Mock chain call in Mockito causes NullPointerException or WrongTypeOfReturnValue

I'm trying to mock this call:

requestTelemetryContext.getHttpRequestTelemetry().getContext().getOperation()

So I tried that:

OperationContext operationContext = new OperationContext(null);
RequestTelemetryContext requestTelemetryContext = mock(RequestTelemetryContext.class);
when(requestTelemetryContext.getHttpRequestTelemetry().getContext().getOperation()).thenReturn(operationContext); //causes java.lang.NullPointerException

but that gives me
java.lang.NullPointerException.

So modified it into:

    OperationContext operationContext = new OperationContext(null);
    RequestTelemetryContext requestTelemetryContext = mock(RequestTelemetryContext.class);
    RequestTelemetry requestTelemetry = new RequestTelemetry();
    when(requestTelemetryContext.getHttpRequestTelemetry()).thenReturn(requestTelemetry);
    when(requestTelemetryContext.getHttpRequestTelemetry().getContext().getOperation()).thenReturn(operationContext);

that gives me:

> org.mockito.exceptions.misusing.WrongTypeOfReturnValue:
> OperationContext cannot be returned by getHttpRequestTelemetry()
> getHttpRequestTelemetry() should return RequestTelemetry
> *** If you're unsure why you're getting above error read on. Due to the nature of the syntax above problem might occur because:
> 1. This exception might occur in wrongly written multi-threaded tests. Please refer to Mockito FAQ on limitations of concurrency
> testing.
> 2. A spy is stubbed using when(spy.foo()).then() syntax. It is safer to stub spies -
> - with doReturn|Throw() family of methods. More in javadocs for Mockito.spy() method.

I do not understand why it can't just mock the whole chain call and I am not trying to return OperationContext by getHttpRequestTelemetry like it states in the error.

EDIT:
I've tried suggested approach:

OperationContext operationContext = new OperationContext(null);

RequestTelemetryContext requestTelemetryContext = mock(RequestTelemetryContext.class);
RequestTelemetry requestTelemetry = mock(RequestTelemetry.class);
TelemetryContext telemetryContext = mock(TelemetryContext.class);

when(requestTelemetryContext.getHttpRequestTelemetry()).thenReturn(requestTelemetry);
when(requestTelemetry.getContext()).thenReturn(telemetryContext);
when(telemetryContext.getOperation()).thenReturn(operationContext);
ThreadContext.setRequestTelemetryContext(requestTelemetryContext);

unfortunately that line RequestTelemetry requestTelemetry = mock(RequestTelemetry.class); causes:

> Mockito cannot mock/spy because :
> - final class

so i need to find a way to mock final class.

You need to return the mock at each stage of your call chain, and each mock should return the next mock object e.g.

a().b().c().d()

so a() should return a mock, which is configured to return a mock b etc.

when(a()).thenReturn(mockA);
when(mockA.b()).thenReturn(mockB);

etc.

You have two issues here. the first one is with the mock of chained operations (requestTelemetryContext.getHttpRequestTelemetry().getContext().getOperation()). to solve this without mocking every class in the middle, you can use RETURNS_DEEP_STUBS when mocking:

OperationContext operationContext = new OperationContext(null);
RequestTelemetryContext requestTelemetryContext = mock(RequestTelemetryContext.class, RETURNS_DEEP_STUBS);
when(requestTelemetryContext.getHttpRequestTelemetry().getContext().getOperation()).thenReturn(operationContext);

The second issue is that mockito cannot mock final classes - this is I'm afraid not solvable, you can't mock a final class