Support strict stubbing for @MockitoBean and @MockitoSpyBean

[sbrannen]

Overview

As explained in #33692, due to limitations in Mockito it is not currently possible to support strict stubbing for mocks and spies created via @MockitoBean and @MockitoSpyBean.

This issue therefore serves as a placeholder to collaborate with the Mockito team to investigate a way to provide such support.

Known Issues

• It is impossible for two frameworks to use the MockitoSession simultaneously. Only one MockitoSession can exist at any given time for the current thread.
• The MockitoTestExecutionListener that existed during the 6.2 milestone phase attempted to emulate the behavior of Mockito's own MockitoExtension; however, a Spring TestExecutionListener cannot achieve the same level of integration into JUnit Jupiter's extension model.
  • A Spring TestExecutionListener cannot access the enclosing test instances for @Nested test classes.
  • A Spring TestExecutionListener cannot store state in a parent TestContext (simply because there is no parent).
  • Consequently, the MockitoTestExecutionListener provided less benefit than using the MockitoExtension when it comes to JUnit Jupiter support.

Results of exploratory research

MockitoSession is effectively a "Unit of Work" that is bound to the current thread via a ThreadLocal. Mocks created between the "start" and "finish" of the session are tracked for the current thread only. Mocks created outside that session -- for example, before the session or in another thread -- are not tracked by the session. In addition, there is no API that allows one to attach an existing mock to a MockitoSession, and there is no API to query whether a MockitoSession already exists for the current thread.

The following provides an overview of how MockitoSession technically works.

• startMocking() invokes: MockitoAnnotations.openMocks()
• startMocking() registers: org.mockito.internal.junit.UniversalTestListener
• finishMocking() unregisters: org.mockito.internal.junit.UniversalTestListener
• finishMocking() invokes: Mockito.validateMockitoUsage()

Hypothetically (and this is not tested in any way, shape, or form), if we wanted to mimic MockitoSession support for mocks created via @MockitoBean and @MockitoSpyBean, we could track the MockCreationSettings in MockitoBeanOverrideMetadata.createMock() and then register a (subclass of) UniversalTestListener via Mockito.framework().addListener() and invoke CustomSubclassOfUniversalTestListener.onMockCreated(Object, MockCreationSettings) with the mock created by MockitoBeanOverrideMetadata and the saved MockCreationSettings in MockitoTestExecutionListener.beforeTestMethod(), and we could then invoke Mockito.framework().removeListener() and Mockito.validateMockitoUsage() in MockitoTestExecutionListener.afterTestMethod().

However, UniversalTestListener is an internal implementation detail of Mockito and resides in the org.mockito.internal.junit package, and we should ideally not rely on Mockito internals that may change unexpectedly.

Instead, we should approach the Mockito team to discuss alternatives to the current MockitoSession API and semantics that would allow us to provide similar support to mocks that we choose and within a scope that we define.

Related Issues

• Offer a way for MockitoTestExecutionListener to enable strict stubbing #33318
• MockitoTestExecutionListener no longer opens mocks in the prepareTestInstance() callback #33690
• Remove @MockitoBeanSettings and support for MockitoSession management #33692