Mastering Web and Blogging

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Servlets are server side components that provide a powerful mechanism for developing server side programs. Servlets provide component-based, platform-independent methods for building Web-based applications, without the performance limitations of CGI programs. Unlike proprietary server extension mechanisms (such as the Netscape Server API or Apache modules), servlets are server as well as platform-independent. This leaves you free to select a "best of breed" strategy for your servers, platforms, and tools. Using servlets web developers can create fast and efficient server side application which can run on any servlet enabled web server. Servlets run entirely inside the Java Virtual Machine. Since the Servlet runs at server side so it does not checks the browser for compatibility. Servlets can access the entire family of Java APIs, including the JDBC API to access enterprise databases. Servlets can also access a library of HTTP-specific calls, receive all the benefits of the mature java language including portability, performance, reusability, and crash protection. Today servlets are the 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 the components 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.

Servlets are not designed for a specific protocols. It is different thing that they are most commonly used with the HTTP protocols Servlets uses the classes in the java packages javax.servlet and javax.servlet.http. Servlets provides a way of creating the sophisticated server side extensions in a server as they follow the standard framework and use the highly portable java language.

HTTP Servlet typically used to:

Priovide dynamic content like getting the results of a database query and returning to the client.

Process and/or store the data submitted by the HTML.

Manage information about the state of a stateless HTTP. e.g. an online shopping car manages request for multiple concurrent customers.


Generic Servlet :

Contains 5 methods

init:

Method Description:

Exceptions : ServletException

public void init(ServletConfig config) throws ServletException

The init() method is called only once by the servlet container throughout the life of a servlet. By this init() method the servlet get to know that it has been placed into service.
The servlet cannot be put into the service if
The init() method does not return within a fix time set by the web server.

Parameters - The init() method takes a ServletConfig object that contains the initialization parameters and servlet's configuration and throws a ServletException if an exception has occurred.

Method 2:

service()

Method Description:

Method Signature: public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException

services the client's request with the help of two objects. These two objects which are javax.servlet.ServletRequest and javax.servlet.ServletResponse are passed by the servlet container.

Once the servlet starts getting the requests, the service() method is called by the servlet container to respond.

The servlet code of the response always should be set for a servlet that throws or sends an error.Parameters - The service() method takes the ServletRequest object that contains the client's request and the object ServletResponse contains the servlet's response. The service() method throws ServletException and IOExceptions exception.

Method 3:

Name: getServletConfig()

Method Signature: public ServletConfig getServletConfig()

This method returns the ServletConfig object. The object obtained is then sent to the init method.When this interface is implemented then it stores the ServletConfig object in order to return it. It is done by the generic class which implements this inetrface.

Method 4:

getServletInfo()

Method Signature: public String getServletInfo()

Description
The information about the servlet is returned by this method like version, author etc. This method returns a string which should be in the form of plain text and not any kind of markup.
Returns - a string that contains the information about the servlet

Method 5:

Name : Destroy()

Method Signature : public void destroy()
Description :

This method is called when we need to close the servlet. That is before removing a servlet instance from service, the servlet container calls the destroy() method. Once the servlet container calls the destroy() method, no service methods will be then called . That is after the exit of all the threads running in the servlet, the destroy() method is called. Hence, the servlet gets a chance to clean up all the resources like memory, threads etc which are being held