Servlets : Basics

Servlet technology is used to create web application/server side application and generates dynamic web page.

Servlet technology is robust and scalable because of java language. Before Servlet, CGI (Common Gateway Interface) scripting language was popular as a server-side programming language.

Disadvantages of CGI

There are many problems in CGI technology:

1.     If number of clients increases, it takes more time for sending response.

2.     For each request, it starts a process and Web server is limited to start processes.

3.     It uses platform dependent language e.g. C, C++, perl.

There are many advantages of servlet over CGI. The web container creates threads for handling the multiple requests to the servlet. Threads have a lot of benefits over the Processes such as they share a common memory area, lightweight, cost of communication between the threads are low. The basic benefits of servlet are as follows:

1.     better performance: because it creates a thread for each request not process.

2.     Portability: because it uses java language.

3.     Robust: Servlets are managed by JVM so we don't need to worry about memory leak, garbage collection etc.

4.     Secure: because it uses java language.

Servlet API

The javax.servlet and javax.servlet.http packages represent interfaces and classes for servlet api.

The javax.servlet package contains many interfaces and classes that are used by the servlet or web container. These are not specific to any protocol.

The javax.servlet.http package contains interfaces and classes that are responsible for http requests only.

Let's see what are the interfaces of javax.servlet package.

Interfaces in javax.servlet package

There are many interfaces in javax.servlet package. They are as follows:

1.     Servlet

2.     ServletRequest

3.     ServletResponse

4.     RequestDispatcher

5.     ServletConfig

6.     ServletContext

7.     SingleThreadModel

8.     Filter

9.     FilterConfig

10.  FilterChain

11.  ServletRequestListener

12.  ServletRequestAttributeListener

13.  ServletContextListener

14.  ServletContextAttributeListener

Classes in javax.servlet package

There are many classes in javax.servlet package. They are as follows:

1.     GenericServlet

2.     ServletInputStream

3.     ServletOutputStream

4.     ServletRequestWrapper

5.     ServletResponseWrapper

6.     ServletRequestEvent

7.     ServletContextEvent

8.     ServletRequestAttributeEvent

9.     ServletContextAttributeEvent

10.  ServletException

11.  UnavailableException

Interfaces in javax.servlet.http package

There are many interfaces in javax.servlet.http package. They are as follows:

1.     HttpServletRequest

2.     HttpServletResponse

3.     HttpSession

4.     HttpSessionListener

5.     HttpSessionAttributeListener

6.     HttpSessionBindingListener

7.     HttpSessionActivationListener

8.     HttpSessionContext (deprecated now)

Classes in javax.servlet.http package

There are many classes in javax.servlet.http package. They are as follows:

1.     HttpServlet

2.     Cookie

3.     HttpServletRequestWrapper

4.     HttpServletResponseWrapper

5.     HttpSessionEvent

6.     HttpSessionBindingEvent

7.     HttpUtils (deprecated now)

Servlet Interface

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

      import java.io.*;  

      import javax.servlet.*;    

      public class First implements Servlet{  

      ServletConfig config=null;  

  

      public void init(ServletConfig config){  

      this.config=config;  

      System.out.println("servlet is initialized");  

}  

    

      public void service(ServletRequest req,ServletResponse res)  

     throws IOException,ServletException{  

  

     res.setContentType("text/html");  

   

    PrintWriter out=res.getWriter();  

    out.print("<html><body>");  

    out.print("<b>hello simple servlet</b>");  

    out.print("</body></html>");  

        

     }  

       

     }  

GenericServlet class

GenericServlet class implements Servlet, ServletConfig and Serializable interfaces. It provides the implementation of all the methods of these interfaces except the service method.

GenericServlet class can handle any type of request so it is protocol-independent.

You may create a generic servlet by inheriting the GenericServlet class and providing the implementation of the service method.

Methods of GenericServlet class

There are many methods in GenericServlet class. They are as follows:

1.     public void init(ServletConfig config) is used to initialize the servlet.

2.     public abstract void service(ServletRequest request, ServletResponse response) provides service for the incoming request. It is invoked at each time when user requests for a servlet.

3.     public void destroy() is invoked only once throughout the life cycle 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.

6.     public void init() it is a convenient method for the servlet programmers, now there is no need to call super.init(config)

7.     public ServletContext getServletContext() returns the object of ServletContext.

8.     public String getInitParameter(String name) returns the parameter value for the given parameter name.

9.     public Enumeration getInitParameterNames() returns all the parameters defined in the web.xml file.

10.  public String getServletName() returns the name of the servlet object.

11.  public void log(String msg) writes the given message in the servlet log file.

12.  public void log(String msg,Throwable t) writes the explanatory message in the servlet log file and a stack trace.

import java.io.*;  

import javax.servlet.*;  

  

public class First extends GenericServlet{  

public void service(ServletRequest req,ServletResponse res)  

throws IOException,ServletException{  

  

res.setContentType("text/html");  

  

PrintWriter out=res.getWriter();  

out.print("<html><body>");  

out.print("<b>hello generic servlet</b>");  

out.print("</body></html>");  

  

}  

}  

HttpServlet class

The HttpServlet class extends the GenericServlet class and implements Serializable interface. It provides http specific methods such as doGet, doPost, doHead, doTrace etc.

Methods of HttpServlet class

There are many methods in HttpServlet class. They are as follows:

1.     public void service(ServletRequest req,ServletResponse res) dispatches the request to the protected service method by converting the request and response object into http type.

2.     protected void service(HttpServletRequest req, HttpServletResponse res) receives the request from the service method, and dispatches the request to the doXXX() method depending on the incoming http request type.

3.     protected void doGet(HttpServletRequest req, HttpServletResponse res) handles the GET request. It is invoked by the web container.

4.     protected void doPost(HttpServletRequest req, HttpServletResponse res) handles the POST request. It is invoked by the web container.

5.     protected void doHead(HttpServletRequest req, HttpServletResponse res) handles the HEAD request. It is invoked by the web container.

6.     protected void doOptions(HttpServletRequest req, HttpServletResponse res) handles the OPTIONS request. It is invoked by the web container.

7.     protected void doPut(HttpServletRequest req, HttpServletResponse res) handles the PUT request. It is invoked by the web container.

8.     protected void doTrace(HttpServletRequest req, HttpServletResponse res) handles the TRACE request. It is invoked by the web container.

9.     protected void doDelete(HttpServletRequest req, HttpServletResponse res) handles the DELETE request. It is invoked by the web container.

10.  protected long getLastModified(HttpServletRequest req) returns the time when HttpServletRequest was last modified since midnight January 1, 1970 GMT.

Life Cycle of a Servlet (Servlet Life Cycle)

The web container maintains the life cycle of a servlet instance. Let's see the life cycle of the servlet:

1.     Servlet class is loaded.

2.     Servlet instance is created.

3.     init method is invoked.

4.     service method is invoked.

5.     destroy method is invoked.

1) Servlet class is loaded

The classloader is responsible to load the servlet class. The servlet class is loaded when the first request for the servlet is received by the web container.

2) Servlet instance is created

The web container creates the instance of a servlet after loading the servlet class. The servlet instance is created only once in the servlet life cycle.

3) init method is invoked

The web container calls the init method only once after creating the servlet instance. The init method is used to initialize the servlet. It is the life cycle method. Syntax of the init method is given below:

public void init(ServletConfig config) throws ServletException  

4) service method is invoked

The web container calls the service method each time when request for the servlet is received. If servlet is not initialized, it follows the first three steps as described above then calls the service method. If servlet is initialized, it calls the service method. Notice that servlet is initialized only once. The syntax of the service method of the Servlet interface is given below:

public void service(ServletRequest request, ServletResponse response)

  throws ServletException, IOException  

5) destroy method is invoked

The web container calls the destroy method before removing the servlet instance from the service. It gives the servlet an opportunity to clean up any resource for example memory, thread etc. The syntax of the destroy method of the Servlet interface is given below:

public void destroy()