Why does `Mockito.mockConstruction` not return the mocked object in my JUnit 5 test?

General Discussions
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1

I’m trying to replace powermock and use mockConstruction in Mockito to mock object construction.

However, the mocked object isn’t returned, and the code inside the try-with-resources block isn’t executed as expected.

How can I properly use mockconstruction so that the constructed objects are mocked and verifiable in my tests?

2

I ran into this exact issue recently. The key is that mockConstruction in Mockito works with a try-with-resources block, and the mocked objects are only available inside that block.

For example:

try (MockedConstruction<MyClass> mocked = Mockito.mockConstruction(MyClass.class)) {
    MyClass obj = new MyClass(); // this is mocked
    Mockito.when(obj.someMethod()).thenReturn("mocked value");

    // Your assertions go here
}

If you try to access obj outside the try block, it won’t be mocked. I missed this at first, and nothing seemed to work until I wrapped the construction properly.

3

I faced the same confusion and realized you can capture all constructed mocks via mocked.constructed(). This is handy for verifying behavior after construction:

try (MockedConstruction<MyClass> mocked = Mockito.mockConstruction(MyClass.class)) {
    MyClass obj = new MyClass(); // mocked
    // interact with obj
    List<MyClass> mocks = mocked.constructed(); 
    Mockito.verify(mocks.get(0)).someMethod();
}

This variation is helpful when multiple objects are being constructed and you want to verify interactions on all of them.

4

From my experience, the main reasons mockConstruction doesn’t return the mocked object are:

  • Instantiating the object outside the try-with-resources block.

  • Forgetting that the mocking only affects new constructions, not existing instances.

  • Not using the returned MockedConstruction to verify interactions.

  • Once I wrapped the constructor in the try block and used mocked.constructed() for assertions, everything worked smoothly.