Tutorial: Reactive Spring Boot Part 9 – Java RSocket Client

In this lesson we add an RSocket client that can talk to the RSocket server we created in the last lesson.

This is the ninth part of our tutorial showing how to build a Reactive application using Spring Boot, Kotlin, Java and JavaFX.  The original inspiration was a 70 minute live demo.

This blog post contains a video showing the process step-by-step and a textual walk-through (adapted from the transcript of the video) for those who prefer a written format.

This tutorial is a series of steps during which we will build a full Spring Boot application featuring a Kotlin back end, a Java client and a JavaFX user interface.

By now, we have an application that works end to end via Spring’s WebClient. Last lesson, we introduced a new RSocket server, which is currently running, and in this lesson we’re going to see how to create a client to connect to it.

Creating an integration test

As with the WebClientStockClient, we’re going to drive the RSocket client with an integration test.  The test looks almost identical to the WebClientStockClientIntegrationTest.

  1. Create a copy of WebClientStockClientIntegrationTest in the same test directory and rename it to RSocketStockClientIntegrationTest.
  2. Change the WebClientStockClient variable to an RSocketStockClient variable and rename the variable to rSocketStockClient.
  3. (Tip; using IntelliJ IDEA’s rename refactoring will change the name of this variable everywhere it’s used so there’s no need to find/replace).
  4. We know this isn’t going to need a web client, because that was only needed for the WebClient stock client. Remove the constructor argument and the field declaration.

(note: this code will not compile yet)

Creating the RSocket stock client

  1. RSocketStockClient doesn’t exist yet, so create it as an empty class.
  2. (Tip: pressing Alt+Enter on the red RSocketStockClient code gives the option of creating this class.)
  3. The test assumes a pricesFor method, so create the missing method on RSocketStockClient.
  4. (Tip: pressing Alt+Enter on the red pricesFor method gives the option of creating this method with the correct signature on RSocketStockClient.)

Introducing a StockClient interface

Of course the method declaration looks exactly the same as it does in WebClientStockClient, so it feels like a good time to introduce an interface that both clients can implement.

  1. Create a new interface called StockClient. We want the pricesFor method to appear on the interface since this is the method that has the same signature on both client classes.
  2. (Tip: use IntelliJ IDEA’s Extract Interface feature on WebClientStockClient to automatically create the new interface with a pricesFor method.)

  1. Make sure the WebClientStockClient class has been updated to implement the new StockClient, and has the @Override annotation on the pricesFor method.

  1. Do the same in RSocketStockClient.

  1. Now the test is compiling run it to see it fail.  It should fail on the assertNotNull assertion, since we’re returning null from the pricesFor method.

Implementing the RSocket connection

Normally with Test Driven Development we’d take small steps to make the tests pass and have more granular tests.  To keep this lesson focused we’re going to jump right in and implement the working RSocket client.

  1. Add spring-boot-starter-rsocket to the pom.xml file for the stock-client module.

  1. Add an RSocketRequester field called rSocketRequester to RSocketStockClient.
  2. Add a constructor parameter for it.
  3. (Tip: IntelliJ IDEA can automatically generate constructor parameters for fields.)

  1. In pricesFor, call rSocketRequester.route. For the route, we want to use the same route we defined in the back-end RSocket service, which in our case was “stockPrices”.
  2. Send the stock symbol to the server, via the data method.
  3. We expect the call to return a Flux of StockPrice, so pass StockPrice.class into the retrieveFlux method.
  4. Return the result of these calls from the pricesFor method instead of null.

Creating the RSocketRequester

The test doesn’t compile because we added an rSocketRequester constructor parameter and we don’t have an instance of RSocketRequester in our test.

  1. Create a private method called createRSocketRequester in the test near the top where other objects are typically initialised.
  2. Create a field for an RSocketRequester.Builder.  If we add the @Autowired annotation, Spring will inject an instance of this into our test.
  3. To tell Spring to manage the test we need to annotate it with @SpringBootTest.
  4. Inside createRSocketRequester, use the rSocketRequester to connect via TCP to our RSocketServer, which is running on localhost at port 7000.
  5. Call block to wait until we’re connected.

A passing integration test

We expect this test to work when we run it, but actually we’re missing something important.  We get an error that we’re missing a SpringBootConfiguration, which might be a little puzzling. In fact, this module doesn’t have a SpringBootApplication at all, because it was designed to be library code shared among other application code, it’s not an application in its own right.  Let’s look at one way to get our test to work.

  1. Create a class TestApplication in the same directory as the test.
  2. Annotate this with @SpringBootApplication.
  3. Re-run the integration test, everything should start up as expected, and the test should pass.

Testing with StepVerifier

Since the test passes we can assume the client successfully connects to the server via RSocket, gets a Flux of StockPrice objects, can take the first five of these and checks the first one has the correct symbol.  This is a slightly simplistic approach to testing reactive applications.  There are other approaches, one of which is to use the StepVerifier.  Using this approach, we can code our expectations for the events that we see.

  1. Create a new StepVerifier with five prices from the prices Flux.
  2. Use expectNextMatches to check the symbol for all five prices is correct.
  3. Call verifyComplete to not only check these expectations are met, but also that there are no more StockPrice objects after these five.
  4. Delete the old assertions (the StepVerifier replaces them all).

This approach can support much more than this simple example, and is also very useful for testing time-based publishers like ours.

Adding a retry backoff and error handling strategies

We have one final piece to consider.  Our WebClientStockClient defined a retryBackoff strategy, and a simple approach for handling errors.  We can actually apply exactly the same approach to our RSocketStockClient as well.

  1. Copy the retryBackoff and doOnError steps from WebClientStockClient and paste into RSocketStockClient.pricesFor.
  2. Re-run the test, it should all still pass.

Now we have an RSocket server on the back end emitting stock prices and an RSocket client that can connect to it and see those prices. In the next lesson we’re going to see how to switch from using the WebClientStockClient to using our new RSocketStockClient.

Full code is available on GitHub.

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