JUnit 5 parallel testing

Prepare test cases

public class TestClassBase {
    private static final Logger log = LoggerFactory.getLogger(TestClassBase.class);

    @BeforeEach
    public void beforeEach(final TestInfo testInfo) {
        log.info("Running {} in {}", testInfo.getDisplayName(), Thread.currentThread().getName());
    }

    @AfterEach
    public void afterEach(final TestInfo testInfo) {
        log.info("Finished {} in {}", testInfo.getDisplayName(), Thread.currentThread().getName());
    }
}

public class TestClassA extends TestClassBase {
    @ParameterizedTest(name = "test: {arguments}")
    @ValueSource(strings = { "A1", "A2" })
    public void timeConsumingTest(final String name) throws Exception {
        Thread.sleep(Duration.ofSeconds(1).toMillis());
    }
}

public class TestClassB extends TestClassBase {
    @ParameterizedTest(name = "test: {arguments}")
    @ValueSource(strings = { "B1", "B2" })
    public void timeConsumingTest(final String name) throws Exception {
        Thread.sleep(Duration.ofSeconds(1).toMillis());
    }
}

Default output

[Test worker] INFO TestClassBase - Running test: A1 in Test worker
[Test worker] INFO TestClassBase - Finished test: A1 in Test worker
[Test worker] INFO TestClassBase - Running test: A2 in Test worker
[Test worker] INFO TestClassBase - Finished test: A2 in Test worker
[Test worker] INFO TestClassBase - Running test: B1 in Test worker
[Test worker] INFO TestClassBase - Finished test: B1 in Test worker
[Test worker] INFO TestClassBase - Running test: B2 in Test worker
[Test worker] INFO TestClassBase - Finished test: B2 in Test worker

Enable JUnit 5 parallel testing

Add junit.jupiter.execution.parallel System Properties to gradle.build:

test {
    //...
    systemProperty("junit.jupiter.execution.parallel.enabled", true)
    systemProperty("junit.jupiter.execution.parallel.mode.default", "concurrent")
    systemProperty("junit.jupiter.execution.parallel.mode.classes.default", "concurrent")
}

Output

[ForkJoinPool-1-worker-5] INFO TestClassBase - Running test: A2 in ForkJoinPool-1-worker-5
[ForkJoinPool-1-worker-2] INFO TestClassBase - Running test: A1 in ForkJoinPool-1-worker-2
[ForkJoinPool-1-worker-6] INFO TestClassBase - Running test: B2 in ForkJoinPool-1-worker-6
[ForkJoinPool-1-worker-4] INFO TestClassBase - Running test: B1 in ForkJoinPool-1-worker-4
[ForkJoinPool-1-worker-5] INFO TestClassBase - Finished test: A2 in ForkJoinPool-1-worker-5
[ForkJoinPool-1-worker-6] INFO TestClassBase - Finished test: B2 in ForkJoinPool-1-worker-6
[ForkJoinPool-1-worker-2] INFO TestClassBase - Finished test: A1 in ForkJoinPool-1-worker-2
[ForkJoinPool-1-worker-4] INFO TestClassBase - Finished test: B1 in ForkJoinPool-1-worker-4

As you can see, the test classes are executed in parallel, and each @Test method within each class is also executed concurrently.

Class-level only parallel testing

You can parallelize the testing of classes while keeping the execution of @Test methods within each class in a single-threaded, sequential order.

Modify gradle.build:

test {
    //...
    systemProperty("junit.jupiter.execution.parallel.mode.default", "same_thread")
    systemProperty("junit.jupiter.execution.parallel.mode.classes.default", "concurrent")
}

Output

[ForkJoinPool-1-worker-3] INFO TestClassBase - Running test: A1 in ForkJoinPool-1-worker-3
[ForkJoinPool-1-worker-1] INFO TestClassBase - Running test: B1 in ForkJoinPool-1-worker-1
[ForkJoinPool-1-worker-3] INFO TestClassBase - Finished test: A1 in ForkJoinPool-1-worker-3
[ForkJoinPool-1-worker-1] INFO TestClassBase - Finished test: B1 in ForkJoinPool-1-worker-1
[ForkJoinPool-1-worker-1] INFO TestClassBase - Running test: B2 in ForkJoinPool-1-worker-1
[ForkJoinPool-1-worker-3] INFO TestClassBase - Running test: A2 in ForkJoinPool-1-worker-3
[ForkJoinPool-1-worker-1] INFO TestClassBase - Finished test: B2 in ForkJoinPool-1-worker-1
[ForkJoinPool-1-worker-3] INFO TestClassBase - Finished test: A2 in ForkJoinPool-1-worker-3

You can see that the test classes are executed in parallel. A1 and A2 run sequentially in worker-3, while B1 and B2 run sequentially in worker-1.

Set a Fixed Parallelism Value

By default, the strategy adjusts parallelism based on the number of CPU cores.

Add the following System Properties to gradle.build:

test {
    //...
    systemProperty("junit.jupiter.execution.parallel.config.strategy", "fixed")
    systemProperty("junit.jupiter.execution.parallel.config.fixed.parallelism", 4)
}

to set the parallelism to a fixed value.

Common Issues in Multi-threaded Environment Testing

Sometimes, a single test runs fine, but errors occur when all tests run simultaneously. Setting parallelism to 1 can help troubleshoot multi-threading issues.

Common multi-threaded environment testing problems include shared resources, such as:

• Sharing the same database with tables, views, locks, etc.
• Sharing the same static variable that isn't thread-safe
• Sharing the same directory/file

Common Solutions

• Different test classes should use different databases. If sharing is necessary, add prefixes or suffixes to the resource names being used.
• Use ResourceLock to add locks to tests that have resource conflicts.
• Minimize the use of static variables. Instead, use thread-safe shared variables, the singleton design pattern, or ThreadLocal.
• Each test should use its own @TempDir

Reference

• JUnit 5 User Guide/Parallel Execution