Servlet: What is HttpSession?

HttpSession object is used to store entire session with a specific client. We can store, retrieve and remove attribute from HttpSession object. Any servlet can have access to HttpSession object throughout the getSession() method of the HttpServletRequest object.

Servlet: How HttpSession works

1. On client's first request, the Web Container generates a unique session ID and gives it back to the client with response. This is a temporary session created by web container.
2. The client sends back the session ID with each request. Making it easier for the web container to identify where the request is coming from.
3. The Web Container uses this ID, finds the matching session with the ID and associates the session with the request.

Servlet: HttpSession Interface

Some Important Methods of Servlet HttpSession

Complete Example demonstrating usage of HttpSession

All the files mentioned below are required for the example.

index.html

<form method="post" action="Validate">
    User: <input type="text" name="user" /><br/>
    Password: <input type="text" name="pass" ><br/>
    <input type="submit" value="submit">
</form>

web.xml

<web-app..>
    
    <servlet>
        <servlet-name>Validate</servlet-name>
        <servlet-class>Validate</servlet-class>
    </servlet>
    <servlet>
        <servlet-name>Welcome</servlet-name>
        <servlet-class>Welcome</servlet-class>
    </servlet>
   
    <servlet-mapping>
        <servlet-name>Validate</servlet-name>
        <url-pattern>/Validate</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>Welcome</servlet-name>
        <url-pattern>/Welcome</url-pattern>
    </servlet-mapping>
    
    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
    </welcome-file-list>
    
</web-app>

Validate.java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class Validate extends HttpServlet {

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        
        String name = request.getParameter("user");
        String pass = request.getParameter("pass");
        
        if(pass.equals("1234"))
        {
            //creating a session
            HttpSession session = request.getSession();
            session.setAttribute("user", name);
            response.sendRedirect("Welcome");
        }
    }
}

Welcome.java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class Welcome extends HttpServlet {
 
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        HttpSession session = request.getSession();
        String user = (String)session.getAttribute("user");
        out.println("Hello "+user);
    }
}