Dinesh on Java

Servlets are Java classes which service HTTP requests and implement the javax.servlet.Servlet interface. Web application developers typically write servlets that extend javax.servlet.http.HttpServlet, an abstract class that implements the Servlet interface and is specially designed to handle HTTP requests.

Servlet Interface-

Servlet interface provides common behavior to all the servlets.

Servlet interface needs to be implemented for creating any servlet (either directly or indirectly). It provides 3 life cycle methods that are used to initialize the servlet, to service the requests, and to destroy the servlet and 2 non-life cycle methods.

Methods of Servlet interface

There are 5 methods in Servlet interface. The init, service and destroy are the life cycle methods of servlet. These are invoked by the web container.

Method Description

1. public void init(ServletConfig config) initializes the servlet. It is the life cycle method of servlet and invoked by the web container only once.
2. public void service(ServletRequest request,ServletResponse response) provides response for the incoming request. It is invoked at each request by the web container.
3. public void destroy() is invoked only once and indicates that servlet is being destroyed.
4. public ServletConfig getServletConfig() returns the object of ServletConfig.
5. public String getServletInfo() returns information about servlet such as writer, copyright, version etc.

 

Hello World Example in Servlet:

Following is the sample source code structure of a servlet example to write Hello World:

// Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

// Extend HttpServlet class
public class HelloWorld extends HttpServlet {
 
  private String message;

  public void init() throws ServletException
  {
      // Do required initialization
      message = "Hello World";
  }

  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // Set response content type
      response.setContentType("text/html");

      // Actual logic goes here.
      PrintWriter out = response.getWriter();
      out.println("<h1>" + message + "</h1>");
  }
  
  public void destroy()
  {
      // do nothing.
  }
}

Compiling a Servlet:

Let us put above code if HelloWorld.java file and put this file in D:Servlet (Windows) or /usr/Servlet (Unix) then you would need to add these directories as well in CLASSPATH.

Assuming your environment is setup properly, go in Servlet directory and compile HelloWorld.java as follows:

If the servlet depends on any other libraries, you have to include those JAR files on your CLASSPATH as well. I have included only servlet-api.jar JAR file because I’m not using any other library in Hello World program.

If everything goes fine, above compilation would produce HelloWorld.class file in the same directory. Next section would explain how a compiled servlet would be deployed in production.

Servlet Deployment:

By default, a servlet application is located at the path C:Program Files (x86)Apache Software FoundationTomcat 7.0webappsROOT and the class file would reside in C:Program Files (x86)Apache Software FoundationTomcat 7.0webappsROOTWEB-INFclasses.

If you have a fully qualified class name of com.dineshonjava.MyServlet, then this servlet class must be located in C:Program Files (x86)Apache Software FoundationTomcat 7.0webappsROOTWEB-INF/classes/com/dineshonjava/MyServlet.class.

For now, let us copy HelloWorld.class into /webapps/ROOT/WEB-INF/classes and create following entries in web.xml file located in C:Program Files (x86)Apache Software FoundationTomcat 7.0webappsROOT/webapps/ROOT/WEB-INF/

<servlet>
        <servlet-name>HelloWorld</servlet-name>
        <servlet-class>HelloWorld</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloWorld</servlet-name>
        <url-pattern>/HelloWorld</url-pattern>
    </servlet-mapping>

Now let us start tomcat server using C:Program Files (x86)Apache Software FoundationTomcat 7.0binstartup.bat (on windows) or /bin/startup.sh (on Linux/Solaris etc.) and finally type http://localhost:8080/HelloWorld in browser’s address box. If everything goes fine, you would get following result: