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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).

This guide shows the functional way of using Spring WebFlux. You can also use annotations with WebFlux.

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:

http://localhost:8080/hello

What You Will Need

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:

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.

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.

Create a WebFlux Handler

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

src/main/java/hello/Greeting.java

Unresolved directive in <stdin> - include::complete/src/main/java/hello/Greeting.java[]

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

src/main/java/hello/GreetingHandler.java

Unresolved directive in <stdin> - include::complete/src/main/java/hello/GreetingHandler.java[]

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:

src/main/java/hello/GreetingRouter.java

Unresolved directive in <stdin> - include::complete/src/main/java/hello/GreetingRouter.java[]

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:

src/main/java/hello/GreetingClient.java

Unresolved directive in <stdin> - include::complete/src/main/java/hello/GreetingClient.java[]

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.

Make the Application Executable

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

src/main/java/hello/Application.java

Unresolved directive in <stdin> - include::complete/src/main/java/hello/Application.java[]

@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. 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.

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:

java -jar build/libs/gs-reactive-rest-service-0.1.0.jar

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.

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.

src/test/java/hello/GreetingRouterTest.java

Unresolved directive in <stdin> - include::complete/src/test/java/hello/GreetingRouterTest.java[]

Summary

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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