Q: What is a output comment?
A: 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
Q: What is a Hidden Comment?
A: A comments that documents the JSP page but is not sent to the client. The JSP engine
ignores a hidden comment, and does not process any code within hidden comment tags. A
hidden comment is not sent to the client, either in the displayed JSP page or the HTML page
source. The hidden comment is useful when you want to hide or “comment out” part of your JSP
page.
You can use any characters in the body of the comment except the closing –%> combination. If
you need to use –%> in your comment, you can escape it by typing –%\>.
JSP Syntax
<%-- comment --%>
Examples
<%@ page language="java" %>
<%-- This comment will not be visible to the colent in the page source --%>
Q: What is a Expression?
21
A: An expression tag contains a scripting language expression that is evaluated, converted
to a String, and inserted where the expression appears in the JSP file. Because the value of an
expression is converted to a String, you can use an expression within text in a JSP file. Like
<%= someexpression %>
<%= (new java.util.Date()).toLocaleString() %>
You cannot use a semicolon to end an expression
Q: What is a Declaration?
A: A declaration declares one or more variables or methods for use later in the JSP source
file.
A declaration must contain at least one complete declarative statement. You can declare any
number of variables or methods within one declaration tag, as long as they are separated by
semicolons. The declaration must be valid in the scripting language used in the JSP file.
<%! somedeclarations %>
<%! int i = 0; %>
<%! int a, b, c; %>
Q: What is a Scriptlet?
A: 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
1.Declare variables or methods to use later in the file (see also Declaration).
2.Write expressions valid in the page scripting language (see also Expression).
3.Use any of the JSP implicit objects or any object declared with a
You must write plain text, HTML-encoded text, or other JSP tags outside the scriptlet.
Scriptlets are executed at request time, when the JSP engine processes the client request. If the
scriptlet produces output, the output is stored in the out object, from which you can display it.
Q: What are implicit objects? List them?
A: Certain objects that are available for the use in JSP documents without being declared
first. These objects are parsed by the JSP engine and inserted into the generated servlet. The
implicit objects re listed below
• request
• response
22
• pageContext
• session
• application
• out
• config
• page
• exception
Q: Difference between forward and sendRedirect?
A: 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.
Q: What are the different scope valiues for the
A: The different scope values for
1. page
2. request
3.session
4.application
Q: Explain the life-cycle mehtods in JSP?
A: 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.
23
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.
Q: How do I prevent the output of my JSP or Servlet pages from being cached by the
browser?
A: You will need to set the appropriate HTTP header attributes to prevent the dynamic
content output by the JSP page from being cached by the browser. Just execute the following
scriptlet at the beginning of your JSP pages to prevent them from being cached at the browser.
You need both the statements to take care of some of the older browser versions.
<%
response.setHeader(“Cache-Control”,”no-store”); //HTTP 1.1
response.setHeader(“Pragma\”,”no-cache”); //HTTP 1.0
response.setDateHeader (“Expires”, 0); //prevents caching at the proxy server
%>
Q: How does JSP handle run-time exceptions?
A: 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.
Q: How can I implement a thread-safe JSP page? What are the advantages and
Disadvantages of using it?
A: 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. 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
24
example above of not being able to use <%! %>. You should try really hard to make them threadsafe
the old fashioned way: by making them thread-safe .
Q: How do I use a scriptlet to initialize a newly instantiated bean?
A: 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.
value=”<%=java.text.DateFormat.getDateInstance().format(new java.util.Date()) %>” / >
<%-- scriptlets calling bean setter methods go here --%>
Q: 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?
A: You could make a simple wrapper function, like
<%!
String blanknull(String s) {
return (s == null) ? \”\” : s;
}
%>
then use it inside your JSP form, like
” >
Q: What’s a better approach for enabling thread-safe servlets and JSPs?
SingleThreadModel Interface or Synchronization?
A: Although the SingleThreadModel technique is easy to use, and works well for low volume
sites, it does not scale well. If you anticipate your users to increase in the future, you may be
better off implementing explicit synchronization for your shared data. The key however, is to
effectively minimize the amount of code that is synchronzied so that you take maximum
advantage of multithreading.
25
Also, note that SingleThreadModel is pretty resource intensive from the server\’s perspective. The
most serious issue however is when the number of concurrent requests exhaust the servlet
instance pool. In that case, all the unserviced requests are queued until something becomes free
- which results in poor performance. Since the usage is non-deterministic, it may not help much
even if you did add more memory and increased the size of the instance pool.
Q: How can I enable session tracking for JSP pages if the browser has disabled
cookies?
A: 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”);
%>
26
\’>hello2.jsp
hello2.jsp
<%@ page session="true" %>
<%
Integer i= (Integer )session.getValue(“num”);
out.println(“Num value in session is ” + i.intValue());
%>
Q: What is the difference b/w variable declared inside a declaration part and variable
declared in scriplet part?
A: 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.
Q: How does JSP handle run-time exceptions?
A: 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.
Q: How can I implement a thread-safe JSP page? What are the advantages and
Disadvantages of using it?
A: 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. 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
27
example above of not being able to use <%! %>. You should try really hard to make them threadsafe
the old fashioned way: by making them thread-safe .
Q: How do I use a scriptlet to initialize a newly instantiated bean?
A: 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.
value=”<%=java.text.DateFormat.getDateInstance().format(new java.util.Date()) %>” / >
<%-- scriptlets calling bean setter methods go here --%>
Q: 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?
A: You could make a simple wrapper function, like
<%!
String blanknull(String s) {
return (s == null) ? \”\” : s;
}
%>
then use it inside your JSP form, like
” >
Q: What’s a better approach for enabling thread-safe servlets and JSPs?
SingleThreadModel Interface or Synchronization?
A: Although the SingleThreadModel technique is easy to use, and works well for low volume
sites, it does not scale well. If you anticipate your users to increase in the future, you may be
better off implementing explicit synchronization for your shared data. The key however, is to
effectively minimize the amount of code that is synchronzied so that you take maximum
advantage of multithreading.
28
Also, note that SingleThreadModel is pretty resource intensive from the server\’s perspective. The
most serious issue however is when the number of concurrent requests exhaust the servlet
instance pool. In that case, all the unserviced requests are queued until something becomes free
- which results in poor performance. Since the usage is non-deterministic, it may not help much
even if you did add more memory and increased the size of the instance pool.
Q: How can I enable session tracking for JSP pages if the browser has disabled
cookies?
A: 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”);
%>
29
\’>hello2.jsp
hello2.jsp
<%@ page session="true" %>
<%
Integer i= (Integer )session.getValue(“num”);
out.println(“Num value in session is ” + i.intValue());
%>
Q: Is there a way to execute a JSP from the comandline or from my own application?
A: There is a little tool called JSPExecutor that allows you to do just that. The developers
(Hendrik Schreiber
a full blown servlet engine, but to provide means to use JSP for generating source code or
reports. Therefore most HTTP-specific features (headers, sessions, etc) are not implemented, i.e.
no reponseline or header is generated. Nevertheless you can use it to precompile JSP for your
website.
