5.5. Test Instance Post-processing

[TestInstancePostProcessor](https://junit.org/junit5/docs/current/api/org.junit.jupiter.api/org/junit/jupiter/api/extension/TestInstancePostProcessor.html) defines the API for Extensions that wish to post process test instances.

Common use cases include injecting dependencies into the test instance, invoking custom initialization methods on the test instance, etc.

For a concrete example, consult the source code for the [MockitoExtension](https://github.com/mockito/mockito/blob/release/2.x/subprojects/junit-jupiter/src/main/java/org/mockito/junit/jupiter/MockitoExtension.java) and the [SpringExtension](https://github.com/spring-projects/spring-framework/tree/HEAD/spring-test/src/main/java/org/springframework/test/context/junit/jupiter/SpringExtension.java).