Servlets Interview Questions And Answers - DoAnswers.com

Servlets are the Java platform technology of choice for extending and enhancing Web servers. Servlets provide a component-based, platform-independent method for building Web-based applications, without the performance limitations of CGI programs. And unlike proprietary server extension mechanisms (such as the Netscape Server API or Apache modules), servlets are server- and platform-independent. This leaves you free to select a "best of breed" strategy for your servers, platforms, and tools.

Servlets have access to the entire family of Java APIs, including the JDBC API to access enterprise databases. Servlets can also access a library of HTTP-specific calls and receive all the benefits of the mature Java language, including portability, performance, reusability, and crash protection.

Today servlets are a popular choice for building interactive Web applications. Third-party servlet containers are available for Apache Web Server, Microsoft IIS, and others. Servlet containers are usually a component of Web and application servers, such as BEA WebLogic Application Server, IBM WebSphere, Sun Java System Web Server, Sun Java System Application Server, and others.

You might want to check out the latest information on JavaServer Pages (JSP) technology. JSP technology is an extension of the servlet technology created to support authoring of HTML and XML pages. It makes it easier to combine fixed or static template data with dynamic content. Even if you're comfortable writing servlets, there are several compelling reasons to investigate JSP technology as a complement to your existing work.

* Servlets are used to process the client request. * A Servlet can handle multiple request concurrently and be used to develop high performance system * A Servlet can be used to load balance among serveral servers, as Servlet can easily forward request.

J2EE Web components can be either servlets or JSP pages. Servlets are Java programming language classes that dynamically process requests and construct responses. JSP pages are textAnswer:based documents that execute as servlets but allow a more natural approach to creating static content.

Servlet mapping defines an association between a URL pattern and a servlet. You can use one servlet to process a number of url pattern (request pattern). For example in case of Struts *.do url patterns are processed by Struts Controller Servlet.

A servlet is a program written in the Java programming language that runs on the server, as opposed to the browser (applets). Detailed information can be found at http://java.sun.com/products/servlet.

A Session refers to all the request that a single client makes to a server. A session is specific to the user and for each user a new session is created to track all the request from that user. Every user has a separate session and separate session variable is associated with that session. In case of web applications the default time-out value for session variable is 20 minutes, which can be changed as per the requirement.

A deployment descriptor is an Extensible Markup Language (XML) textAnswer:based file with an .xml extension that describes a component?s deployment settings. A J2EE application and each of its modules has its own deployment descriptor. For example, an enterprise bean module deployment descriptor declares transaction attributes and security authorizations for an enterprise bean. Because deployment descriptor information is declarative, it can be changed without modifying the bean source code. At run time, the J2EE server reads the deployment descriptor and acts upon the component accordingly.

HttpSession Class provides a way to identify a user across across multiple request. The servlet container uses HttpSession interface to create a session between an HTTP client and an HTTP server. The session lives only for a specified time period, across more than one connection or page request from the user.

When servlet container is loaded, all the servlets defined in the web.xml file does not initialized by default. But the container receives the request it loads the servlet. But in some cases if you want your servlet to be initialized when context is loaded, you have to use a concept called pre-initialization of Servlet. In case of Pre-initialization, the servlet is loaded when context is loaded. You can specify 1 in between the tag.

The controller is responsible for intercepting and translating user input into actions to be performed by the model. The controller is responsible for selecting the next view based on user input and the outcome of model operations. The Controller receives the request from the browser, invoke a business operation and coordinating the view to return to the client. The controller is implemented by a java servlet, this servlet is centralized point of control for the web application. In struts framework the controller responsibilities are implemented by several different components like The ActionServlet Class The RequestProcessor Class The Action Class The ActionServlet extends the javax.servlet.http.httpServlet class. The ActionServlet class is not abstract and therefore can be used as a concrete controller by your application. The controller is implemented by the ActionServlet class. All incoming requests are mapped to the central controller in the deployment descriptor as follows. action org.apache.struts.action.ActionServlet All request URIs with the pattern *.do are mapped to this servlet in the deployment descriptor as follows. action *.do *.do A request URI that matches this pattern will have the following form. http://www.my_site_name.com/mycontext/actionName.do The preceding mapping is called extension mapping, however, you can also specify path mapping where a pattern ends with /* as shown below. action /do/* *.do A request URI that matches this pattern will have the following form. http://www.my_site_name.com/mycontext/do/action_Name The class org.apache.struts.action.requestProcessor process the request from the controller. You can sublass the RequestProcessor with your own version and modify how the request is processed. Once the controller receives a client request, it delegates the handling of the request to a helper class.

50. What is Server Side Push and how is it implemented and when is it useful?

Server Side push is useful when data needs to change regularly on the clients application or browser, without intervention from client. Standard examples might include apps like Stock's Tracker, Current News etc. As such server cannot connect to client's application automatically. The mechanism used is, when client first connects to Server, (Either through login etc..), then Server keeps the TCP/IP connection open. It's not always possible or feasible to keep the connection to Server open. So another method used is, to use the standard HTTP protocols ways of refreshing the page, which is normally supported by all browsers. This will refresh the page in the browser automatically and loads the new data every 5 seconds.