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).
| This guide shows the functional way of using Spring WebFlux. You can also use annotations with WebFlux. |
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:
http://localhost:8080/helloLike 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:
When you finish, you can check your results against the code in gs-reactive-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:
| If your IDE has the Spring Initializr integration, you can complete this process from your IDE. |
| You can also fork the project from Github and open it in your IDE or other editor. |
We’re going to start with a Greeting POJO that will be serialized as JSON by our RESTful service:
package com.example.reactivewebservice; 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 com.example.reactivewebservice; 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.
In this application, we use a router to handle the only route we expose ( /hello ), as shown in the following example:
package com.example.reactivewebservice; 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.
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 com.example.reactivewebservice; 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.
| You can use WebClient to communicate with non-reactive, blocking services, too. |
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
| The steps described here create a runnable JAR. You can also build a classic WAR file. |
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:
>> message = Hello, Spring!
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.
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 com.example.reactivewebservice; 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!"); >); > >Congratulations! You have developed a Reactive Spring application that includes a WebClient to consume a RESTful service!
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| All guides are released with an ASLv2 license for the code, and an Attribution, NoDerivatives creative commons license for the writing. |
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