Servlet Interface Example (Java EE 10)

Servlets in Java are used to handle web requests and generate dynamic responses. The Servlet interface from the jakarta.servlet package defines the essential methods that all servlets must implement. This tutorial will guide you through an example demonstrating the Servlet interface and its primary methods.

For more detailed information, refer to the official documentation.

Introduction

The Servlet interface defines methods to initialize a servlet, process requests, and perform cleanup. The three main methods are:

init(ServletConfig config): Called once when the servlet is initialized.
service(ServletRequest req, ServletResponse res): Called to handle each request.
destroy(): Called once when the servlet is about to be destroyed.

In this tutorial, we will create a simple servlet to manage and display user feedback using the Servlet interface.

Prerequisites

Before we start, ensure you have the following:

Basic understanding of Java and web development.
Java web application development environment set up (e.g., Apache Tomcat).
Maven for managing project dependencies.

Project Structure

Here's the structure of our project:

servlet-interface-example/
├── src/
│   ├── main/
│   │   ├── java/
│   │   │   └── com/
│   │   │       └── example/
│   │   │           └── servlet/
│   │   │               └── FeedbackServlet.java
│   │   ├── resources/
│   │   └── webapp/
│   │       ├── WEB-INF/
│   │       └── index.jsp
└── pom.xml

Maven Dependencies

Add the following dependencies to your pom.xml:

<dependencies>
    <!-- Servlet API -->
    <dependency>
        <groupId>jakarta.servlet</groupId>
        <artifactId>jakarta.servlet-api</artifactId>
        <version>6.1.0</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

Creating the Servlet

Let's create a FeedbackServlet that implements the Servlet interface. This servlet will handle user feedback by storing it in memory and displaying it upon request.

FeedbackServlet.java

package com.example.servlet;

import jakarta.servlet.Servlet;
import jakarta.servlet.ServletConfig;
import jakarta.servlet.ServletException;
import jakarta.servlet.ServletRequest;
import jakarta.servlet.ServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;

public class FeedbackServlet implements Servlet {

    private ServletConfig config;
    private List<String> feedbackList;

    @Override
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        feedbackList = new ArrayList<>();
    }

    @Override
    public ServletConfig getServletConfig() {
        return config;
    }

    @Override
    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        String feedback = req.getParameter("feedback");
        if (feedback != null && !feedback.isEmpty()) {
            feedbackList.add(feedback);
        }

        res.setContentType("text/html");
        PrintWriter out = res.getWriter();
        out.println("<html><body>");
        out.println("<h1>Feedback List</h1>");
        out.println("<ul>");
        for (String fb : feedbackList) {
            out.println("<li>" + fb + "</li>");
        }
        out.println("</ul>");
        out.println("</body></html>");
    }

    @Override
    public String getServletInfo() {
        return "FeedbackServlet";
    }

    @Override
    public void destroy() {
        feedbackList = null;
    }
}

Explanation of Servlet Methods

init(ServletConfig config)

Purpose: The init method is called once when the servlet is first initialized. It is used to perform any servlet-specific initialization tasks.
Example in FeedbackServlet: In this example, the init method initializes an empty list to store feedback messages.

service(ServletRequest req, ServletResponse res)

Purpose: The service method is called for each request to the servlet. It processes the request and generates a response.
Example in FeedbackServlet: The service method retrieves the feedback parameter from the request, adds it to the feedback list if it's not empty, and generates an HTML response displaying all feedback messages.

destroy()

Purpose: The destroy method is called once when the servlet is about to be destroyed. It is used to perform any cleanup tasks.
Example in FeedbackServlet: In this example, the destroy method sets the feedback list to null, effectively releasing any resources used by the servlet.

Configuring the Servlet

Instead of using the web.xml file, we will use annotations to configure the servlet.

Complete FeedbackServlet with Annotations

package com.example.servlet;

import jakarta.servlet.ServletConfig;
import jakarta.servlet.ServletException;
import jakarta.servlet.ServletRequest;
import jakarta.servlet.ServletResponse;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;

@WebServlet("/feedback")
public class FeedbackServlet extends HttpServlet {
    
    private List<String> feedbackList;

    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config);
        feedbackList = new ArrayList<>();
    }

    @Override
    protected void doPost(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        String feedback = req.getParameter("feedback");
        if (feedback != null && !feedback.isEmpty()) {
            feedbackList.add(feedback);
        }
        res.setContentType("text/html");
        PrintWriter out = res.getWriter();
        out.println("<html><body>");
        out.println("<h1>Feedback List</h1>");
        out.println("<ul>");
        for (String fb : feedbackList) {
            out.println("<li>" + fb + "</li>");
        }
        out.println("</ul>");
        out.println("</body></html>");
    }

    @Override
    public void destroy() {
        feedbackList = null;
    }
}

Creating the JSP

Create a simple JSP page to submit feedback.

index.jsp

<!DOCTYPE html>
<html>
<head>
    <title>Feedback Form</title>
</head>
<body>
    <h1>Feedback Form</h1>
    <form action="feedback" method="post">
        <label for="feedback">Enter your feedback:</label><br>
        <input type="text" id="feedback" name="feedback"><br><br>
        <input type="submit" value="Submit">
    </form>
</body>
</html>

Running the Application

Build your project with Maven:
```
mvn clean install
```
Deploy the WAR file to your servlet container (e.g., Apache Tomcat).
Access the application at http://localhost:8080/servlet-interface-example.

Conclusion

This tutorial demonstrated how to implement the Servlet interface in a Java web application using the Jakarta Servlet API. We created a simple feedback management system to showcase the init, service, and destroy methods. For more advanced applications, consider extending HttpServlet and using its built-in methods for handling HTTP-specific functionalities.

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