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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:

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If you use Maven, visit the Spring Initializr to generate a new project with the required dependency (Spring Web Reactive).

The following listing shows the pom.xml file that is created when you choose Maven:

If you use Gradle, visit the Spring Initializr to generate a new project with the required dependency (Spring Web Reactive).

The following listing shows the build.gradle file that is created when you choose Gradle:

Manual Initialization (optional)

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

This simple reactive class always returns “Hello, Spring!” 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:

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.

The Spring MVC 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 implementation to consume our RESTful service:

src/main/java/hello/GreetingWebClient.java

The WebClient class uses reactive features, in the form of a Mono to hold the content of the URI we specify and a function (in the getResult method) to turn that content into a string. If we had different requirements, we might turn it into something other than a string. Since we want to put the result into System.out, a string works here.

Although you can 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 Reactive Spring’s support for embedding the Netty server as the HTTP runtime, instead of deploying to an external instance.

src/main/java/hello/Application.java

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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:

>> result = 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.

src/test/java/hello/GreetingRouterTest.java

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

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