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

// 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.