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Building a Reactive RESTful Web Service

This guide walks you through the process of creating a «Hello, Spring!» RESTful web service with Spring WebFlux (new as of Spring Boot 2.0) and then consumes that service with a WebClient (also new as of Spring Boot 2.0).

What You Will Build

You will build a RESTful web service with Spring Webflux and a WebClient consumer of that service. You will be able to see output in both System.out and at:

What You Will Need

• About 15 minutes
• A favorite text editor or IDE
• Java 1.8 or later
• Gradle 7.5+ or Maven 3.5+
• You can also import the code straight into your IDE:

How to complete this guide

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-reactive-rest-service.git
• cd into gs-reactive-rest-service/initial
• Jump ahead to Create a WebFlux Handler.

When you finish, you can check your results against the code in gs-reactive-rest-service/complete .

Starting with Spring Initializr

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:

1. 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.
2. Choose either Gradle or Maven and the language you want to use. This guide assumes that you chose Java.
3. Click Dependencies and select Spring Reactive Web.
4. Click Generate.
5. Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.

Create a WebFlux Handler

We’re going to start with a Greeting POJO that will be serialized as JSON by our RESTful service:

package hello; public class Greeting < private String message; public Greeting() < >public Greeting(String message) < this.message = message; >public String getMessage() < return this.message; >public void setMessage(String message) < this.message = message; >@Override public String toString() < return "Greeting'; > >

In the Spring Reactive approach, we use a handler to handle the request and create a response, as shown in the following example:

package hello; import org.springframework.http.MediaType; import org.springframework.stereotype.Component; import org.springframework.web.reactive.function.BodyInserters; import org.springframework.web.reactive.function.server.ServerRequest; import org.springframework.web.reactive.function.server.ServerResponse; import reactor.core.publisher.Mono; @Component public class GreetingHandler < public Monohello(ServerRequest request) < return ServerResponse.ok().contentType(MediaType.APPLICATION_JSON) .body(BodyInserters.fromValue(new Greeting("Hello, Spring!"))); >>

This simple reactive class always returns a JSON body with a “Hello, Spring!” greeting. It could return many other things, including a stream of items from a database, a stream of items that were generated by calculations, and so on. Note the reactive code: a Mono object that holds a ServerResponse body.

Create a Router

In this application, we use a router to handle the only route we expose ( /hello ), as shown in the following example:

package hello; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import org.springframework.http.MediaType; import org.springframework.web.reactive.function.server.RouterFunction; import org.springframework.web.reactive.function.server.RouterFunctions; import org.springframework.web.reactive.function.server.ServerResponse; import static org.springframework.web.reactive.function.server.RequestPredicates.GET; import static org.springframework.web.reactive.function.server.RequestPredicates.accept; @Configuration(proxyBeanMethods = false) public class GreetingRouter < @Bean public RouterFunctionroute(GreetingHandler greetingHandler) < return RouterFunctions .route(GET("/hello").and(accept(MediaType.APPLICATION_JSON)), greetingHandler::hello); >>

The router listens for traffic on the /hello path and returns the value provided by our reactive handler class.

Create a WebClient

The Spring RestTemplate class is, by nature, blocking. Consequently, we do not want to use it in a reactive application. For reactive applications, Spring offers the WebClient class, which is non-blocking. We use a WebClient-based implementation to consume our RESTful service:

package hello; import reactor.core.publisher.Mono; import org.springframework.http.MediaType; import org.springframework.stereotype.Component; import org.springframework.web.reactive.function.client.WebClient; @Component public class GreetingClient < private final WebClient client; // Spring Boot auto-configures a `WebClient.Builder` instance with nice defaults and customizations. // We can use it to create a dedicated `WebClient` for our component. public GreetingClient(WebClient.Builder builder) < this.client = builder.baseUrl("http://localhost:8080").build(); >public Mono getMessage() < return this.client.get().uri("/hello").accept(MediaType.APPLICATION_JSON) .retrieve() .bodyToMono(Greeting.class) .map(Greeting::getMessage); >>

The WebClient class uses reactive features, in the form of a Mono to hold the content of the message (returned by the getMessage method). This is using a function API, rather than an imperative one, to chain reactive operators.

It can take time to get used to Reactive APIs, but the WebClient has interesting features and can also be used in traditional Spring MVC applications.

Make the Application Executable

We’re going to use the main() method to drive our application and get the Greeting message from our endpoint.

• @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 hello package, letting it find the controllers.

If you use Maven, you can run the application by using ./mvnw spring-boot:run . Alternatively, you can build the JAR file with ./mvnw clean package and then run the JAR file, as follows:

java -jar target/gs-reactive-rest-service-0.1.0.jar

Logging output is displayed. The service should be up and running within a few seconds.

Once the service has started, you can see a line that reads:

That line comes from the reactive content being consumed by the WebClient. Naturally, you can find something more interesting to do with your output than put it in System.out.

Test the Application

Now that the application is running, you can test it. To start with, you can open a browser and go to http://localhost:8080/hello and see, “Hello, Spring!” For this guide, we also created a test class to get you started on testing with the WebTestClient class.

package hello; import org.junit.jupiter.api.Test; import org.junit.jupiter.api.extension.ExtendWith; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.boot.test.context.SpringBootTest; import org.springframework.http.MediaType; import org.springframework.test.context.junit.jupiter.SpringExtension; import org.springframework.test.web.reactive.server.WebTestClient; import static org.assertj.core.api.Assertions.assertThat; @ExtendWith(SpringExtension.class) // We create a `@SpringBootTest`, starting an actual server on a `RANDOM_PORT` @SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT) public class GreetingRouterTest < // Spring Boot will create a `WebTestClient` for you, // already configure and ready to issue requests against "localhost:RANDOM_PORT" @Autowired private WebTestClient webTestClient; @Test public void testHello() < webTestClient // Create a GET request to test an endpoint .get().uri("/hello") .accept(MediaType.APPLICATION_JSON) .exchange() // and use the dedicated DSL to test assertions against the response .expectStatus().isOk() .expectBody(Greeting.class).value(greeting ->< assertThat(greeting.getMessage()).isEqualTo("Hello, Spring!"); >); > >

Summary

Congratulations! You have developed a Reactive Spring application that includes a WebClient to consume a RESTful service!

Want to write a new guide or contribute to an existing one? Check out our contribution guidelines.

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