Ah, now I understand it perfectly. It is a crucial detail and a very common pattern in unit tests with Mockito: when populatePisRecords is a static method in a utility class, we cannot spy on the service with @Spy, but rather we must mock the call to the static method itself.
To achieve this, a different Mockito feature is used, available through the mockito-inline dependency. This dependency allows temporarily mocking static methods during the execution of a specific test.
Dependency verification (essential): ensure that mockito-inline is included in the project's pom.xml or build.gradle. The standard spring-boot-starter-test package may not include it by default. If errors appear when trying to mock static methods, explicitly add the dependency with groupId org.mockito and artifactId mockito-inline in test scope.
Updated test and recommended approach: use Mockito.mockStatic within a try-with-resources block. That is the modern and safe way to mock static methods; it guarantees that the mock is only active within the block and is automatically cleaned up upon exit.
Key steps in the test: 1) create the test objects and the expected ProducerRecord; 2) open a try-with-resources block with MockedStatic of the utility class; 3) use mockedIntakeUtils.when with lambda syntax to stub IntakeUtils.populatePisRecords and return the desired map; 4) execute the method under test inside the try block so that the mock is active; 5) verify interactions with kafkaTemplate.
Use of try-with-resources: it is the recommended syntax because it prevents the static mock from leaking into other tests and causing unexpected and difficult-to-debug failures.
Lambda syntax when stubbing static methods: you should use mockedIntakeUtils.when(() -> IntakeUtils.populatePisRecords(...)).thenReturn(...) or the equivalent form with matchers. It is important that the act and assert are executed within the try block so that the mock is in effect.
Suggested test cases: send a successful record and verify that kafkaTemplate.send was invoked; handle the case where the ProducerRecord has a null topic and verify that send is not called; simulate that kafkaTemplate throws an exception and verify that the service catches it and does not propagate it; return an empty map and verify that no message is sent.
Good practices and brief explanation: 1 Do not use @Spy on the class under test when only external dependencies need to be replaced. 2 Use @InjectMocks to inject mocks into the class being tested. 3 Use Mockito.mockStatic to control static methods of a utility class. 4 Ensure that the scope of the mock is the minimum necessary using try-with-resources.
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