JSP Interview Questions
What is a output comment?
What is a Hidden comment?
What is an Expression?
What is a Declaration ?
What is a Scriptlet?
What are implicit objects? List
them?
Difference between forward and
sendRedirect?
What are the different scope
values for the <jsp:useBean>?
Explain the life-cycle methods in
JSP?
How does JSP handle run-time
exceptions?
How can I implement a thread-safe
JSP page? What are the advantages and Disadvantages of using it?
How do I use a scriptlet to
initialize a newly instantiated bean?
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?
What's a better approach for
enabling thread-safe servlets and JSPs? SingleThreadModel Interface or
Synchronization?
How can I enable session tracking
for JSP pages if the browser has disabled cookies?
What is a output comment?
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() %>
-->
What is a Hidden Comment?
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" %>
<html>
<head><title>A Hidden
Comment </title></head>
<body>
<%-- This comment will not be
visible to the colent in the page source --%>
</body>
</html>
What is a Expression?
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
What is a Declaration?
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; %>
What is a Scriptlet?
1.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
can1.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 <jsp:useBean> tag.
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.
What are implicit objects? List
them?
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
pageContext
session
application
out
config
page
exception
Difference between forward and
sendRedirect?
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.
What are the different scope
valiues for the <jsp:useBean>?
The different scope values for
<jsp:useBean> are
1. page
2. request
3.session
4.application
Explain the life-cycle mehtods in
JSP?
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() andjspDestroy() 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.
How do I prevent the output of my
JSP or Servlet pages from being cached by the browser?
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 1.1response.setHeader("Pragma\","no-cache"); //HTTP
1.0response.setDateHeader ("Expires", 0); //prevents caching at the
proxy server%>
How does JSP handle run-time
exceptions?
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.
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 <%@ 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 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 .
How do I use a scriptlet to
initialize a newly instantiated bean?
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.
<jsp:useBean id="foo"
class="com.Bar.Foo" >
<jsp:setProperty name="foo"
property="today"
value="<%=java.text.DateFormat.getDateInstance().format(new
java.util.Date()) %>" / >
<%-- scriptlets calling bean setter methods go here
--%></jsp:useBean >
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
<%!String blanknull(String s)
{return (s == null) ? \"\" : s;}%>
then use it inside your JSP form,
like
<input type="text"
name="lastName" value="<%=blanknull(lastName)% >"
>
What's a better approach for
enabling thread-safe servlets and JSPs? SingleThreadModel Interface or
Synchronization?
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.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.
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
<%@ page session=\"true\"
%>
<%
Integer num = new Integer(100);
session.putValue("num",num);
String url
=response.encodeURL("hello2.jsp");
%>
<a
href=\'<%=url%>\'>hello2.jsp</a>hello2.jsp<%@ page
session="true" %>
<%Integer i= (Integer
)session.getValue("num");out.println("Num value in session is
" + i.intValue());
%>
JSP Interview Questions
What is the difference between
variable declared inside a declaration part and variable declared in scriplet
part?Is there a way to execute a JSP from the comandline or from my own
application?
What is the difference b/w
variable declared inside a declaration part and variable declared in scriplet
part?
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.
Is there a way to execute a JSP
from the comandline or from my own application?
There is a little tool called
JSPExecutor that allows you to do just that. The developers (Hendrik Schreiber
<hs@webapp.de> & Peter Rossbach <pr@webapp.de>) aim was not to
write 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.
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