Spring Boot Building a RESTful Web Service

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In this article, we will learn how to develop a simple RESTFul web service application using Spring Boot. We use Maven to build this project since most IDEs support it.

What we’ll build?

We’ll build a service that will accept HTTP GET requests at:

http://localhost:8080/greeting

and respond with a JSON representation of a greeting:

{"id":1,"content":"Hello, World!"}

We can customize the greeting with an optional name parameter in the query string:

http://localhost:8080/greeting?name=User

The name parameter value overrides the default value of "World" and is reflected in the response:

{"id":1,"content":"Hello, User!"}

Tools and Technologies Used

• Spring Boot - 3
• JDK - 17 or later
• Spring Framework - 6+
• Maven - 3.2+
• IDE - Eclipse or Spring Tool Suite (STS)

Create Spring boot Project

There are many ways to create a Spring Boot application. The simplest way is to use Spring Initializr at http://start.spring.io/, which is an online Spring Boot application generator.

Look at the above diagram, we have specified the following details:

• Generate: Maven Project
• Java Version: 17 (Default)
• Spring Boot: 3.0.4
• Group: net.javaguides.springboot
• Artifact: Springboot-helloworld-application
• Name: Springboot-helloworld-application
• Description: Rest API using Spring Boot
• Package Name: net.javaguides.springboot.Springboothelloworldapplication
• Packaging: jar (This is the default value)
• Dependencies: Web

Once, all the details are entered, clicking on Generate Project button will generate a spring boot project and downloads it. Next, Unzip the downloaded zip file and import it into your favorite IDE.

Project Directory Structure

Following is the packing structure of this application for your reference-

The pom.xml File

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>

 <groupId>net.javaguides.springboot</groupId>
 <artifactId>Springboot-helloworld-application</artifactId>
 <version>0.0.1-SNAPSHOT</version>
 <packaging>jar</packaging>

 <name>Springboot-helloworld-application</name>
 <description>Demo project for Spring Boot</description>

 <parent>
     <groupId>org.springframework.boot</groupId>
     <artifactId>spring-boot-starter-parent</artifactId>
     <version>3.0.4</version>
     <relativePath/> <!-- lookup parent from repository -->
 </parent>

 <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
     <java.version>17</java.version>
 </properties>

 <dependencies>
     <dependency>
          <groupId>org.springframework.boot</groupId>
          <artifactId>spring-boot-starter-web</artifactId>
     </dependency>

     <dependency>
          <groupId>org.springframework.boot</groupId>
          <artifactId>spring-boot-starter-test</artifactId>
          <scope>test</scope>
     </dependency>
 </dependencies>

 <build>
     <plugins>
         <plugin>
              <groupId>org.springframework.boot</groupId>
              <artifactId>spring-boot-maven-plugin</artifactId>
         </plugin>
     </plugins>
 </build>
</project>

From the above pom.xml, let's understand a few important spring boot features.

Spring Boot Maven plugin

The Spring Boot Maven plugin provides many convenient features:

• It collects all the jars on the classpath and builds a single, runnable "über-jar", which makes it more convenient to execute and transport your service.
• It searches for the public static void main() method to flag as a runnable class.
• It provides a built-in dependency resolver that sets the version number to match Spring Boot dependencies. You can override any version you wish, but it will default to Boot’s chosen set of versions.

spring-boot-starter-parent

All Spring Boot projects typically use spring-boot-starter-parent as the parent in pom.xml.

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.0.4</version>
    </parent>

Parent Poms allow you to manage the following things for multiple child projects and modules:

• Configuration - Java Version and Other Properties
• Dependency Management - Version of dependencies
• Default Plugin Configuration

Create a resource representation class - Greeting.java

Let's create Greeting to hold the response of the REST API:

package net.javaguides.springboot.Springboothelloworldapplication;

public class Greeting {
    private final long id;
    private final String content;

    public Greeting(long id, String content) {
         this.id = id;
         this.content = content;
    }

    public long getId() {
        return id;
    }

    public String getContent() {
         return content;
    }
}

Spring uses the Jackson JSON library to automatically marshal instances of type Greeting into JSON.

Next, we create a resource controller that will serve these greetings.

Create a resource controller - GreetingController.java

In Spring’s approach to building RESTful web services, HTTP requests are handled by a controller. These components are easily identified by the @RestController annotation, and the GreetingController below handles GET requests for /greeting by returning a new instance of the Greeting class:

package net.javaguides.springboot.Springboothelloworldapplication;

import java.util.concurrent.atomic.AtomicLong;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class GreetingController {

    private static final String template = "Hello, %s!";
    private final AtomicLong counter = new AtomicLong();

    @RequestMapping("/greeting")
    public Greeting greeting(@RequestParam(value = "name", defaultValue = "World") String name) {
         return new Greeting(counter.incrementAndGet(), String.format(template, name));
    }
}

Let's understand the above Controller step by step.

• The @RequestMapping annotation ensures that HTTP requests to /greeting are mapped to the greeting() method.
• The above example does not specify GET vs. PUT, POST, and so forth, because of @RequestMapping maps all HTTP operations by default. Use @RequestMapping(method=GET) to narrow this mapping.
• @RequestParam binds the value of the query string parameter name into the name parameter of the greeting() method. If the name parameter is absent in the request, the defaultValue of "World" is used.
• The implementation of the method body creates and returns a new Greeting object with id and content attributes based on the next value from the counter and formats the given name by using the greeting template.
• A key difference between a traditional MVC controller and the RESTful web service controller above is the way that the HTTP response body is created. Rather than relying on a view technology to perform server-side rendering of the greeting data to HTML, this RESTful web service controller simply populates and returns a Greeting object. The object data will be written directly to the HTTP response as JSON.
• This code uses Spring 4’s new @RestController annotation, which marks the class as a controller where every method returns a domain object instead of a view. It’s shorthand for @Controller and @ResponseBody rolled together.
• The Greeting object must be converted to JSON. Thanks to Spring’s HTTP message converter support, you don’t need to do this conversion manually. Because Jackson 2 is on the classpath, Spring’s MappingJackson2HttpMessageConverter is automatically chosen to convert the Greeting instance to JSON.

Make the application executable - SpringbootHelloworldApplication.java

Although it is possible to package this service as a traditional WAR file for deployment to an external application server, the simpler approach demonstrated below creates a standalone application. You package everything in a single, executable JAR file, driven by a good old Java main() method. Along the way, you use Spring’s support for embedding the Tomcat servlet container as the HTTP runtime, instead of deploying to an external instance.

package net.javaguides.springboot.Springboothelloworldapplication;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class SpringbootHelloworldApplication {

    public static void main(String[] args) {
         SpringApplication.run(SpringbootHelloworldApplication.class, args);
    }
}

@SpringBootApplication is a convenience annotation that adds all of the following:

@Configuration tags the class as a source of bean definitions for the application context.

@EnableAutoConfiguration tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings.

Normally you would add @EnableWebMvc for a Spring MVC app, but Spring Boot adds it automatically when it sees spring-webmvc on the classpath. This flags the application as a web application and activates key behaviors such as setting up a DispatcherServlet.

@ComponentScan tells Spring to look for other components, configurations, and services in the hello package, allowing it to find the controllers.

The main() method uses Spring Boot’s SpringApplication.run() method to launch an application. Did you notice that there wasn’t a single line of XML? No web.xml file either. This web application is 100% pure Java and you didn’t have to deal with configuring any plumbing or infrastructure.

Running the Application

In this article, the two ways we can start the standalone Spring boot application.

1. From the root directory of the application and type the following command to run it -

$ mvn spring-boot:run

2. From your IDE, run the SpringbootHelloworldApplication.main() method as a standalone Java class that will start the embedded Tomcat server on port 8080 and point the browser to http://localhost:8080/.

Test the service

Now that the service is up, visit http://localhost:8080/greeting, where you see:

Provide a name query string parameter with http://localhost:8080/greeting?name=User. Notice how the value of the content attribute changes from "Hello, World!" to "Hello User!":

This change demonstrates that the @RequestParam arrangement in GreetingController is working as expected. The name parameter has been given a default value of "World", but can always be explicitly overridden through the query string.

Summary

Congratulations! You’ve just developed a RESTful web service with Spring. Learn more about Spring Boot on Spring Boot Tutorial

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