In this example, we will show you how to create REST APIs using JavaEE Servlet which returns JSON to the client.

We use below HttpServlet class methods to perform CRUD operations:

protected void doDelete(HttpServletRequest req, HttpServletResponse resp) - This method is called by the server (via the service method) to allow a servlet to handle a DELETE request.
protected void doGet(HttpServletRequest req, HttpServletResponse resp) - This method is called by the server (via the service method) to allow a servlet to handle a GET request.
protected void doPost(HttpServletRequest req, HttpServletResponse resp) - This method is called by the server (via the service method) to allow a servlet to handle a POST request.
protected void doPut(HttpServletRequest req, HttpServletResponse resp) - This method is called by the server (via the service method) to allow a servlet to handle a PUT request.

1. Create Maven Web Application

Use the below article to create a Maven web project in Eclipse IDE:

How to Create a Web Project Using Maven in Eclipse

2. Add Maven Dependencies

Open the pom.xml file and add the following dependencies:

                <dependency>
			<groupId>javax.servlet</groupId>
			<artifactId>servlet-api</artifactId>
			<version>3.0-alpha-1</version>
			<scope>provided</scope>
		</dependency>
		<dependency>
			<groupId>com.google.code.gson</groupId>
			<artifactId>gson</artifactId>
			<version>2.8.5</version>
		</dependency>
		<dependency>
			<groupId>org.assertj</groupId>
			<artifactId>assertj-core</artifactId>
			<version>3.11.1</version>
			<scope>test</scope>
		</dependency>
		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<version>4.12</version>
			<scope>test</scope>
		</dependency>

3. Create Model - Todo.java

Let's create a Todo model class that Servlet returns to the client as JSON:

public class Todo {

    private Long id;
    private String text;

    public Todo() {
    }

    public Todo(Long id, String text) {
        this.id = id;
        this.text = text;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }
}

4. Create Static Data - Todos.java

Let's create a Todos class that creates static data:

import java.util.HashMap;
import java.util.Map;

public class Todos {

    public static Map<Long, Todo> todos = new HashMap<>();

    static {
        todos.put(1L, new Todo(1L, "first todo"));
        todos.put(2L, new Todo(2L, "second todo"));
    }

    public static Long nextId() {
        return todos.keySet().stream().reduce(Math::max).orElse(0L) + 1L;
    }
}

5. Create Util Class

Util class to convert Stream to String:

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;

import static java.util.stream.Collectors.joining;

public class Util {

	public static String readInputStream(InputStream stream) {
		return new BufferedReader(new InputStreamReader(stream)).lines().collect(joining("\n"));
	}
}

6. Create Servlet - CRUD Operations

Let's create a Servlet named TodoServlet that performs CRUD operations:

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

import static java.util.stream.Collectors.joining;

public class TodoServlet extends HttpServlet {

	private static final Gson GSON = new GsonBuilder().create();

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
		String uri = req.getRequestURI();
		Long id = Long.parseLong(uri.substring("/todos/".length()));

		String json = GSON.toJson(Todos.todos.get(id));

		resp.setStatus(200);
		resp.setHeader("Content-Type", "application/json");
		resp.getOutputStream().println(json);
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException {
		String uri = req.getRequestURI();
		Long id = Long.parseLong(uri.substring("/todos/".length()));

		if (Todos.todos.containsKey(id)) {
			resp.setStatus(422);
			resp.getOutputStream().println("You cannot created Todo with id " + id + " because it exists!");
		}

		String json = Util.readInputStream(req.getInputStream());
		Todo todo = GSON.fromJson(json, Todo.class);
		todo.setId(id);

		Todos.todos.put(todo.getId(), todo);

		resp.setStatus(201);
		resp.setHeader("Content-Type", "application/json");
		resp.getOutputStream().println(GSON.toJson(todo));
	}

	@Override
	protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws IOException {
		String uri = req.getRequestURI();
		Long id = Long.parseLong(uri.substring("/todos/".length()));

		if (!Todos.todos.containsKey(id)) {
			resp.setStatus(422);
			resp.getOutputStream().println("You cannot update Todo with id " + id + " because it doesn't exists!");
		}

		String json = Util.readInputStream(req.getInputStream());
		Todo todo = GSON.fromJson(json, Todo.class);
		todo.setId(id);

		Todos.todos.put(todo.getId(), todo);

		resp.setStatus(200);
		resp.setHeader("Content-Type", "application/json");
		resp.getOutputStream().println(GSON.toJson(todo));
	}

	@Override
	protected void doDelete(HttpServletRequest req, HttpServletResponse resp) throws IOException {
		String uri = req.getRequestURI();
		Long id = Long.parseLong(uri.substring("/todos/".length()));

		Todo todo = Todos.todos.remove(id);
		String json = GSON.toJson(todo);

		resp.setStatus(200);
		resp.setHeader("Content-Type", "application/json");
		resp.getOutputStream().println(json);
	}
}

protected void doDelete(HttpServletRequest req, HttpServletResponse resp) - This method is called by the server (via the service method) to allow a servlet to handle a DELETE request.
protected void doGet(HttpServletRequest req, HttpServletResponse resp) - This method is called by the server (via the service method) to allow a servlet to handle a GET request.
protected void doPost(HttpServletRequest req, HttpServletResponse resp) - This method is called by the server (via the service method) to allow a servlet to handle a POST request.
protected void doPut(HttpServletRequest req, HttpServletResponse resp) - This method is called by the server (via the service method) to allow a servlet to handle a PUT request.

7. Configure Servlet in Web.xml

Now let's configure Servlet in web.xml file:

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE web-app
		PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
		"http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
	<servlet>
		<servlet-name>TodoServlet</servlet-name>
		<servlet-class>com.sourcecodeexamples.rest.TodoServlet</servlet-class>
	</servlet>

	<servlet-mapping>
		<servlet-name>TodoServlet</servlet-name>
		<url-pattern>/todos/*</url-pattern>
	</servlet-mapping>
</web-app>

Run in this project in Tomcat server and Test all the APIs using /todos URL.

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