java - How to Re-run failed JUnit tests immediately?

Question

Is there a way to have an JUnit Rule or something similar that gives every failing test a second chance, just by trying to run it once again.

Background: I have a large Set of Selenium2-WebDriver tests written with JUnit. Due to a very aggressive timing (only short wait periods after the clicks) some tests (1 out of 100, and always a different one) can fail because the server sometimes responds a bit slower. But I can not make the wait period so long that it is definitely long enough, because then the tests will take for ever.) -- So I think it is acceptable for this use case that a test is green even if it needs a second try.

Of course it would be better to have a 2 out of 3 majority (repeat a failing test 3 times, and take them as correct, if two of the tests are correct), but this would be a future improvement.

Answer 1

You can do this with a TestRule. This will give you the flexibility you need. A TestRule allows you to insert logic around the test, so you would implement the retry loop:

public class RetryTest {
    public class Retry implements TestRule {
        private int retryCount;

        public Retry(int retryCount) {
            this.retryCount = retryCount;
        }

        public Statement apply(Statement base, Description description) {
            return statement(base, description);
        }

        private Statement statement(final Statement base, final Description description) {
            return new Statement() {
                @Override
                public void evaluate() throws Throwable {
                    Throwable caughtThrowable = null;

                    // implement retry logic here
                    for (int i = 0; i < retryCount; i++) {
                        try {
                            base.evaluate();
                            return;
                        } catch (Throwable t) {
                            caughtThrowable = t;
                            System.err.println(description.getDisplayName() + ": run " + (i+1) + " failed");
                        }
                    }
                    System.err.println(description.getDisplayName() + ": giving up after " + retryCount + " failures");
                    throw caughtThrowable;
                }
            };
        }
    }

    @Rule
    public Retry retry = new Retry(3);

    @Test
    public void test1() {
    }

    @Test
    public void test2() {
        Object o = null;
        o.equals("foo");
    }
}

The heart of a TestRule is the base.evaluate(), which calls your test method. So around this call you put a retry loop. If an exception is thrown in your test method (an assertion failure is actually an AssertionError), then the test has failed, and you'll retry.

There is one other thing that may be of use. You may only want to apply this retry logic to a set of tests, in which case you can add into the Retry class above a test for a particular annotation on the method. Description contains a list of annotations for the method. For more information about this, see my answer to How to run some code before each JUnit @Test method individually, without using @RunWith nor AOP?.

Using a custom TestRunner

This is the suggestion of CKuck, you can define your own Runner. You need to extend BlockJUnit4ClassRunner and override runChild(). For more information see my answer to How to define JUnit method rule in a suite?. This answer details how to define how to run code for every method in a Suite, for which you have to define your own Runner.