JSP Interview questions-part3

Question: How do I use comments within a JSP page?

Answer: You can use "JSP-style" comments to selectively block out code while debugging or simply to comment your scriptlets. JSP comments are not visible at the client. For example:

<%-- the scriptlet is now commented out

<% out.println("Hello World"); %> --%>

You can also use HTML-style comments anywhere within your JSP page. These comments are visible at the client. For example:

Of course, you can also use comments supported by your JSP scripting language within your scriptlets. For example, assuming Java is the scripting language, you can have:

<% //some comment /** yet another comment **/ %>

Question: Response has already been commited error. What does it mean?

Answer: This error show only when you try to redirect a page after you already have written something in your page. This happens because HTTP specification force the header to be set up before the lay out of the page can be shown (to make sure of how it should be displayed...content-type="text/html" or "text/xml" or "plain-text" or "image/jpg", etc...) When you try to send a redirect status (Number is line_status_402), your HTTP server cannot send it right now if it hasn't finished to set up the header. If not starter to set up the header, there are no problems, but if it 's already begin to set up the header, then your HTTP server expects these headers to be finished setting up and it cannot be the case if the stream of the page is not over... In this last case it's like you have a file started with some output (like testing your variables...) ... and before you indicate that the file is over (and before the size of the page can be setted up in the header), you try to send a redirect status... It s simply impossible due to the specification of HTTP 1.0 and 1.1

Question: How do I use a scriptlet to initialize a newly instantiated bean?

Answer: A jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated. Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone.

The following example shows the "today" property of the Foo bean initialized to the current date when it is instantiated. Note that here, we make use of a JSP expression within the jsp:setProperty action.

"/ >

<%-- scriptlets calling bean setter methods go here --%>

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

Answer: 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

<%@ page session="true" %>

<% Integer num = new Integer(100);

session.putValue("num",num);

String url =response.encodeURL("hello2.jsp"); %>

'>hello2.jsp

hello2.jsp

<%@ page session="true" %>

<% Integer i= (Integer )session.getValue("num");

out.println("Num value in session is "+i.intValue());