Java Server Pages (JSP) Interview Questions And Answers - DoAnswers.com

JavaServer Pages (JSP) technology enables Web developers and designers to rapidly develop and easily maintain, information-rich, dynamic Web pages that leverage existing business systems. As part of the Java technology family, JSP technology enables rapid development of Web-based applications that are platform independent. JSP technology separates the user interface from content generation, enabling designers to change the overall page layout without altering the underlying dynamic content.

Benefits for Developers

If you are a Web page developer or designer who is familiar with HTML, you can:

Use JSP technology without having to learn the Java language: You can use JSP technology without learning how to write Java scriplets. Although scriptlets are no longer required to generate dynamic content, they are still supported to provide backward compatibility.

Extend the JSP language: Java tag library developers and designers can extend the JSP language with "simple tag handlers," which utilize a new, much simpler and cleaner, tag extension API. This spurs the growing number of pluggable, reusable tag libraries available, which in turn reduces the amount of code needed to write powerful Web applications.

Easily write and maintain pages: The JavaServer Pages Standard Tag Library (JSTL) expression language is now integrated into JSP technology and has been upgraded to support functions. The expression language can now be used instead of scriptlet expressions.

When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completly with in the web container. When a sendRedirtect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completly new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward.

JSP pages are typically compiled into Java platform servlet classes. As a result, JSP pages require a Java virtual machine that supports the Java platform servlet specification.

THe generated servlet class for a JSP page implements the HttpJspPage interface of the javax.servlet.jsp package. Hte HttpJspPage interface extends the JspPage interface which inturn extends the Servlet interface of the javax.servlet package. the generated servlet class thus implements all the methods of the these three interfaces. The JspPage interface declares only two mehtods - jspInit() and jspDestroy() that must be implemented by all JSP pages regardless of the client-server protocol. However the JSP specification has provided the HttpJspPage interfaec specifically for the JSp pages serving HTTP requests. This interface declares one method _jspService(). The jspInit()- The container calls the jspInit() to initialize te servlet instance.It is called before any other method, and is called only once for a servlet instance. The _jspservice()- The container calls the _jspservice() for each request, passing it the request and the response objects. The jspDestroy()- The container calls this when it decides take the instance out of service. It is the last method called n the servlet instance.

14. How can I enable session tracking for JSP pages if the browser has disabled cookies?

We know that session tracking uses cookies by default to associate a session identifier with a unique user. If the browser does not support cookies, or if cookies are disabled, you can still enable session tracking using URL rewriting. URL rewriting essentially includes the session ID within the link itself as a name/value pair. However, for this to be effective, you need to append the session ID for each and every link that is part of your servlet response. Adding the session ID to a link is greatly simplified by means of of a couple of methods: response.encodeURL() associates a session ID with a given URL, and if you are using redirection, response.encodeRedirectURL() can be used by giving the redirected URL as input. Both encodeURL() and encodeRedirectedURL() first determine whether cookies are supported by the browser; if so, the input URL is returned unchanged since the session ID will be persisted as a cookie. Consider the following example, in which two JSP files, say hello1.jsp and hello2.jsp, interact with each other. Basically, we create a new session within hello1.jsp and place an object within this session. The user can then traverse to hello2.jsp by clicking on the link present within the page. Within hello2.jsp, we simply extract the object that was earlier placed in the session and display its contents. Notice that we invoke the encodeURL() within hello1.jsp on the link used to invoke hello2.jsp; if cookies are disabled, the session ID is automatically appended to the URL, allowing hello2.jsp to still retrieve the session object. Try this example first with cookies enabled. Then disable cookie support, restart the brower, and try again. Each time you should see the maintenance of the session across pages. Do note that to get this example to work with cookies disabled at the browser, your JSP engine has to support URL rewriting. hello1.jsp \'>hello2.jsp hello2.jsp

15. How can I get to print the stacktrace for an exception occuring within my JSP page?

By printing out the exception's stack trace, you can usually diagonse a problem better when debugging JSP pages. By looking at a stack trace, a programmer should be able to discern which method threw the exception and which method called that method. However, you cannot print the stacktrace using the JSP out implicit variable, which is of type JspWriter. You will have to use a PrintWriter object instead. The following snippet demonstrates how you can print a stacktrace from within a JSP error page:

16. How can I get to view any compilation/parsing errors at the client while developing JSP pages?

With JSWDK 1.0, set the following servlet initialization property within the \WEB-INF\servlets.properties file for your application: jsp.initparams=sendErrToClient=true This will cause any compilation/parsing errors to be sent as part of the response to the client.

You can make your JSPs thread-safe by having them implement the SingleThreadModel interface. This is done by adding the directive <%@ page isThreadSafe="false" % > within your JSP page. EJB interview questions Is is possible for an EJB client to marshal an object of class java.lang.Class to an EJB? Answer: Technically yes, spec. compliant NO! - The enterprise bean must not attempt to query a class to obtain information about the declared members that are not otherwise accessible to the enterprise bean because of the security rules of the Java language. Is it legal to have static initializer blocks in EJB? Answer: Although technically it is legal, static initializer blocks are used to execute some piece of code before executing any constructor or method while instantiating a class. Static initializer blocks are also typically used to initialize static fields - which may be illegal in EJB if they are read/write - In EJB this can be achieved by including the code in either the ejbCreate(), setSessionContext() or setEntityContext() methods. Is it possible to stop the execution of a method before completion in a SessionBean? Answer: Stopping the execution of a method inside a Session Bean is not possible without writing code inside the Session Bean. This is because you are not allowed to access Threads inside an EJB.

18. How can I implement a thread-safe JSP page? What are the advantages and Disadvantages of using it?

You can make your JSPs thread-safe by having them implement the SingleThreadModel interface. This is done by adding the directive within your JSP page. With this, instead of a single instance of the servlet generated for your JSP page loaded in memory, you will have N instances of the servlet loaded and initialized, with the service method of each instance effectively synchronized. You can typically control the number of instances (N) that are instantiated for all servlets implementing SingleThreadModel through the admin screen for your JSP engine. More importantly, avoid using the tag for variables. If you do use this tag, then you should set isThreadSafe to true, as mentioned above. Otherwise, all requests to that page will access those variables, causing a nasty race condition. SingleThreadModel is not recommended for normal use. There are many pitfalls, including the example above of not being able to use . You should try really hard to make them thread-safe the old fashioned way: by making them thread-safe .

19. How can I prevent the word "null" from appearing in my HTML input text fields when I populate them with a resultset that has null values?

You could make a simple wrapper function, like then use it inside your JSP form, like