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.

Practice

1. What does the @TestInstance(TestInstance.Lifecycle.PER_CLASS) annotation do in a JUnit test class?

easy

A. Creates a single test instance shared by all test methods in the class.

B. Creates a new test instance before each test method runs.

C. Disables test instance creation for the class.

D. Runs tests in parallel threads automatically.

Solution

1. Step 1: Understand the annotation purpose

   The @TestInstance(TestInstance.Lifecycle.PER_CLASS) annotation tells JUnit to create only one instance of the test class for all test methods.

2. Step 2: Compare with default behavior

   By default, JUnit creates a new instance before each test method, but PER_CLASS changes this to reuse the same instance.

3. Final Answer:

   Creates a single test instance shared by all test methods in the class. -> Option A

4. Quick Check:

   PER_CLASS means one instance per class [OK]

Hint: PER_CLASS means one instance for all tests in class [OK]

Common Mistakes:

• Confusing PER_CLASS with default PER_METHOD lifecycle
• Thinking PER_CLASS disables instance creation
• Assuming PER_CLASS runs tests in parallel

2. Which of the following is the correct syntax to set the test instance lifecycle to PER_CLASS in JUnit 5?

easy

A. @TestInstance(TestInstance.Lifecycle.PER_CLASS)

B. @TestInstance(Lifecycle.PER_CLASS)

C. @TestInstance(PER_CLASS)

D. @TestInstance(Lifecycle.PER_METHOD)

Solution

1. Step 1: Recall the full annotation syntax

   The correct syntax uses the full enum path: @TestInstance(TestInstance.Lifecycle.PER_CLASS).

2. Step 2: Check other options

   @TestInstance(Lifecycle.PER_CLASS) misses the TestInstance. prefix, @TestInstance(PER_CLASS) is incomplete, and @TestInstance(Lifecycle.PER_METHOD) uses PER_METHOD which is default, not PER_CLASS.

3. Final Answer:

   @TestInstance(TestInstance.Lifecycle.PER_CLASS) -> Option A

4. Quick Check:

   Full enum path required in annotation [OK]

Hint: Use full enum path: TestInstance.Lifecycle.PER_CLASS [OK]

Common Mistakes:

• Omitting TestInstance prefix in annotation
• Using PER_METHOD instead of PER_CLASS
• Using incorrect enum names or syntax

3. Consider the following JUnit 5 test class with @TestInstance(TestInstance.Lifecycle.PER_CLASS):

import org.junit.jupiter.api.*;

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

    @Test
    void testIncrement() {
        count++;
        Assertions.assertEquals(1, count);
    }

    @Test
    void testIncrementAgain() {
        count++;
        Assertions.assertEquals(2, count);
    }
}

What will be the result when running these tests?

medium

A. Tests fail with NullPointerException.

B. Both tests fail because count resets to 0 before each test.

C. First test passes, second test fails due to count reset.

D. Both tests pass because count is shared and increments correctly.

Solution

1. Step 1: Understand PER_CLASS effect on instance

   With PER_CLASS, the same test instance is used for both tests, so the field count keeps its value between tests.

2. Step 2: Trace the count value changes

   First test increments count from 0 to 1 and asserts 1. Second test increments count from 1 to 2 and asserts 2. Both assertions are correct.

3. Final Answer:

   Both tests pass because count is shared and increments correctly. -> Option D

4. Quick Check:

   Shared instance means shared state [OK]

Hint: PER_CLASS shares instance; state persists across tests [OK]

Common Mistakes:

• Assuming count resets before each test
• Expecting independent test instances
• Confusing PER_CLASS with PER_METHOD lifecycle

4. Given this test class using PER_CLASS lifecycle:

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class SampleTest {
    private List<String> list = new ArrayList<>();

    @BeforeEach
    void setup() {
        list.clear();
        list.add("start");
    }

    @Test
    void testOne() {
        list.add("one");
        Assertions.assertEquals(2, list.size());
    }

    @Test
    void testTwo() {
        list.add("two");
        Assertions.assertEquals(2, list.size());
    }
}

If the list.clear() call is removed from setup(), what problem will occur?

medium

A. Tests will pass normally without issues.

B. Tests will fail because list keeps growing between tests.

C. Tests will throw NullPointerException.

D. JUnit will create new instances despite PER_CLASS.

Solution

1. Step 1: Analyze list state without clearing

   Without list.clear(), the list keeps accumulating elements from previous tests because the same instance is reused.

2. Step 2: Understand assertion impact

   Each test expects list size 2, but after first test, list size grows beyond 2, causing assertions to fail in subsequent tests.

3. Final Answer:

   Tests will fail because list keeps growing between tests. -> Option B

4. Quick Check:

   Shared state must be reset to avoid test interference [OK]

Hint: Clear shared state in setup to avoid test interference [OK]

Common Mistakes:

• Forgetting to reset mutable fields in PER_CLASS tests
• Assuming new instance per test with PER_CLASS
• Ignoring side effects between tests

5. You want to optimize a test class by sharing expensive setup across tests using @TestInstance(TestInstance.Lifecycle.PER_CLASS). Which of the following is the best practice to avoid flaky tests?

hard

A. Remove all assertions to speed up tests.

B. Avoid using @BeforeEach and rely on constructor initialization only.

C. Reset all mutable shared state in a @BeforeEach method.

D. Use static fields to hold shared state instead of instance fields.

Solution

1. Step 1: Understand shared state risks

   With PER_CLASS lifecycle, the same instance is reused, so mutable fields keep their values across tests, risking interference.

2. Step 2: Apply best practice for test isolation

   Resetting mutable shared state in @BeforeEach ensures each test starts fresh, preventing flaky or dependent tests.

3. Final Answer:

   Reset all mutable shared state in a @BeforeEach method. -> Option C

4. Quick Check:

   Reset shared state before each test to avoid flaky tests [OK]

Hint: Always reset mutable state in @BeforeEach with PER_CLASS [OK]

Common Mistakes:

• Relying only on constructor for setup with PER_CLASS
• Using static fields causing global state issues
• Ignoring test isolation principles