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Building a RESTful Web Service
This guide walks you through the process of creating a “Hello, World” RESTful web service with Spring.
What You Will Build
You will build a service that will accept HTTP GET requests at
http://localhost:8080/greeting
.
It will respond with a JSON representation of a greeting, as the following listing shows:
Java 17 or later
You can also import the code straight into your IDE:
Like most Spring Getting Started guides , you can start from scratch and complete each step or you can bypass basic setup steps that are already familiar to you. Either way, you end up with working code.
To start from scratch , move on to Starting with Spring Initializr .
To skip the basics , do the following:
Download
and unzip the source repository for this guide, or clone it using
Git
:
git clone
https://github.com/spring-guides/gs-rest-service.git
cd into
gs-rest-service/initial
Jump ahead to Create a Resource Representation Class .
When you finish
, you can check your results against the code in
gs-rest-service/complete
.
You can use this pre-initialized project and click Generate to download a ZIP file. This project is configured to fit the examples in this tutorial.
To manually initialize the project:
Navigate to https://start.spring.io . This service pulls in all the dependencies you need for an application and does most of the setup for you.
Choose either Gradle or Maven and the language you want to use. This guide assumes that you chose Java.
Click Dependencies and select Spring Web .
Click Generate .
Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.
The
id
field is a unique identifier for the greeting, and
content
is the textual representation of the greeting.
To model the greeting representation, create a resource representation class. To do so, provide a Java record class for the
id
and
content
data, as the following listing (from
src/main/java/com/example/restservice/Greeting.java
) shows:
package com.example.restservice;
public record Greeting(long id, String content) { }
In Spring’s approach to building RESTful web services, HTTP requests are handled by a controller. These components are identified by the
@RestController
annotation, and the
GreetingController
shown in the following listing (from
src/main/java/com/example/restservice/GreetingController.java
) handles
GET
requests for
/greeting
by returning a new instance of the
Greeting
class:
This controller is concise and simple, but there is plenty going on under the hood. We break it down step by step.
The
@GetMapping
annotation ensures that HTTP GET requests to
/greeting
are mapped to the
greeting()
method.
@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 shown earlier 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 populates and returns a
Greeting
object. The object data will be written directly to the HTTP response as JSON.
This code uses Spring
@RestController
annotation, which marks the class as a controller where every method returns a domain object instead of a view. It is shorthand for including both
@Controller
and
@ResponseBody
.
The
Greeting
object must be converted to JSON. Thanks to Spring’s HTTP message converter support, you need not do this conversion manually. Because
Jackson 2
is on the classpath, Spring’s
MappingJackson2HttpMessageConverter
is automatically chosen to convert the
Greeting
instance to JSON.
@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. For example, if
spring-webmvc
is on the classpath, this annotation 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
com/example
package, letting it find the controllers.
The
main()
method uses Spring Boot’s
SpringApplication.run()
method to launch an application. Did you notice that there was not a single line of XML? There is no
web.xml
file, either. This web application is 100% pure Java and you did not have to deal with configuring any plumbing or infrastructure.
Build an executable JAR
You can run the application from the command line with Gradle or Maven. You can also build a single executable JAR file that contains all the necessary dependencies, classes, and resources and run that. Building an executable jar makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.
If you use Gradle, you can run the application by using
./gradlew bootRun
. Alternatively, you can build the JAR file by using
./gradlew build
and then run the JAR file, as follows:
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 be explicitly overridden through the query string.
Notice also how the
id
attribute has changed from
1
to
2
. This proves that you are working against the same
GreetingController
instance across multiple requests and that its
counter
field is being incremented on each call as expected.
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