This guide walks you through the process of creating a "Hello, Spring!" RESTful web service with Spring WebFlux (new as of version 5) and then consumes that service with a WebClient (also new as of version 5).
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Note
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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/helloFor all Spring applications, you should start with the Spring Initializr. The Initializr offers a fast way to pull in all the dependencies you need for an application and does a lot of the set up for you. This example needs only the Spring Reactive Web dependency.
You can get a Maven build file with the necessary dependencies directly from the Spring Initializr.
The following listing shows the pom.xml file that is created when you choose Maven:
link:initial/pom.xml[role=include]
You can get a Gradle build file with the necessary dependencies directly from the Spring Initializr.
The following listing shows the build.gradle file that is created when you choose Gradle:
link:initial/build.gradle[role=include]
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
link:complete/src/main/java/hello/GreetingHandler.java[role=include]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
link:complete/src/main/java/hello/GreetingRouter.java[role=include]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
link:complete/src/main/java/hello/GreetingWebClient.java[role=include]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.
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Tip
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You can use WebClient to communicate with non-reactive, blocking services, too.
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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
link:complete/src/main/java/hello/Application.java[role=include]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
link:complete/src/test/java/hello/GreetingRouterTest.java[role=include]Congratulations! You have developed a Reactive Spring application that includes a WebClient to consume a RESTful service!