In Servlet doGet() and doPost(), both are methods of the HttpServlet class. These methods are used to handle client requests, proceed with those requests, and generate corresponding responses.

doGet() method is used for retrieving data from the server.
doPost() method is used for sending data to the server.

doGet()

This method does not provide security because the values are shown in the URL bar. By using a GET request, we can send only Character Data (ASCII) and we cannot send Binary Data like Images.

It is less secure for sensitive data .
It is used for Displaying data.
Used for sending small amounts of data.

Java

public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException{
    PrintWriter out = resp.getWriter();
    String mess = req.getParameter("mess");
    out.println(mess);
}

doPost()

This method provide more security as compare to doGet(). By using POST request we can send both Binary and Character Data to the Server.

It is more secure for sensitive data.
POST requires more server-side processing, so it's slightly slower
Use large amount of data.

Java

@Override
public void doPost(HttpServletRequest req, HttpServletResponse res)
        throws Exception {

    String username = req.getParameter("username");
    String password = req.getParameter("password");

    response.setContentType("text/html");
    PrintWriter out = res.getWriter();

    if (username.equals("vishnu") && password.equals("vishnu123")) {
        out.println("<h3>Welcome to user page</h3>");
        System.out.println("welcome");
    } else {
        out.println("<h2>Please enter correct details</h2>");
        System.out.println("Login failed");
    }
}

Example: We will create a simple form to get the details from user like below:

Step-by-Step Implementation: Servlet Login Page

Create an HTML form to take the input data from the user.
Create a Servlet for handling http get and post request.

Step 1: Create Project

Create a Dynamic Web Application in Eclipse
Project structure:

YourProjectName/
└── WebContent/
└── User.html

Step 2: Create HTML Form

Create a login form to collect user input
Takes email and password from user
Sends data to servlet using POST method

HTML

<!DOCTYPE html>
<html>
<head>
    <title>Servlet Login Form</title>
</head>
<body>

    <h3>Login Form</h3>
    <form action="myLogin" method="post">
        <label>User Email:</label><br>
        <input type="text" name="username" placeholder="Enter Email" required><br><br>

        <label> Enter password:</label><br>
        <input type="password" name="password" placeholder="Enter Password" required><br><br>

        <input type="submit" value="Login">
    </form>

</body>
</html>

Step 3: Create the Servlet (FirstDemo.java)

Create servlet class inside src folder
Handle request and validate user credentials

Java

package com.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

@WebServlet("/myLogin")
public class FirstDemo extends HttpServlet {

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        String email = request.getParameter("username");
        String password = request.getParameter("password");

        if (email.equals("admin@gmail.com") && password.equals("admin123")) {
            out.print("<html><body>");
            out.print("<h2>login successfull </h2>");
            out.print("Email " + email + "<br/>");
            out.print("Password: " + password + "<br/>");
            out.print("</body></html>");
        } else {
            out.print("<html><body>");
            out.print("<h3>failed</h3>");
            out.print("<p>Enter correct  user details.</p>");
            out.print("</body></html>");
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.sendRedirect("User.html");
    }
}

Explanation:

Here, we have used the @WebServlet annotation to map URL requests to the servlet.
When the user clicks Login, the form data is sent to the server.
Once the form page is submitted, the 'doPost()' method of the Servlet class will be invoked.
doPost() method is called automatically when the server receives a POST request because in html page method type is post.
Inside doPost, method read the input using request.getparameter() and this method match the attribute in the form.
And then compare the credentials provided by you.
and then generate the response according to our credentials
Suppose if we provide the user email and password then we have to show a message on the browser

Step 4: Run the Application

Right-click project -> Run on Server
Open browser and run:

http://localhost:8080/FirstDemo/myLogin

Servlet Request Handling: doGet() vs doPost()

doGet()

doPost()

Step-by-Step Implementation: Servlet Login Page

Step 1: Create Project

Step 2: Create HTML Form

Step 3: Create the Servlet (FirstDemo.java)

Step 4: Run the Application

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