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

Question

Assume I have made a simple client in my application that uses a remote web service that is exposing a RESTful API at some URI /foo/bar/{baz}. Now I wish to unit test my client that makes calls to this web service.

Ideally, in my tests, I’d like to mock the responses I get from the web service, given a specific request like /foo/bar/123 or /foo/bar/42. My client assumes the API is actually running somewhere, so I need a local "web service" to start running on http://localhost:9090/foo/bar for my tests.

I want my unit tests to be self-contained, similar to testing Spring controllers with the Spring MVC Test framework.

Some pseudo-code for a simple client, fetching numbers from the remote API:

// Initialization logic involving setting up mocking of remote API at 
// http://localhost:9090/foo/bar

@Autowired
NumberClient numberClient // calls the API at http://localhost:9090/foo/bar

@Test
public void getNumber42() {
    onRequest(mockAPI.get("/foo/bar/42")).thenRespond("{ \"number\" : 42 }");
    assertEquals(42, numberClient.getNumber(42));
}

// ..

What are my alternatives using Spring?

Answer 1

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