Parallel test configuration in JUnit - Build an Automation Script

Configure and verify parallel test execution in JUnit

Preconditions (2)

Step 1: Create two simple test classes with multiple test methods each

Step 2: Configure JUnit to run tests in parallel using junit-platform.properties file

Step 3: Run the tests using the IDE or command line

Step 4: Observe the test execution order and timing

✅ Expected Result: Tests from different classes run in parallel, reducing total execution time compared to sequential run

Automation Requirements - JUnit 5

Assertions Needed:

Verify all tests pass successfully

Verify tests run in parallel by measuring execution time or thread names

Best Practices:

Use junit-platform.properties for parallel configuration

Keep tests independent to avoid flaky results

Use @Test annotations properly

Avoid shared mutable state between tests

Automated Solution

JUnit

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertTrue;

public class ParallelTest1 {

    @Test
    void testOne() throws InterruptedException {
        System.out.println("ParallelTest1 - testOne - Thread: " + Thread.currentThread().getName());
        Thread.sleep(1000); // simulate work
        assertTrue(true);
    }

    @Test
    void testTwo() throws InterruptedException {
        System.out.println("ParallelTest1 - testTwo - Thread: " + Thread.currentThread().getName());
        Thread.sleep(1000);
        assertTrue(true);
    }
}

public class ParallelTest2 {

    @Test
    void testThree() throws InterruptedException {
        System.out.println("ParallelTest2 - testThree - Thread: " + Thread.currentThread().getName());
        Thread.sleep(1000);
        assertTrue(true);
    }

    @Test
    void testFour() throws InterruptedException {
        System.out.println("ParallelTest2 - testFour - Thread: " + Thread.currentThread().getName());
        Thread.sleep(1000);
        assertTrue(true);
    }
}

// junit-platform.properties file content (place in src/test/resources):
junit.jupiter.execution.parallel.enabled = true
junit.jupiter.execution.parallel.mode.default = concurrent
junit.jupiter.execution.parallel.config.strategy = fixed
junit.jupiter.execution.parallel.config.fixed.parallelism = 2

// To run tests, use your IDE or command line: mvn test or gradle test

// Observe console output for thread names and timing to confirm parallel execution.

The two test classes ParallelTest1 and ParallelTest2 each have two test methods. Each test prints the current thread name and sleeps for 1 second to simulate work.

The junit-platform.properties file enables parallel execution and sets the parallelism to 2 threads.

When running the tests, you will see that tests from different classes run on different threads concurrently, reducing total test time.

This setup uses JUnit 5's built-in parallel execution feature configured via properties, which is the recommended way to enable parallel tests.

Assertions verify tests pass, and printing thread names helps confirm parallelism.

Common Mistakes - 4 Pitfalls

{'mistake': 'Not enabling parallel execution in junit-platform.properties', 'why_bad': 'Tests will run sequentially, so no speed benefit is gained.', 'correct_approach': "Add 'junit.jupiter.execution.parallel.enabled = true' in junit-platform.properties."}

Sharing mutable state between tests

Using @BeforeAll or @AfterAll without static modifier

Assuming parallelism without verifying thread usage

Bonus Challenge

Now add data-driven testing with 3 different inputs to one of the test methods and verify parallel execution still works.

Show Hint

Practice

1. What is the main purpose of configuring parallel test execution in JUnit?

easy

A. To disable all tests temporarily

B. To run tests one after another in a fixed order

C. To run multiple tests at the same time and save testing time

D. To automatically fix failing tests

Solution

1. Step 1: Understand parallel test execution

   Parallel test execution means running tests simultaneously instead of one by one.

2. Step 2: Identify the benefit

   Running tests at the same time saves overall testing time.

3. Final Answer:

   To run multiple tests at the same time and save testing time -> Option C

4. Quick Check:

   Parallel tests = faster testing [OK]

Hint: Parallel tests run simultaneously to save time [OK]

Common Mistakes:

• Thinking parallel means disabling tests
• Confusing parallel with sequential execution
• Assuming parallel fixes test failures

2. Which annotation enables parallel execution of tests in JUnit 5?

easy

A. @Execution(ExecutionMode.CONCURRENT)

B. @RunWith(ParallelRunner.class)

C. @ParallelTest

D. @ConcurrentExecution

Solution

1. Step 1: Recall JUnit 5 parallel test annotation

   JUnit 5 uses @Execution with ExecutionMode.CONCURRENT to run tests in parallel.

2. Step 2: Check other options

   @RunWith is from JUnit 4, and @ParallelTest or @ConcurrentExecution do not exist in JUnit 5.

3. Final Answer:

   @Execution(ExecutionMode.CONCURRENT) -> Option A

4. Quick Check:

   @Execution(CONCURRENT) enables parallel tests [OK]

Hint: Use @Execution(ExecutionMode.CONCURRENT) for parallel tests [OK]

Common Mistakes:

• Using JUnit 4 annotations in JUnit 5
• Assuming @ParallelTest exists
• Confusing annotation names

3. Given the following JUnit 5 test class, what will be the output when tests run with @Execution(ExecutionMode.CONCURRENT)?

import org.junit.jupiter.api.*;
import org.junit.jupiter.api.parallel.Execution;
import org.junit.jupiter.api.parallel.ExecutionMode;

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
@Execution(ExecutionMode.CONCURRENT)
public class SampleTest {

    @Test
    void testA() throws InterruptedException {
        Thread.sleep(100);
        System.out.println("Test A completed");
    }

    @Test
    void testB() {
        System.out.println("Test B completed");
    }
}

medium

A. Test A completed
Test B completed (always in this order)

B. Test B completed
Test A completed (order may vary due to concurrency)

C. Compilation error due to @Execution usage

D. Tests run sequentially ignoring @Execution

Solution

1. Step 1: Understand @Execution(ExecutionMode.CONCURRENT)

   This annotation runs tests in parallel, so testA and testB start at the same time.

2. Step 2: Analyze test behavior

   testA sleeps 100ms before printing, testB prints immediately. So testB likely prints first, but order can vary.

3. Final Answer:

   Test B completed Test A completed (order may vary due to concurrency) -> Option B

4. Quick Check:

   Parallel tests print order varies [OK]

Hint: Sleep delays show concurrency by changing print order [OK]

Common Mistakes:

• Assuming tests run sequentially despite @Execution
• Expecting fixed print order
• Thinking @Execution causes compile errors

4. You configured @Execution(ExecutionMode.CONCURRENT) but tests fail intermittently due to shared data. What is the best fix?

medium

A. Make tests independent by avoiding shared mutable state

B. Remove @Execution and run tests sequentially

C. Add Thread.sleep() to all tests

D. Use @Disabled on all tests

Solution

1. Step 1: Identify cause of intermittent failures

   Shared mutable data causes race conditions when tests run in parallel.

2. Step 2: Apply best practice for parallel tests

   Tests must be independent and avoid shared mutable state to run safely in parallel.

3. Final Answer:

   Make tests independent by avoiding shared mutable state -> Option A

4. Quick Check:

   Independent tests avoid parallel conflicts [OK]

Hint: Avoid shared data for stable parallel tests [OK]

Common Mistakes:

• Just removing parallel execution instead of fixing tests
• Adding sleeps does not fix race conditions
• Disabling tests hides the problem

5. You want to run tests in parallel but only within each test class, not across classes. Which configuration achieves this in JUnit 5?

hard

A. Annotate test classes with @Execution(ExecutionMode.SAME_THREAD)

B. Use @Execution(ExecutionMode.CONCURRENT) on each test method only

C. Set junit.jupiter.execution.parallel.enabled to false

D. Set junit.jupiter.execution.parallel.mode.default to concurrent and junit.jupiter.execution.parallel.mode.classes.default to same_thread

Solution

1. Step 1: Understand JUnit 5 parallel config properties

   Property junit.jupiter.execution.parallel.mode.default controls method-level parallelism; junit.jupiter.execution.parallel.mode.classes.default controls class-level parallelism.

2. Step 2: Configure methods parallel, classes sequential

   Setting methods to concurrent and classes to same_thread runs tests in parallel within classes but not across classes.

3. Final Answer:

   Set junit.jupiter.execution.parallel.mode.default to concurrent and junit.jupiter.execution.parallel.mode.classes.default to same_thread -> Option D

4. Quick Check:

   Methods parallel + classes sequential = parallel in class only [OK]

Hint: Configure method and class parallel modes separately [OK]

Common Mistakes:

• Assuming @Execution on methods controls class-level parallelism
• Disabling parallel execution entirely
• Using SAME_THREAD on classes disables all parallelism