What is @TestInstance and how does it change test class lifecycle?

// Default: PER_METHOD (new instance per test)
class DefaultLifecycleTest {
    int count = 0;

    @Test void first()  { count++; assertEquals(1, count); }  // pass
    @Test void second() { count++; assertEquals(1, count); }  // pass
    // Each @Test gets its own instance, so count starts at 0 each time
}

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

    @Test @Order(1) void first()  { count++; assertEquals(1, count); }
    @Test @Order(2) void second() { count++; assertEquals(2, count); }
    // Same instance: count accumulates across tests

    // Benefits of PER_CLASS:
    // 1. @BeforeAll and @AfterAll can be NON-STATIC
    @BeforeAll
    void setUpAll() {  // no static required!
        database = Database.connect();
    }

    @AfterAll
    void tearDownAll() {  // no static required!
        database.close();
    }

    // 2. Shared expensive state (database, server)
    //    without static fields
    Database database;
}