TestInstance lifecycle per class in JUnit - Test Execution Trace

Test Overview

This test demonstrates how JUnit creates a single test instance for all test methods when using @TestInstance(Lifecycle.PER_CLASS). It verifies that the same object is used across tests by incrementing a counter.

Test Code - JUnit 5

JUnit

import org.junit.jupiter.api.*;
import static org.junit.jupiter.api.Assertions.*;

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
public class CounterTest {
    private int counter = 0;

    @BeforeAll
    void setup() {
        counter = 1;
    }

    @Test
    void testIncrement1() {
        counter++;
        assertTrue(counter > 1, "Counter should be greater than 1");
    }

    @Test
    void testIncrement2() {
        counter++;
        assertTrue(counter > 2, "Counter should be greater than 2");
    }
}

Execution Trace - 4 Steps

Step	Action	System State	Assertion	Result
1	JUnit creates a single test instance of CounterTest for all tests due to @TestInstance(PER_CLASS)	CounterTest instance created with counter = 0	-	PASS
2	@BeforeAll method setup() runs and sets counter = 1	CounterTest instance counter is now 1	-	PASS
3	testIncrement1() runs, increments counter to 2	CounterTest instance counter is 2	assertTrue(counter > 1) verifies counter is 2	PASS
4	testIncrement2() runs, increments counter to 3	CounterTest instance counter is 3	assertTrue(counter > 2) verifies counter is 3	PASS

Failure Scenario

Failing Condition: If @TestInstance(Lifecycle.PER_CLASS) is missing, JUnit defaults to PER_METHOD lifecycle, requiring @BeforeAll methods to be static, but setup() is non-static.

Execution Trace Quiz - 3 Questions

What does @TestInstance(Lifecycle.PER_CLASS) do in this test?

A Runs tests in parallel threads

B Creates a new test instance for each test method

C Creates one test instance shared by all test methods

D Disables the test methods

Key Result

Using @TestInstance(Lifecycle.PER_CLASS) allows sharing state between test methods by using a single test class instance, which is useful for tests that depend on shared mutable state.