Parallel test configuration in JUnit

Introduction

Running tests in parallel saves time by checking many things at once instead of one by one.

When you have many tests and want to finish testing faster.

When tests do not depend on each other and can run safely at the same time.

When you want to use your computer's multiple CPU cores efficiently.

When you want to reduce waiting time in continuous integration pipelines.

Syntax

JUnit

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

@Execution(ExecutionMode.CONCURRENT)
public class MyParallelTests {

    @Test
    void testOne() {
        // test code here
    }

    @Test
    void testTwo() {
        // test code here
    }
}

Use @Execution(ExecutionMode.CONCURRENT) on the test class or method to enable parallel execution.

JUnit 5 supports parallel execution natively with this annotation and configuration.

Examples

This runs all tests in the class at the same time.

JUnit

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

@Execution(ExecutionMode.CONCURRENT)
public class ParallelTests {

    @Test
    void testA() {
        // test A code
    }

    @Test
    void testB() {
        // test B code
    }
}

Only testX runs in parallel; testY runs normally.

JUnit

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

public class MixedTests {

    @Test
    @Execution(ExecutionMode.CONCURRENT)
    void testX() {
        // test X runs in parallel
    }

    @Test
    @Execution(ExecutionMode.SAME_THREAD)
    void testY() {
        // test Y runs sequentially
    }
}

Sample Program

This test class runs two tests at the same time. Each test waits 1 second. Running in parallel means total time is about 1 second, not 2.

JUnit

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

@Execution(ExecutionMode.CONCURRENT)
public class SampleParallelTest {

    @Test
    void testOne() throws InterruptedException {
        System.out.println("Start testOne");
        Thread.sleep(1000);
        System.out.println("End testOne");
    }

    @Test
    void testTwo() throws InterruptedException {
        System.out.println("Start testTwo");
        Thread.sleep(1000);
        System.out.println("End testTwo");
    }
}

Important Notes

Make sure tests do not share or change the same data to avoid conflicts when running in parallel.

You can configure parallelism level in JUnit platform properties if needed.

Parallel tests help speed but can make debugging harder if tests interfere.

Summary

Parallel test configuration runs tests at the same time to save time.

Use @Execution(ExecutionMode.CONCURRENT) to enable parallel tests in JUnit.

Ensure tests are independent to avoid errors when running in parallel.