JMockit Fakes

If you are working with microservices, you may find that you are having a lot of dependencies on other services. If you want your tests to be autonomous, that is, running in isolation of those dependencies, you can either use a tool like WireMock, or you can use a mocking framework.

Recently, I came across JMockit, a mocking framework for Java.

To mock external dependencies, you can use so-called “fakes”. Say you use a service that knows the credentials of the customers of your e-store, and you make requests to this service by using a driver provided by this service, say CredentialsHttpService. In your development environment (and Jenkins), you don’t have access to a running service. A solution to this would be a fake implementation of CredentialsHttpService, where we would mock the methods that we actually call from our code.

public class CredentialsHttpService {
    ...
    public Optional<CustomerAccount> getCustomerAccount(String id) {
        // does HTTP request to service
    }
    ...
}

In our test code, we can now implement the fake:

public class ServiceTest {
    @Tested
    Service service;

    @Test
    void test() {
        new MockUp<CredentialsHttpService>() {
            final static AtomicInteger counter = new AtomicInteger();

            @Mock
            Optional<CustomerAccount> getCustomerAccount(String id) {
                return Optional.of(CustomerAccount.builder()
                                        .accountNumber(counter.incrementAndGet())
                                        .build());
            }
        }

        service.doFoo();  // eventually invokes our fake
                          // getCustomerAccount() implementation
    }
}

The class to be faked is the type parameter of the MockUp class. The fake CredentialsHttpService will only mock the methods that are annotated with @Mock. All other methods of the faked class will have their real implementation. Mocked methods are not restricted by access modifiers: the method may have a private, protected, package-private or public access modifier, as long as the method name and number of parameters and parameter types are the same.

There is a lot of other things that fakes can do in JMockit, see the documentation.