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Testing / JUnit6 Interview Questions

What is the TestInstance lifecycle in JUnit 6 and what are the two modes?

By default JUnit 6 creates a new test class instance for each test method (PER_METHOD). The @TestInstance annotation lets you change this to PER_CLASS, where one instance is shared across all methods in the class.

Test instance lifecycle modes
ModeAnnotationBehaviourImplication
PER_METHOD (default)None neededNew instance per @Test methodTest isolation guaranteed; @BeforeAll/@AfterAll must be static
PER_CLASS@TestInstance(Lifecycle.PER_CLASS)Single instance for entire classInstance state can accumulate across tests; @BeforeAll/@AfterAll can be non-static
// PER_METHOD (default): new instance per test
class DefaultLifecycleTest {
    int counter = 0;

    @Test void firstTest()  { counter++; assertEquals(1, counter); }
    @Test void secondTest() { counter++; assertEquals(1, counter); }  // passes!
    // Each test gets a fresh instance with counter=0
}

// PER_CLASS: single shared instance
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class SharedInstanceTest {
    int counter = 0;

    @Test void firstTest()  { counter++; assertEquals(1, counter); }
    @Test void secondTest() { counter++; assertEquals(2, counter); }  // passes!
    // Shared instance: counter carries over

    // @BeforeAll and @AfterAll can be non-static:
    @BeforeAll
    void initAll() {
        System.out.println("Runs once, non-static -- only possible with PER_CLASS");
    }
}

// PER_CLASS is common for @Nested tests where
// the outer class must hold shared state accessed by inner classes.

Why can @BeforeAll and @AfterAll methods be non-static when using @TestInstance(Lifecycle.PER_CLASS)?
What is the default TestInstance lifecycle in JUnit 6?

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