Tuesday, 26 June 2012

servlets interview questions pdf

Servlets Faqs-->6


90) I have a global variable in a servlet class. What will happen to this global variable if two requests hit on the same time?

What will happen is an unforeseeable event.
The best way to establish a default occurrence (the servlet handles a request at a time) is to synchronize the access to the global variable or alternatively to create a servlet that implements the SingleThreadModel interface.

91) Suppose I have 2 servers, server1 and server2. How can I take data in a cookie from server1, and send it to server2?

You'll have to create a (new) similar cookie on server 2.
Have a ReadCookieServlet running on server1 that
  • Reads the cookie, using request.getCookies()
  • Redirects to WriteCookieServlet running on server2, passing the cookie name, value and expiration date as request parameters, using response.sendRedirect().
Have a WriteCookieServlet running on server2 that
  • Reads the cookie name, value and expiration date request parameters, using request.getParameter().
  • Creates a similar cookie, using response.addCookie().
92) How can I pass data from a servlet running in one context (webapp) to a servlet running in another context?

There are three ways I can think of off the top of my head:
  1. Store the information you want to share in a persistant format, such as in a file system or database. That way, any servlet that is running in a JVM that can "see" these resources can get to this information.
  2. If persisting this information is not an option, you can bind this information to a context that is accessible to all servlet contexts, such as the application server's context. This way, you can keep the data you want to share in memory.
  3. Use the old fashion way of passing information to a servlet - HTTP. One servlet could foward a request to another servlet and include the data that needs to be shared as parameters in the request.
93) How can I write an "error page" -- that is, a servlet or JSP to report errors of other servlets?

The Servlet 2.2 specification allows you to specify an error page (a servlet or a JSP) for different kinds of HTTP errors or ServletExceptions. You can specify this in deployment descriptor of the web application as:

        <error-page>
           <exception-type>FooException</exception-type>
           <location>/error.jsp</location>
         </error-page>
where FooException is a subclass of ServletException.
The web container invokes this servlet in case of errors, and you can access the following information from the request object of error servlet/JSP: error code, exception type, and a message.

94) What is the difference between ServletContext and ServletConfig?

A ServletContext represents the context in a servlet container of a servlet instance operates. A servlet container can have several contexts (or web applications) at one time. Each servlet instance is running in one of these contexts. All servlets instances running in the same context are part of the same web application and, therefore, share common resources. A servlet accesses these shared resource (such as a RequestDispatcher and application properties) through the ServletContext object.
This notion of a web application became very significant upon the Servlet 2.1 API, where you could deploy an entire web application in a WAR file. Notice that I always said "servlet instance", not servlet. That is because the same servlet can be used in several web applications at one time. In fact, this may be common if there is a generic controller servlet that can be configured at run time for a specific application. Then, you would have several instances of the same servlet running, each possibly having different configurations.
This is where the ServletConfig comes in. This object defines how a servlet is to be configured is passed to a servlet in its init method. Most servlet containers provide a way to configure a servlet at run-time (usually through flat file) and set up its initial parameters. The container, in turn, passes these parameters to the servlet via the ServetConfig.

95) Under what circumstances will a servlet be reloaded?

That depends on the Servlet container.
Most of the Servlet containers reload the servlet only it detects the code change in the Servlet, not in the referenced classes.
In Tomcat's server.xml deployment descriptor, if you have mentioned

        <Context path="/myApp"
                docBase="D:/myApp/webDev"
                crossContext="true"
                 debug="0"
                reloadable="true"
                trusted="false" >
        </Context>
The reloadable = true makes the magic. Every time the Servlet container detects that the Servlet code is changed, it will call the destroy on the currently loaded Servlet and reload the new code.
But if the class that is referenced by the Servlet changes, then the Servlet will not get loaded. You will have to change the timestamp of the servlet or stop-start the server to have the new class in the container memory.

96) What is a Servlet Filter?

A filter is basically a component that is invoked whenever a resource is invoked for which the filter is mapped. The resource can be something like a servlet, or a URL pattern. A filter normally works on the request, response, or header attributes, and does not itself send a response to the client.

97) I am using the RequestDispatcher's forward() method to redirect to a JSP. The problem is that the jsp's url is now relative to the servlet's url and all my url's in the jsp such as <img src="pic.gif"> will be corrupt. How do I solve this problem?

You can use absolute urls like:

        <BODY>
           <% String base = request.getContextPath(); %>
           <IMG src="/img/pic.gif">
        </BODY>
or write out a BASE tag like:

        <% String base = request.getContextPath(); %>
        <HEAD>
           <BASE HREF="<%=base%>">
        </HEAD>
        <BODY>
           <IMG src="img/pic.gif">
        </BODY>
That should take care of the problem.

98) How can I return a readily available (static) HTML page to the user instead of generating it in the servlet?

To solve your problem, you can either send a "Redirect" back to the client or use a RequestDispatcher and forward your request to another page:
  1. Redirect:
A redirection is made using the HttpServletResponse object:

  if(condition) {
     response.sendRedirect("page1.html");
  } else {
           response.sendRedirect("page2.html");
  }
  1. RequestDispatcher:
A request dispatcher can be obtained through the ServletContext. It can be used to include another page or to forward to it.

  if(condition) {
     this.getServletContext()
         .getRequestDispatcher("page1.html").forward();
  } else {
    this.getServletContext()
         .getRequestDispatcher("page2.html").forward();
  }
Both solutions require, that the pages are available in you document root. If they are located somewhere else on your filesystem, you have to open the file manually and copy their content to the output writer.
If your application server is set up in combination with a normal web server like Apache, you should use solution (1), because the the web server usually serves static files much faster than the application server.

99) What is the difference between static variables and instance variables in a servlet?

According to the Java Language definition, a static variable is shared among all instances of a class, where a non-static variable -- also called an instance variable -- is specific to a single instance of that class.
According to the Servlet specification, a servlet that does not declare SingleThreadModel usually has one and only one instance, shared among all concurrent requests hitting that servlet.
That means that, in servlets (and other multithreaded applications), an instance variable behaves very much like a static variable, since it is shared among all threads. You have to be very careful about synchronizing access to shared data.
The big difference between instance variables and static variables comes when you have configured your servlet engine to instantiate two instances of the same servlet class, but with different init parameters. In this case, there will be two instances of the same servlet class, which means two sets of instance variables, but only one set of static variables.
Remember that you can store data in lots of different places in a servlet. To wit:
  • Local variables - for loop iterators, result sets, and so forth
  • Request attributes - for data that must be passed to other servlets invoked with the RequestDispatcher
  • Session attributes - persists for all future requests from the current user only
  • Instance variables - for data that persists for the life of the servlet, shared with all concurrent users
  • Static variables - for data that persists for the life of the application, shared with all concurrent users -- including any other servlet instances that were instantiated with different init parameters
  • Context attributes - for data that must persist for the life of the application, and be shared with all other servlets
100) How can I share data between two different web applications?

Different servlets may share data within one application via ServletContext. If you have a compelling to put the servlets in different applications, you may wanna consider using EJBs.

Reactions:

0 comments:

Post a Comment