How to use ReentrantLock method of org.mockito.internal.creation.bytebuddy.MockMethodInterceptor class

You can't do this with Mockito, but Apache Commons actually has a way to do this using one of its built in utilities. You can put this in a function in JUnit that is run after Mockito injects the rest of the mocks but before your test cases run, like this:

@InjectMocks
MyClass myClass;

@Before
public void before() throws Exception {
    FieldUtils.writeField(myClass, "fieldName", fieldValue, true);
}

For these kinds of tests I like to use a little library called awaitility. You can do it yourself with a countdown latch, but as you have seen you have to hack up your test with a machete to make that work.

In this test you should call verify after you have awaited the latch.

Another problem in your code is the private Boolean response. Since you're changing it in another thread you should make that an AtomicBoolean or at least declare it volatile.

This should work

import static org.mockito.ArgumentMatchers.any;
import static org.mockito.Mockito.verify;

verify(bar).DoStuff(any(Foo[].class));

I agree with what @fge said, more over. Lets look at example. Consider you have a method:

class A {
    public void foo(OtherClass other) {
        SomeData data = new SomeData("Some inner data");
        other.doSomething(data);
    }
}

Now if you want to check the inner data you can use the captor:

// Create a mock of the OtherClass
OtherClass other = mock(OtherClass.class);

// Run the foo method with the mock
new A().foo(other);

// Capture the argument of the doSomething function
ArgumentCaptor<SomeData> captor = ArgumentCaptor.forClass(SomeData.class);
verify(other, times(1)).doSomething(captor.capture());

// Assert the argument
SomeData actual = captor.getValue();
assertEquals("Some inner data", actual.innerData);

Yes, spy() is fit for your purpose. The warning is due to the fact that real methods are invoked, and hence you can get unexpected results (for example - real money being withdrawn from a bank account)

Mockito 1.8.1 introduced anyVararg() matcher:

when(a.b(anyInt(), anyInt(), Matchers.<String>anyVararg())).thenReturn(b);

Also see history for this: https://code.google.com/archive/p/mockito/issues/62

Edit new syntax after deprecation:

when(a.b(anyInt(), anyInt(), ArgumentMatchers.<String>any())).thenReturn(b);

As a starting point and demonstration for the principal you can start with the following snippet.

// define the headers you want to be returned
Map<String, String> headers = new HashMap<>();
headers.put(null, "HTTP/1.1 200 OK");
headers.put("Content-Type", "text/html");

// create an Enumeration over the header keys
Enumeration<String> headerNames = Collections.enumeration(headers.keySet());

// mock HttpServletRequest
HttpServletRequest request = mock(HttpServletRequest.class);
// mock the returned value of request.getHeaderNames()
when(request.getHeaderNames()).thenReturn(headerNames);

System.out.println("demonstrate output of request.getHeaderNames()");
while (headerNames.hasMoreElements()) {
    System.out.println("header name: " + headerNames.nextElement());
}
    
// mock the returned value of request.getHeader(String name)
doAnswer(new Answer<String>() {
    @Override
    public String answer(InvocationOnMock invocation) throws Throwable {
        Object[] args = invocation.getArguments();
        return headers.get((String) args[0]);
    }
}).when(request).getHeader("Content-Type");

System.out.println("demonstrate output of request.getHeader(String name)");
String headerName = "Content-Type";
System.out.printf("header name: [%s]   value: [%s]%n", 
        headerName, request.getHeader(headerName));
}

Output

demonstrate output of request.getHeaderNames()
header name: null
header name: Content-Type

demonstrate output of request.getHeader(String name)
header name: [Content-Type]   value: [text/html]

Since Mockito 1.9.5+ and Java 8+

You can use a lambda expression, like:

when(myMock.myFunction(anyString())).thenAnswer(i -> i.getArguments()[0]);

Where i is an instance of InvocationOnMock.

For older versions

You can create an Answer in Mockito. Let's assume, we have an interface named MyInterface with a method myFunction.

public interface MyInterface {
    public String myFunction(String abc);
}

Here is the test method with a Mockito answer:

public void testMyFunction() throws Exception {
    MyInterface mock = mock(MyInterface.class);
    when(mock.myFunction(anyString())).thenAnswer(new Answer<String>() {
    @Override
    public String answer(InvocationOnMock invocation) throws Throwable {
        Object[] args = invocation.getArguments();
        return (String) args[0];
    }
    });

    assertEquals("someString",mock.myFunction("someString"));
    assertEquals("anotherString",mock.myFunction("anotherString"));
}

Let me quote the official documentation:

Important gotcha on spying real objects!

Sometimes it's impossible to use when(Object) for stubbing spies. Example:

List list = new LinkedList();
List spy = spy(list);

// Impossible: real method is called so spy.get(0) throws IndexOutOfBoundsException (the list is yet empty)
when(spy.get(0)).thenReturn("foo");

// You have to use doReturn() for stubbing
doReturn("foo").when(spy).get(0);

In your case it goes something like:

doReturn(resultsIWant).when(myClassSpy).method1();