GenericServlet vs HttpServlet

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Difference between HttpServlet vs Generic Severlet

javax.servlet.Servlet----is an interface defining what a servlet must implement.it defines methods for all the implementations - that‘s what interfaces usually do.

javax.servlet.GenericServlet------is just that, a generic, protocol-independent servlet.  It is abstract, so it is not to be directly instantiated. It is the usable class to extend  if you some day have to write servlet for protocol other than HTTP.

javax.servlet.http.HttpServlet------- is abstract class to be extended if you want to communicate over HTTP protocol. Most likely you only have to care about this one.

Servlet:-

1. The Servlets runs as a thread in a web-container instead of in a seperate OS process.
2. Only one object is created first time when first request comes, other request share the same object.
3. Servlet is platform independent.
4. Servlet is fast.

GenericServlet:-

1. General for all protocol.
2. Implements Servlet Interface.
3. Use Service method.

HttpServlet:-

1. Only for HTTP Protocol.
2. Inherit GenericServlet class.
3. Use doPost, doGet method instead of service method.

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HttpServlet

public abstract class HttpServlet

extends GenericServlet

implements java.io.Serializable

Provides an abstract class to be subclassed to create an HTTP servlet suitable for a Web site. A subclass of HttpServlet must override at least one method, usually one of these:

• doGet, if the servlet supports HTTP GET requests
• doPost, for HTTP POST requests
• doPut, for HTTP PUT requests
• doDelete, for HTTP DELETE requests
• init and destroy, to manage resources that are held for the life of the servlet
• getServletInfo, which the servlet uses to provide information about itself

why httpservlet is abstract?

If you extend the httpservlet class without overriding any methods, you will get a useless servlet;

GenericServlet

public abstract class GenericServlet

extends java.lang.Object

implements Servlet, ServletConfig, java.io.Serializable

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When we should use Service() or toGet(), toPost()?

The differences between the doGet() and doPost() methods are that they are called in theHttpServlet that your servlet extends by its service() method when it recieves a GET or a POST request from a HTTP protocol request.

A GET request is a request to get a resource from the server. This is the case of a browser requesting a web page. It is also possible to specify parameters in the request, but the length of the parameters on the whole is limited. This is the case of a form in a web page declared this way in html: <form method="GET"> or <form>.

A POST request is a request to post (to send) form data to a resource on the server. This is the case of of a form in a web page declared this way in html: <form method="POST">. In this case the size of the parameters can be much greater.

The GenericServlet has a service() method that gets called when a client request is made. This means that it gets called by both incoming requests and the HTTP requests are given to the servlet as they are (you must do the parsing yourself).

The HttpServlet instead has doGet() and doPost() methods that get called when a client request is GET or POST. This means that the parsing of the request is done by the servlet: you have the appropriate method called and have convenience methods to read the request parameters.

NOTE: the doGet() and doPost() methods (as well as other HttpServlet methods) are called by the service() method.

Concluding,

if you must respond to GET or POST requests made by a HTTP protocol client (usually a browser) don‘t hesitate to extend HttpServlet and use its convenience methods.
If you must respond to requests made by a client that is not using the HTTP protocol, you must useservice().

protected void service(HttpServletRequest req, HttpServletResponse resp) {
    String method = req.getMethod();

    if (method.equals(METHOD_GET)) {
            doGet(req, resp);
    } else if (method.equals(METHOD_HEAD)) {
        doHead(req, resp);
    } else if (method.equals(METHOD_POST)) {
        doPost(req, resp);

    } else if (method.equals(METHOD_PUT)) {
        doPut(req, resp);   

    } else if (method.equals(METHOD_DELETE)) {
        doDelete(req, resp);

    } else if (method.equals(METHOD_OPTIONS)) {
        doOptions(req,resp);

    } else if (method.equals(METHOD_TRACE)) {
        doTrace(req,resp);

    } else {
        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
    }
}