Mock an interface by proxying to a a real class

README.md

Mocking complex behavior with Mockito's doAnswer() and ArgumentCaptor can quickly get messy. But creating test doubles for interfaces that have many methods requires a lot of boilerplate and increases maintenance overhead.

This code allows you to create a test double without implementing a common interface with the class to be mocked.

MockUtils.java

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Proxy;

public class MockUtils {

    /**
     * Proxy an interface to a real implementation. The implementation doesn't have to implement the interface,
     * which can save on a lot of boilerplate for test doubles.
     */
    public static <T> T proxyToFake(Class<T> iface, Object forwardTo) {
        var forwarder = Proxy.newProxyInstance(
            iface.getClassLoader(),
            new Class[]{iface},
            (proxy, method, args) -> {
                try {
                    var delegateMethod = forwardTo.getClass().getMethod(method.getName(), method.getParameterTypes());
                    return delegateMethod.invoke(forwardTo, args);
                } catch (NoSuchMethodException e) {
                    throw new UnsupportedOperationException("Method not implemented in forwarding class: " + method);
                } catch (InvocationTargetException e) {
                    throw e.getCause();
                } catch (IllegalAccessException e) {
                    throw new RuntimeException(e);
                }
            }
        );
        return (T) forwarder;
    }
}

Usage.java

class FakeSpringRepositoryDelete {

    private final List<Long> deleted = new ArrayList<>();
  
    public void deleteAllById(Iterable<Long> ids) {
        deleted.addAll(ids);
    }
}


@Test
void myTest() {
  var testDouble = new FakeSpringRepositoryDelete();
  var testDoubleProxy = MockUtils.proxyToFake(MySpringDataRepository.class, testDouble);

  var classUnderTest = new MyService();
  classUnderTest.doSomething();
  
  assertThat(testDouble).containsOnly(1, 2, 3)
}