How to run tests in parallel JUnit

JunitHow-ToBeginner · 3 min read

How to Run Tests in Parallel with JUnit 5

In JUnit 5, you can run tests in parallel by configuring the junit.jupiter.execution.parallel.enabled property to true in the junit-platform.properties file or programmatically. This enables parallel execution of test classes and methods, improving test speed without extra dependencies.

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Syntax

To enable parallel test execution in JUnit 5, set the following properties in junit-platform.properties:

junit.jupiter.execution.parallel.enabled=true - turns on parallel execution.
junit.jupiter.execution.parallel.mode.default=concurrent - runs test methods in parallel.
junit.jupiter.execution.parallel.mode.classes.default=concurrent - runs test classes in parallel.

You can also configure parallelism level and strategy with other properties.

properties

junit.jupiter.execution.parallel.enabled = true
junit.jupiter.execution.parallel.mode.default = concurrent
junit.jupiter.execution.parallel.mode.classes.default = concurrent

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Example

This example shows two test classes with multiple test methods running in parallel using JUnit 5 configuration.

java

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

public class ParallelTest1 {
    @Test
    void testA() throws InterruptedException {
        Thread.sleep(500);
        assertTrue(true);
    }

    @Test
    void testB() throws InterruptedException {
        Thread.sleep(500);
        assertTrue(true);
    }
}

class ParallelTest2 {
    @Test
    void testC() throws InterruptedException {
        Thread.sleep(500);
        assertTrue(true);
    }

    @Test
    void testD() throws InterruptedException {
        Thread.sleep(500);
        assertTrue(true);
    }
}

Output

All tests pass, and total execution time is about 1 second instead of 2 seconds, showing parallel execution.

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Common Pitfalls

Common mistakes when running JUnit tests in parallel include:

Not enabling parallel execution in junit-platform.properties.
Sharing mutable state between tests causing flaky failures.
Using thread-unsafe resources without synchronization.
Expecting parallelism in JUnit 4, which lacks built-in support.

Always design tests to be independent and thread-safe for parallel runs.

properties

/* Wrong: No parallel enabled, tests run sequentially */
// junit-platform.properties
junit.jupiter.execution.parallel.enabled = false

/* Right: Enable parallel execution */
junit.jupiter.execution.parallel.enabled = true
junit.jupiter.execution.parallel.mode.default = concurrent
junit.jupiter.execution.parallel.mode.classes.default = concurrent

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Quick Reference

Property	Description	Example Value
junit.jupiter.execution.parallel.enabled	Enable parallel test execution	true
junit.jupiter.execution.parallel.mode.default	Parallel mode for test methods	concurrent
junit.jupiter.execution.parallel.mode.classes.default	Parallel mode for test classes	concurrent
junit.jupiter.execution.parallel.config.strategy	Parallelism strategy (fixed or dynamic)	fixed
junit.jupiter.execution.parallel.config.fixed.parallelism	Number of threads for fixed strategy	4

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Key Takeaways

Enable parallel execution by setting junit.jupiter.execution.parallel.enabled to true.

Configure parallel mode for both test methods and classes to run concurrently.

Ensure tests are independent and thread-safe to avoid flaky failures.

JUnit 5 supports parallel execution natively; JUnit 4 does not.

Adjust parallelism strategy and thread count for optimal performance.