Jsp Interview Questions

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Q: What is JSP?

A: JSP is a dynamic scripting capability for web pages that allows Java as well as a few special tags to be embedded into a web file (HTML/XML, etc). The suffix traditionally ends with .jsp to indicate to the web server that the file is a JSP files. JSP is a server side technology - you can’t do any client side validation with it.The advantages are:

a) The JSP assists in making the HTML more functional. Servlets on the other hand allow outputting of HTML but it is a tedious process.

b) It is easy to make a change and then let the JSP capability of the web server you are using deal with compiling it into a servlet and running it.

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 Example 1 Displays in the page source:

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" %> <html> <head> <title>A Hidden Comment</title> </head> <body> <%-- This comment will not be visible to the colent in the page source --%> </body> </html>

Q: What is a Expression?

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 do the following

  • 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 <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.

Q: What are JSP Directives?

A: A JSP directive affects the overall structure of the servlet class. It usually has the following form:

<%@ directive attribute="value"%>

However, you can also combine multiple attribute settings for a single directive, as follows:

<%@ directive attribute1="value1"
attribute 2="value2"...
attributeN ="valueN" %>

There are two main types of directive: page, which lets to do things like import classes, customize the servlet superclass, and the like; and include, which lets to insert a file into the servlet class at the time the JSP file is translated into a servlet

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
pageContext
session
application
out
config
page
exception

Q: What are JSP ACTIONS?

A: JSP actions use constructs in XML syntax to control the behavior of the servlet engine. You can dynamically insert a file, reuse JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Available actions include:

jsp:include - Include a file at the time the page is requested.
jsp:useBean - Find or instantiate a JavaBean.
jsp:setProperty - Set the property of a JavaBean.
jsp:getProperty - Insert the property of a JavaBean into the output.
jsp:forward - Forward the requester to a newpage.
Jsp: plugin - Generate browser-specific code that makes an OBJECT or EMBED

Q: How do you pass data (including JavaBeans) to a JSP from a servlet?

A: (1) Request Lifetime: Using this technique to pass beans, a request dispatcher (using either "include" or "forward") can be called. This bean will disappear after processing this request has been completed.

Servlet:

request.setAttribute("theBean", myBean);
RequestDispatcher rd =
getServletContext().getRequestDispatcher("thepage.jsp");
rd.forward(request, response);

JSP PAGE:

jsp:useBean id="theBean" scope="request" class="....."

(2) Session Lifetime: Using this technique to pass beans that are relevant to a particular session (such as in individual user login) over a number of requests. This bean will disappear when the session is invalidated or it times out, or when you remove it.

Servlet:

HttpSession session = request.getSession(true); //Default false
session.putValue("theBean", myBean);
/* You can do a request dispatcher here, or just let the bean
be visible on the next request */

JSP Page:

jsp:useBean id="theBean" scope="session" class="..."

(3) Application Lifetime: Using this technique to pass beans that are relevant to all servlets and JSP pages in a particular app, for all users. For example, I use this to make a JDBC connection pool object available to the various servlets and JSP pages in my apps. This bean will disappear when the servlet engine is shut down, or when you remove it.

Servlet:

GetServletContext().setAttribute("theBean", myBean);

JSP PAGE:

jsp:useBean id="theBean" scope="application" class="..."

Q: How can I set a cookie in JSP?

A: Setting cookies from within a JSP page is similar to the way they are done within servlets. For example, the following scriptlet sets a cookie "mycookie" at the client:

<% Cookie mycookie = new Cookie("aName","aValue");
response.addCookie(mycookie); %>

Typically, cookies are set at the beginning of a JSP page, as they are sent out as part of the HTTP headers.

Q: How can I delete a cookie with JSP?

A: Say that I have a cookie called "foo," that I set a while ago and I want it to go away. I simply:

<% Cookie killCookie = new Cookie("foo", null);
killCookie.setPath("/");
killCookie.setMaxAge(0);
response.addCookie(killCookie); %>

Q: How are Servlets and JSP Pages related?

A: JSP pages are focused around HTML (or XML) with Java codes and JSP tags inside them. When a web server that has JSP support is asked for a JSP page, it checks to see if it has already compiled the page into a servlet. Thus, JSP pages become servlets and are transformed into pure Java and then compiled, loaded into the server and executed.

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 <jsp:useBean>?

A: The different scope values for <jsp:useBean> are 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. 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 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 .

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. <jsp:useBean id="foo" class="com.Bar.Foo"> <jsp:setProperty name="foo" property="today" value="function"/ > <%-- scriptlets calling bean setter methods go here --%> </jsp:useBean>

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 below

<%! String
blanknull(String s) { return (s == null) ? \"\" : s; } %>
then use it inside your JSP form, like below
<input type="text" name="lastName" value="">

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. 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"); %> <a href="#">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: 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 & Peter Rossbach ) 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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