1. What is a output comment?
 
Answer: A comment that is sent to the client in the viewable page source.The JSP engine handles an output comment as uninterpreted HTML text, returning the comment in the HTML output sent to the client. You can see the comment by viewing the page source from your Web browser.

JSP Syntax
<!-- comment [ <%= expression %> ] -->

Example 1
<!-- This is a commnet sent to client on
<%= (new java.util.Date()).toLocaleString() %>
-->

Displays in the page source:
<!-- This is a commnet sent to client on January 24, 2004 -->

2. What is the difference b/w variable declared inside a declaration part and variable declared in scriplet part?
 
Answer: Variable declared inside declaration part is treated as a global variable.that means after convertion jsp file into servlet that variable will be in outside of service method or it will be declared as instance variable.And the scope is available to complete jsp and to complete in the converted servlet class.where as if u declare a variable inside a scriplet that variable will be declared inside a service method and the scope is with in the service method. 

3. What is a translation unit?
Answer: JSP page can include the contents of other HTML pages or other JSP files. This is done by using the include directive. When the JSP engine is presented with such a JSP page it is converted to one servlet class and this is called a translation unit, Things to remember in a translation unit is that page directives affect the whole unit, one variable declaration cannot occur in the same unit more than once, the standard action jsp:useBean cannot declare the same bean twice in one unit.

4. What is a Expression
Answer: Expressions are act as place holders for language expression, expression is evaluated each time the page is accessed.

5. What is a Scriptlet
Answer: A scriptlet can contain any number of language statements, variable or method declarations, or expressions that are valid in the page scripting language. Within scriptlet tags, you can declare variables or methods to use later in the file, write expressions valid in the page scripting language, use any of the JSP implicit objects or any object declared with a .

6. What are the different scope values for the
Answer: The different scope values for are:

   1. page
   2. request
   3. session
   4. application

7. How does JSP handle run-time exceptions
Answer: You can use the errorPage attribute of the page directive to have uncaught run-time exceptions automatically forwarded to an error processing page. For example: <%@ page errorPage="error.jsp" %>
redirects the browser to the JSP page error.jsp if an uncaught exception is encountered during request processing. Within error.jsp, if you indicate that it is an error-processing page, via the directive: <%@ page isErrorPage="true" %> Throwable object describing the exception may be accessed within the error page via the exception implicit object. Note: You must always use a relative URL as the value for the errorPage attribute.

8. 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.) 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

9. How can I set a cookie and delete a cookie from within a JSP page
Answer: A cookie, mycookie, can be deleted using the following scriptlet:
    <%
    //creating a cookie
    Cookie mycookie = new Cookie("aName","aValue");
    response.addCookie(mycookie);
    //delete a cookie
    Cookie killMyCookie = new Cookie("mycookie", null);
    killMyCookie.setMaxAge(0);
    killMyCookie.setPath("/");
    response.addCookie(killMyCookie);
    %>

10. How do I have the JSP-generated servlet subclass my own custom servlet class, instead of the default
Answer: One should be very careful when having JSP pages extend custom servlet classes as opposed to the default one generated by the JSP engine. In doing so, you may lose out on any advanced optimization that may be provided by the JSP engine. In any case, your new superclass has to fulfill the contract with the JSP engine by:
Implementing the HttpJspPage interface, if the protocol used is HTTP, or implementing JspPage otherwise Ensuring that all the methods in the Servlet interface are declared final Additionally, your servlet superclass also needs to do the following:

The service() method has to invoke the _jspService() method

The init() method has to invoke the jspInit() method
The destroy() method has to invoke jspDestroy()
If any of the above conditions are not satisfied, the JSP engine may throw a translation error.
Once the superclass has been developed, you can have your JSP extend it as follows:

    <%@ page extends="packageName.ServletName" %>