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Java Servlet Example

Jakob Jenkov
Last update: 2014-05-25

A Java Servlet is just an ordinary Java class which implements the interface

javax.servlet.Servlet;

The easiest way to implement this interface is to extend either the class GenericServlet or HttpServlet.

import javax.servlet.GenericServlet;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import java.io.IOException;


public class SimpleServlet extends GenericServlet {

  public void service(ServletRequest request, ServletResponse response)
        throws ServletException, IOException {

       // do something in here
  }
}

When an HTTP request arrives at the web server, targeted for your Servlet, the web server calls your Servlet's service() method.

The service() method then reads the request, and generates a response which is sent back to the client (e.g. a browser).

Here is an example service() implementation:

public void service(ServletRequest request, ServletResponse response)
        throws ServletException, IOException {

  String yesOrNoParam = request.getParameter("param");

  if("yes".equals(yesOrNoParam) ){

      response.getWriter().write(
        "<html><body>You said yes!</body></html>");
  }

  if("no".equals(yesOrNoParam) ){
    
      response.getWriter().write(
        "<html><body>You said no!</body></html>");
  }
}

This service() method first reads the request parameter "param". Then it checks if the param is equal to the text "yes" or "no", and writes an HTML response back to the browser.

Jakob Jenkov




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