spring-guides/gs-reactive-rest-service

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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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.

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

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 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:

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 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

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.

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

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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:

>> 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.

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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