JMockit: Mocking all implementations of an interface - unit-testing

Question

Is it possible to mock all implementations of an interface?
I want to mock the WatchService interface like the following
public class ServiceTest {
#Test
public void callTest(
#Capturing
#Injectable
final WatchService ws
) throws Exception {
final CountDownLatch latch = new CountDownLatch(1);
new MockUp<ServiceTest>() {
#Mock(invocations = 1)
public void onChange() {
latch.countDown();
}
};
new NonStrictExpectations() {
{
ws.take();
result = new Delegate() {
WatchKey take(Invocation inv) {
System.out.println("> " + inv.getInvokedInstance());
try {
new File("target/newFile").createNewFile();
} catch (IOException e) {
e.printStackTrace();
}
return inv.proceed();
}
};
}
};
final Thread thread = new Thread() {
#Override
public void run() {
final Path target = Paths.get("target");
final FileSystem fs = target.getFileSystem();
try {
try (WatchService watcher = fs.newWatchService()) {
target.register(watcher, ENTRY_CREATE);
while (!Thread.currentThread().isInterrupted()) {
WatchKey take = watcher.take();
onChange();
System.out.println("take " + take);
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
};
thread.start();
assertTrue("", latch.await(5, TimeUnit.SECONDS));
thread.interrupt();
}
private void onChange() {
System.out.println("CHANGE");
}
How can I accomplish that?

Answer 1

You can use the #Capturing annotation on a mock field or mock parameter of the interface type. Below we have a complete example test (minus imports).
public class CapturingAndProceedingTest {
static class WatchKey { String key; WatchKey(String k) {key = k;} }
public interface WatchService { public abstract WatchKey take(); }
static class WatchServiceImpl1 implements WatchService {
#Override public WatchKey take() { return new WatchKey("Abc"); }
}
static class WatchServiceImpl2 implements WatchService {
#Override public WatchKey take() { return new WatchKey("123"); }
}
#Test
public void mockAllImplementationsOfAnInterface(
#Capturing // so that all implementing classes are mocked
#Injectable // so that Invocation#proceed() is supported
final WatchService watchService
) {
final List<WatchService> services = new ArrayList<>();
// Record an expectation that will match all calls to
// WatchService#take(), on any class implementing the interface.
new NonStrictExpectations() {{
watchService.take();
result = new Delegate() {
WatchKey take(Invocation inv) throws IOException {
// Do something here...
WatchService service = inv.getInvokedInstance();
services.add(service);
// ...then proceed to the real implementation.
return inv.proceed();
}
};
}};
// Instantiate and use different implementations of the interface.
WatchServiceImpl1 impl1 = new WatchServiceImpl1();
assertEquals("Abc", impl1.take().key);
WatchServiceImpl2 impl2 = new WatchServiceImpl2();
assertEquals("123", impl2.take().key);
assertEquals(Arrays.asList(impl1, impl2), services);
System.out.println(services);
}
}
See the JMockit Tutorial for more examples.