When should you override a Spring bean with a mock in a test?

Overrides unstable external boundaries intentionally while keeping the framework behavior and real collaborators the test claims to prove.

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I override a bean with a mock when the test needs a Spring context but a specific collaborator is outside the boundary, such as a paid external API or nondeterministic notification gateway. Current Spring test support can register a Mockito bean override, while older Boot suites may use @MockBean. I stub only behavior required by the scenario and assert the public result of the component under test.

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MockitoBean and test overrides, mock boundaries, context variation, behavior coupling, adapters, and realistic alternatives.

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Core answer
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A Spring bean override changes the application context; it should remove one irrelevant dependency, not hollow out the system.
I override a bean with a mock when the test needs a Spring context but a specific collaborator is outside the boundary, such as a paid external API or nondeterministic notification gateway. Current Spring test support can register a Mockito bean override, while older Boot suites may use @MockBean. I stub only behavior required by the scenario and assert the public result of the component under test.
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Mocking the repository in a test named integration test means it cannot prove queries, transactions, or constraints. Mocking security services can bypass the filter chain the test claims to cover. I prefer a fake or controllable server when protocol behavior matters, and a real container when database semantics matter. Repeated unique mock declarations also create distinct cached contexts, so shared test shapes improve both clarity and speed.

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Mocking every bean beneath the controller.
Calling a test integration while the real integration is mocked.
Verifying incidental call order.

When should you override a Spring bean with a mock in a test?

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