. This allows a clean separation between a component’s design and its use. b)It performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to be repeatedly process unhandled events as is the case of the event-inheritance.
54. What is source and listener?- source : A source is an object that generates an event. This occurs when the internal state of that object changes in some way. listener : A listener is an object that is notified when an event occurs. It has two major requirements. First, it must have been registered with one or more sources to receive notifications about specific types of events. Second, it must implement methods to receive and process these notifications.
55. What is adapter class?- An adapter class provides an empty implementation of all methods in an event listener interface. Adapter classes are useful when you want to receive and process only some of the events that are handled by a particular event listener interface. You can define a new class to act listener by extending one of the adapter classes and implementing only those events in which you are interested. For example, the MouseMotionAdapter class has two methods, mouseDragged()and mouseMoved(). The signatures of these empty are exactly as defined in the MouseMotionListener interface. If you are interested in only mouse drag events, then you could simply extend MouseMotionAdapter and implement mouseDragged() .
56. What is meant by controls and what are different types of controls in AWT?- Controls are components that allow a user to interact with your application and the AWT supports the following types of controls: Labels, Push Buttons, Check Boxes, Choice Lists, Lists, Scrollbars, Text Components. These controls are subclasses of Component.
57. What is the difference between choice and list?- A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices and only one item may be selected from a choice. A List may be displayed in such a way that several list items are visible and it supports the selection of one or more list items.
58. What is the difference between scrollbar and scrollpane?- A Scrollbar is a Component, but not a Container whereas Scrollpane is a Conatiner and handles its own events and perform its own scrolling.
59. What is a layout manager and what are different types of layout managers available in java AWT?- A layout manager is an object that is used to organize components in a container. The different layouts are available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.
60. How are the elements of different layouts organized?- FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion. BorderLayout: The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container. CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards. GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid. GridBagLayout: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different size and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.
61. Which containers use a Border layout as their default layout?- Window, Frame and Dialog classes use a BorderLayout as their layout.
62. Which containers use a Flow layout as their default layout?- Panel and Applet classes use the FlowLayout as their default layout.
63. What are wrapper classes?- Wrapper classes are classes that allow primitive types to be accessed as objects.
64. What are Vector, Hashtable, LinkedList and Enumeration?- Vector : The Vector class provides the capability to implement a growable array of objects. Hashtable : The Hashtable class implements a Hashtable data structure. A Hashtable indexes and stores objects in a dictionary using hash codes as the object’s keys. Hash codes are integer values that identify objects. LinkedList: Removing or inserting elements in the middle of an array can be done using LinkedList. A LinkedList stores each object in a separate link whereas an array stores object references in consecutive locations. Enumeration: An object that implements the Enumeration interface generates a series of elements, one at a time. It has two methods, namely hasMoreElements() and nextElement(). HasMoreElemnts() tests if this enumeration has more elements and nextElement method returns successive elements of the series.
65. What is the difference between set and list?- Set stores elements in an unordered way but does not contain duplicate elements, whereas list stores elements in an ordered way but may contain duplicate elements.
66. What is a stream and what are the types of Streams and classes of the Streams?- A Stream is an abstraction that either produces or consumes information. There are two types of Streams and they are: Byte Streams: Provide a convenient means for handling input and output of bytes. Character Streams: Provide a convenient means for handling input & output of characters. Byte Streams classes: Are defined by using two abstract classes, namely InputStream and OutputStream. Character Streams classes: Are defined by using two abstract classes, namely Reader and Writer.
67. What is the difference between Reader/Writer and InputStream/Output Stream?- The Reader/Writer class is character-oriented and the InputStream/OutputStream class is byte-oriented.
68. What is an I/O filter?- An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
69. What is serialization and deserialization?- Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.
70. What is JDBC?- JDBC is a set of Java API for executing SQL statements. This API consists of a set of classes and interfaces to enable programs to write pure Java Database applications.
71. What are drivers available?- a) JDBC-ODBC Bridge driver b) Native API Partly-Java driver c) JDBC-Net Pure Java driver d) Native-Protocol Pure Java driver
72. What is the difference between JDBC and ODBC?- a) OBDC is for Microsoft and JDBC is for Java applications. b) ODBC can’t be directly used with Java because it uses a C interface. c) ODBC makes use of pointers which have been removed totally from Java. d) ODBC mixes simple and advanced features together and has complex options for simple queries. But JDBC is designed to keep things simple while allowing advanced capabilities when required. e) ODBC requires manual installation of the ODBC driver manager and driver on all client machines. JDBC drivers are written in Java and JDBC code is automatically installable, secure, and portable on all platforms. f) JDBC API is a natural Java interface and is built on ODBC. JDBC retains some of the basic features of ODBC.
73. What are the types of JDBC Driver Models and explain them?- There are two types of JDBC Driver Models and they are: a) Two tier model and b) Three tier model Two tier model: In this model, Java applications interact directly with the database. A JDBC driver is required to communicate with the particular database management system that is being accessed. SQL statements are sent to the database and the results are given to user. This model is referred to as client/server configuration where user is the client and the machine that has the database is called as the server. Three tier model: A middle tier is introduced in this model. The functions of this model are: a) Collection of SQL statements from the client and handing it over to the database, b) Receiving results from database to the client and c) Maintaining control over accessing and updating of the above.
74. What are the steps involved for making a connection with a database or how do you connect to a database?a) Loading the driver : To load the driver, Class. forName() method is used. Class. forName(”sun. jdbc. odbc. JdbcOdbcDriver”); When the driver is loaded, it registers itself with the java. sql. DriverManager class as an available database driver. b) Making a connection with database: To open a connection to a given database, DriverManager. getConnection() method is used. Connection con = DriverManager. getConnection (”jdbc:odbc:somedb”, “user”, “password”); c) Executing SQL statements : To execute a SQL query, java. sql. statements class is used. createStatement() method of Connection to obtain a new Statement object. Statement stmt = con. createStatement(); A query that returns data can be executed using the executeQuery() method of Statement. This method executes the statement and returns a java. sql. ResultSet that encapsulates the retrieved data: ResultSet rs = stmt. executeQuery(”SELECT * FROM some table”); d) Process the results : ResultSet returns one row at a time. Next() method of ResultSet object can be called to move to the next row. The getString() and getObject() methods are used for retrieving column values: while(rs. next()) { String event = rs. getString(”event”); Object count = (Integer) rs. getObject(”count”);
75. What type of driver did you use in project?- JDBC-ODBC Bridge driver (is a driver that uses native(C language) libraries and makes calls to an existing ODBC driver to access a database engine).
76. What are the types of statements in JDBC?- Statement: to be used createStatement() method for executing single SQL statement PreparedStatement — To be used preparedStatement() method for executing same SQL statement over and over. CallableStatement — To be used prepareCall() method for multiple SQL statements over and over.
77. What is stored procedure?- Stored procedure is a group of SQL statements that forms a logical unit and performs a particular task. Stored Procedures are used to encapsulate a set of operations or queries to execute on database. Stored procedures can be compiled and executed with different parameters and results and may have any combination of input/output parameters.
78. How to create and call stored procedures?- To create stored procedures: Create procedure procedurename (specify in, out and in out parameters) BEGIN Any multiple SQL statement; END; To call stored procedures: CallableStatement csmt = con. prepareCall(”{call procedure name(?,?)}”); csmt. registerOutParameter(column no. , data type); csmt. setInt(column no. , column name) csmt. execute();
79. What is servlet?- Servlets are modules that extend request/response-oriented servers, such as java-enabled web servers. For example, a servlet might be responsible for taking data in an HTML order-entry form and applying the business logic used to update a company’s order database.
80. What are the classes and interfaces for servlets?- There are two packages in servlets and they are javax. servlet and
81. What is the difference between an applet and a servlet?- a) Servlets are to servers what applets are to browsers. b) Applets must have graphical user interfaces whereas servlets have no graphical user interfaces.
82. What is the difference between doPost and doGet methods?- a) doGet() method is used to get information, while doPost() method is used for posting information. b) doGet() requests can’t send large amount of information and is limited to 240-255 characters. However, doPost()requests passes all of its data, of unlimited length. c) A doGet() request is appended to the request URL in a query string and this allows the exchange is visible to the client, whereas a doPost() request passes directly over the socket connection as part of its HTTP request body and the exchange are invisible to the client.
83. What is the life cycle of a servlet?- Each Servlet has the same life cycle: a) A server loads and initializes the servlet by init () method. b) The servlet handles zero or more client’s requests through service() method. c) The server removes the servlet through destroy() method.
84. Who is loading the init() method of servlet?- Web server
85. What are the different servers available for developing and deploying Servlets?- a) Java Web Server b) JRun g) Apache Server h) Netscape Information Server i) Web Logic
86. How many ways can we track client and what are they?- The servlet API provides two ways to track client state and they are: a) Using Session tracking and b) Using Cookies.
87. What is session tracking and how do you track a user session in servlets?- Session tracking is a mechanism that servlets use to maintain state about a series requests from the same user across some period of time. The methods used for session tracking are: a) User Authentication - occurs when a web server restricts access to some of its resources to only those clients that log in using a recognized username and password. b) Hidden form fields - fields are added to an HTML form that are not displayed in the client’s browser. When the form containing the fields is submitted, the fields are sent back to the server. c) URL rewriting - every URL that the user clicks on is dynamically modified or rewritten to include extra information. The extra information can be in the form of extra path information, added parameters or some custom, server-specific URL change. d) Cookies - a bit of information that is sent by a web server to a browser and which can later be read back from that browser. e) HttpSession- places a limit on the number of sessions that can exist in memory. This limit is set in the session. maxresidents property.
88. What is Server-Side Includes (SSI)?- Server-Side Includes allows embedding servlets within HTML pages using a special servlet tag. In many servlets that support servlets, a page can be processed by the server to include output from servlets at certain points inside the HTML page. This is accomplished using a special internal SSINCLUDE, which processes the servlet tags. SSINCLUDE servlet will be invoked whenever a file with an. shtml extension is requested. So HTML files that include server-side includes must be stored with an . shtml extension.
89. What are cookies and how will you use them?- Cookies are a mechanism that a servlet uses to have a client hold a small amount of state-information associated with the user. a) Create a cookie with the Cookie constructor: public Cookie(String name, String value) b) A servlet can send a cookie to the client by passing a Cookie object to the addCookie() method of HttpServletResponse: public void HttpServletResponse. addCookie(Cookie cookie) c) A servlet retrieves cookies by calling the getCookies() method of HttpServletRequest: public Cookie[ ] HttpServletRequest. getCookie().
90. Is it possible to communicate from an applet to servlet and how many ways and how?- Yes, there are three ways to communicate from an applet to servlet and they are: a) HTTP Communication(Text-based and object-based) b) Socket Communication c) RMI Communication
91. What is connection pooling?- With servlets, opening a database connection is a major bottleneck because we are creating and tearing down a new connection for every page request and the time taken to create connection will be more. Creating a connection pool is an ideal approach for a complicated servlet. With a connection pool, we can duplicate only the resources we need to duplicate rather than the entire servlet. A connection pool can also intelligently manage the size of the pool and make sure each connection remains valid. A number of connection pool packages are currently available. Some like DbConnectionBroker are freely available from Java Exchange Works by creating an object that dispenses connections and connection Ids on request. The ConnectionPool class maintains a Hastable, using Connection objects as keys and Boolean values as stored values. The Boolean value indicates whether a connection is in use or not. A program calls getConnection() method of the ConnectionPool for getting Connection object it can use; it calls returnConnection() to give the connection back to the pool.
92. Why should we go for interservlet communication?- Servlets running together in the same server communicate with each other in several ways. The three major reasons to use interservlet communication are: a) Direct servlet manipulation - allows to gain access to the other currently loaded servlets and perform certain tasks (through the ServletContext object) b) Servlet reuse - allows the servlet to reuse the public methods of another servlet. c) Servlet collaboration - requires to communicate with each other by sharing specific information (through method invocation)
93. Is it possible to call servlet with parameters in the URL?- Yes. You can call a servlet with parameters in the syntax as (?Param1 = xxx || m2 = yyy).
94. What is Servlet chaining?- Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request. In servlet chaining, one servlet’s output is piped to the next servlet’s input. This process continues until the last servlet is reached. Its output is then sent back to the client.
95. How do servlets handle multiple simultaneous requests?- The server has multiple threads that are available to handle requests. When a request comes in, it is assigned to a thread, which calls a service method (for example: doGet(), doPost() and service()) of the servlet. For this reason, a single servlet object can have its service methods called by many threads at once.
96. What is the difference between TCP/IP and UDP?- TCP/IP is a two-way communication between the client and the server and it is a reliable and there is a confirmation regarding reaching the message to the destination. It is like a phone call. UDP is a one-way communication only between the client and the server and it is not a reliable and there is no confirmation regarding reaching the message to the destination. It is like a postal mail.
97. What is Inet address?- Every computer connected to a network has an IP address. An IP address is a number that uniquely identifies each computer on the Net. An IP address is a 32-bit number.
98. What is Domain Naming Service(DNS)?- It is very difficult to remember a set of numbers(IP address) to connect to the Internet. The Domain Naming Service(DNS) is used to overcome this problem. It maps one particular IP address to a string of characters. For example, www. mascom. com implies com is the domain name reserved for US commercial sites, moscom is the name of the company and www is the name of the specific computer, which is mascom’s server.
99. What is URL?- URL stands for Uniform Resource Locator and it points to resource files on the Internet. URL has four components: http://www. address. com:80/index.html, where http - protocol name, address - IP address or host name, 80 - port number and index.html - file path.
100. What is RMI and steps involved in developing an RMI object?- Remote Method Invocation (RMI) allows java object that executes on one machine and to invoke the method of a Java object to execute on another machine. The steps involved in developing an RMI object are: a) Define the interfaces b) Implementing these interfaces c) Compile the interfaces and their implementations with the java compiler d) Compile the server implementation with RMI compiler e) Run the RMI registry f) Run the application
101. What is RMI architecture?- RMI architecture consists of four layers and each layer performs specific functions: a) Application layer - contains the actual object definition. b) Proxy layer - consists of stub and skeleton. c) Remote Reference layer - gets the stream of bytes from the transport layer and sends it to the proxy layer. d) Transportation layer - responsible for handling the actual machine-to-machine communication.
102. what is UnicastRemoteObject?- All remote objects must extend UnicastRemoteObject, which provides functionality that is needed to make objects available from remote machines.
103. Explain the methods, rebind() and lookup() in Naming class?- rebind() of the Naming class(found in java. rmi) is used to update the RMI registry on the server machine. Naming. rebind(”AddSever”, AddServerImpl); lookup() of the Naming class accepts one argument, the rmi URL and returns a reference to an object of type AddServerImpl.
104. What is a Java Bean?- A Java Bean is a software component that has been designed to be reusable in a variety of different environments.
105. What is a Jar file?- Jar file allows to efficiently deploying a set of classes and their associated resources. The elements in a jar file are compressed, which makes downloading a Jar file much faster than separately downloading several uncompressed files. The package java. util. zip contains classes that read and write jar files.
106. What is BDK?- BDK, Bean Development Kit is a tool that enables to create, configure and connect a set of set of Beans and it can be used to test Beans without writing a code.
107. What is JSP?- JSP is a dynamic scripting capability for web pages that allows Java as well as a few special tags to be embedded into a web file (HTML/XML, etc). The suffix traditionally ends with .jsp to indicate to the web server that the file is a JSP files. JSP is a server side technology - you can’t do any client side validation with it. The advantages are: a) The JSP assists in making the HTML more functional. Servlets on the other hand allow outputting of HTML but it is a tedious process. b) It is easy to make a change and then let the JSP capability of the web server you are using deal with compiling it into a servlet and running it.
108. What are JSP scripting elements?- JSP scripting elements lets to insert Java code into the servlet that will be generated from the current JSP page. There are three forms: a) Expressions of the form <%= expression %> that are evaluated and inserted into the output, b) Scriptlets of the form<% code %>that are inserted into the servlet’s service method, and c) Declarations of the form <%! Code %>that are inserted into the body of the servlet class, outside of any existing methods.
109. What are JSP Directives?- A JSP directive affects the overall structure of the servlet class. It usually has the following form:<%@ directive attribute=”value” %> However, you can also combine multiple attribute settings for a single directive, as follows:<%@ directive attribute1=”value1″ attribute 2=”value2″ . . . attributeN =”valueN” %> There are two main types of directive: page, which lets to do things like import classes, customize the servlet superclass, and the like; and include, which lets to insert a file into the servlet class at the time the JSP file is translated into a servlet
110. What are Predefined variables or implicit objects?- To simplify code in JSP expressions and scriptlets, we can use eight automatically defined variables, sometimes called implicit objects. They are request, response, out, session, application, config, pageContext, and page.
111. What are JSP ACTIONS?- JSP actions use constructs in XML syntax to control the behavior of the servlet engine. You can dynamically insert a file, reuse JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Available actions include: jsp:include - Include a file at the time the page is requested. jsp:useBean - Find or instantiate a JavaBean. jsp:setProperty - Set the property of a JavaBean. jsp:getProperty - Insert the property of a JavaBean into the output. jsp:forward - Forward the requester to a newpage. Jsp: plugin - Generate browser-specific code that makes an OBJECT or EMBED
112. How do you pass data (including JavaBeans) to a JSP from a servlet?- (1) Request Lifetime: Using this technique to pass beans, a request dispatcher (using either “include” or forward”) can be called. This bean will disappear after processing this request has been completed. Servlet: request. setAttribute(”theBean”, myBean); RequestDispatcher rd = getServletContext(). getRequestDispatcher(”thepage. jsp”); rd. forward(request, response); JSP PAGE:<jsp: useBean id=”theBean” scope=”request” class=”. . . . . ” />(2) Session Lifetime: Using this technique to pass beans that are relevant to a particular session (such as in individual user login) over a number of requests. This bean will disappear when the session is invalidated or it times out, or when you remove it. Servlet: HttpSession session = request. getSession(true); session. putValue(”theBean”, myBean); /* You can do a request dispatcher here, or just let the bean be visible on the next request */ JSP Page:<jsp:useBean id=”theBean” scope=”session” class=”. . . ” /> 3) Application Lifetime: Using this technique to pass beans that are relevant to all servlets and JSP pages in a particular app, for all users. For example, I use this to make a JDBC connection pool object available to the various servlets and JSP pages in my apps. This bean will disappear when the servlet engine is shut down, or when you remove it. Servlet: GetServletContext(). setAttribute(”theBean”, myBean); JSP PAGE:<jsp:useBean id=”theBean” scope=”application” class=”. . . ” />
113. How can I set a cookie in JSP?- response. setHeader(”Set-Cookie”, “cookie string”); To give the response-object to a bean, write a method setResponse (HttpServletResponse response) - to the bean, and in jsp-file:<% bean. setResponse (response); %>
114. How can I delete a cookie with JSP?- Say that I have a cookie called “foo, ” that I set a while ago & I want it to go away. I simply: <% Cookie killCookie = new Cookie(”foo”, null); KillCookie. setPath(”/”); killCookie. setMaxAge(0); response. addCookie(killCookie); %>
115. How are Servlets and JSP Pages related?- JSP pages are focused around HTML (or XML) with Java codes and JSP tags inside them. When a web server that has JSP support is asked for a JSP page, it checks to see if it has already compiled the page into a servlet. Thus, JSP pages become servlets and are transformed into pure Java and then compiled, loaded into the server and executed.