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).
Note
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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
Starting with Spring Initializr
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:
link:initial/pom.xml[]
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:
link:initial/build.gradle[]
Manual Initialization (optional)
If you want to initialize the project manually rather than use the links shown earlier, follow the steps given below:
-
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.
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Choose either Gradle or Maven and the language you want to use. This guide assumes that you chose Java.
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Click Dependencies and select Spring Reactive Web.
-
Click Generate.
-
Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.
Note
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If your IDE has the Spring Initializr integration, you can complete this process from your IDE. |
Create a WebFlux Handler
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[]
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.
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
link: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 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[]
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.
Tip
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You can use WebClient to communicate with non-reactive, blocking services, too.
|
Make the Application Executable
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[]
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.
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
link: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!