How to mock remote REST API in unit test with Spring?

Question

Best method is to use WireMock. Add the following dependencies:

    <dependency>
        <groupId>com.github.tomakehurst</groupId>
        <artifactId>wiremock</artifactId>
        <version>2.4.1</version>
    </dependency>
    <dependency>
        <groupId>org.igniterealtime.smack</groupId>
        <artifactId>smack-core</artifactId>
        <version>4.0.6</version>
    </dependency>

Define and use the wiremock as shown below

@Rule
public WireMockRule wireMockRule = new WireMockRule(8089);

String response ="Hello world";
StubMapping responseValid = stubFor(get(urlEqualTo(url)).withHeader("Content-Type", equalTo("application/json"))
            .willReturn(aResponse().withStatus(200)
                    .withHeader("Content-Type", "application/json").withBody(response)));

If you use Spring RestTemplate you can use MockRestServiceServer. An example can be found here REST Client Testing With MockRestServiceServer.

If you want to unit test your client, then you'd mock out the services that are making the REST API calls, i.e. with mockito - I assume you do have a service that is making those API calls for you, right?

If on the other hand you want to "mock out" the rest APIs in that there is some sort of server giving you responses, which would be more in line of integration testing, you could try one of the many framework out there like restito, rest-driver or betamax.

You can easily use Mockito to mock a REST API in Spring Boot.

Put a stubbed controller in your test tree:

@RestController
public class OtherApiHooks {

    @PostMapping("/v1/something/{myUUID}")
    public ResponseEntity<Void> handlePost(@PathVariable("myUUID") UUID myUUID ) {
        assert (false); // this function is meant to be mocked, not called
        return new ResponseEntity<Void>(HttpStatus.NOT_IMPLEMENTED);
    }
}

Your client will need to call the API on localhost when running tests. This could be configured in src/test/resources/application.properties. If the test is using RANDOM_PORT, your client under test will need to find that value. This is a bit tricky, but the issue is addressed here: Spring Boot - How to get the running port

Configure your test class to use a WebEnvironment (a running server) and now your test can use Mockito in the standard way, returning ResponseEntity objects as needed:

@RunWith(SpringRunner.class)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
public class TestsWithMockedRestDependencies {

  @MockBean private OtherApiHooks otherApiHooks;

  @Test public void test1() {
    Mockito.doReturn(new ResponseEntity<Void>(HttpStatus.ACCEPTED))
      .when(otherApiHooks).handlePost(any());
    clientFunctionUnderTest(UUID.randomUUID()); // calls REST API internally
    Mockito.verify(otherApiHooks).handlePost(eq(id));
  }

}

You can also use this for end-to-end testing of your entire microservice in an environment with the mock created above. One way to do this is to inject TestRestTemplate into your test class, and use that to call your REST API in place of clientFunctionUnderTest from the example.

@Autowired private TestRestTemplate restTemplate;
@LocalServerPort private int localPort; // you're gonna need this too

---

How this works

Because OtherApiHooks is a @RestController in the test tree, Spring Boot will automatically establish the specified REST service when running the SpringBootTest.WebEnvironment.

Mockito is used here to mock the controller class -- not the service as a whole. Therefore, there will be some server-side processing managed by Spring Boot before the mock is hit. This may include such things as deserializing (and validating) the path UUID shown in the example.

From what I can tell, this approach is robust for parallel test runs with IntelliJ and Maven.