HTTP is the protocol that allows web servers and browsers to exchange data over the web. It is a request and response protocol. HTTP uses reliable TCP connections—by default on TCP port 80. The client requests a file and the server responds to the request. It's always the client who initiates a transaction by establishing a connection and sending an HTTP request. The server is in no position to contact a client or make a callback connection tothe client. Either the client or the server can prematurely terminate a connection.
We Can normally adopt two main architectures when developing web applications in Java.
The first architecture utilizes servlets and JSP in the middle tier to serve clients and process the business logic. Small to medium-size applications use this design model.Java Training in Bangalore
The second architecture includes the use of J2EE server and Enterprise JavaBeans (EJB) and this is especially useful for large enterprise applications that need scalability.
Creation process of servlet:
A servlet can be created by inheriting from the GenericServlet or HttpServlet Class in a user defined class(The GenericServlet class is available in javax.servlet package and the HttpServlet class is defined in javax.servlet.http package ).
Creation of servlet by using GenericServlet class:
1. Cerate a class by inheriting from GenericServlet class, the user defined class must be public, bcoz the servletr will be accessed by web server.
2. Override the methods of GenericServlet class in user defined class to provide logic for the servlet. These methods are as follows.public void init() : This executes during initialization of servlet class. It is used to initialize resources that the servlet might use. This method gets called only once during the servlet's lifetime. Generally this method will not be overrien by the programmer bcoz the web server will excute this methd.
· public abstract void service(ServletRequest req,ServletResponse res) throws Servlet-Exception, IOException : This method will be called by the web server only when a client will make the http request. This method is also responsible to produce the http response back to the client after processing the client’s requet. This method generally overridden by the programmer.
· public void destroy() : this executes during removal of servlets instance from the servlet container. It is also invoked by the web server.
3. Create an object of java.io.PrintWriter class by calling getWriter() of ServerletResponse interface, so that response can be given back to client in html format to the user within the println().
Life Cycle of Servlet:
· init() : This method executes during initialization of servlet within the servlet container. Whenever the first user of servlet comes to access the servlet then an instance of the servlet is being created and this method executes. This method is required for performing any common job for all users. Ex. Opening connection with DBMS.
· service()/doGet()/doPost() : This method executes to provide response by accepting request from the users. Whenever any user comes to access the servlet then the web- server creates new thread and the thread executes this method. Hence this method executes once for each user by using one instance of the servlet.
· destroy() : This method executes during removal of servlet instance from the web server. If a servlet is not being accessed by any user for a period then the instance of the servlet is being removed and this method executes. This method may perform any task which is common for all users.
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