JUnit has a very cool feature called RunnerScheduler. A custom RunnerScheduler can be set on a ParentRunner to control how child elements are executed. If you are on a Suite, the child elements would be each test class. If you are on a simple class (Junit4 runner) the child elements are all the test methods. Thus, with a RunnerScheduler you are able to control the overall execution of your test flow.

As an example, suppose you want to execute your test methods concurrently on a given test. You could have a runner called ConcurrentJunitRunner.

@RunWith(ConcurrentJunitRunner.class)
@Concurrent(threads = 6)
public final class ATest {

    @Test public void test0() throws Throwable { printAndWait(); }
    @Test public void test1() throws Throwable { printAndWait(); }
    @Test public void test2() throws Throwable { printAndWait(); }
    @Test public void test3() throws Throwable { printAndWait(); }
    @Test public void test4() throws Throwable { printAndWait(); }
    @Test public void test5() throws Throwable { printAndWait(); }
    @Test public void test6() throws Throwable { printAndWait(); }
    @Test public void test7() throws Throwable { printAndWait(); }
    @Test public void test8() throws Throwable { printAndWait(); }
    @Test public void test9() throws Throwable { printAndWait(); }

    void printAndWait() throws Throwable {
        int w = new Random().nextInt(1000);
        System.out.println(String.format("[%s] %s %s %s", Thread.currentThread().getName(), getClass().getName(), new Throwable().getStackTrace()[1].getMethodName(), w));
        Thread.sleep(w);
    }
}

The @Concurrent annotation controls the thread count.

The runner implements a custom RunnerScheduler which delegates to a thread pool and Java Concurrent API each test method. Thus all test are executed concurrently and the RunnerScheduler waits for all tests to finish.

But wait ! There's even more ! This runner just makes the test methods of a class runnable concurrently. But if you have a lot of tests in your project, you would probably want to also run all these tests concurrently ! Here come the ConcurrentSuite runner !

@RunWith(ConcurrentSuite.class)
@Suite.SuiteClasses({ATest.class, ATest2.class, ATest3.class})
public class MySuite {
}

This runner will run all the tests in your suite. If a test class uses the ConcurrentJunitRunner or is annotated by @Concurrent then its method will be run concurrently. Otherwise it will be run sequentially.

The runners provided on this article demonstrates how to use a custom RunnerScheduler, but can be safely used in any projects and be modified according to your needs.

All the code for this article can be found here. You can also checkout the classes:

svn co http://mycila.googlecode.com/svn/sandbox/ sandbox

Mathieu Carbou
http://blog.mycila.com/
http://www.junit.org/node/589