Scott Leberknight's Weblog

Often when writing JUnit tests, I use the fail() method immediately after an Exception should have been thrown, in order to ensure that invalid input or use of the method results in the exception you expect. However, earlier this week I was updating a test case and noticed one test method that was expecting an exception to occur did not have a call to fail(). So this is the code that should have been in the test:

public void testSomethingFailsThatYouExpectToFail() {
    try {
        SomeObject someObject = new SomeObject();
        someObject.someMethodThatShouldFail("invalid input");
        fail("Should have thrown IllegalArgumentException");  // need this line!
    }
    catch (Exception ex) {
        assertEquals(IllegalArgumentException.class, ex.getClass());
    }
}

But the code in the test did not have the fail() method call. Without fail(), this test works as expected only when the method actually throws the exception and the assertion validates the type of exception thrown. If, on the other hand, the exception is not thrown then the test passes but is not actually a valid test!

So as a result I did a quick search through all tests and found a significant number where I or one of the other developers had forgotten to call fail() immediately following the method call where we expect an exception to be thrown, which I then had to fix by adding the call to fail() back in. After doing that I found there were actually three tests that then failed. So even though these were exceptional test cases I had to fix the offending code. Maybe I can write a Checkstyle or PMD rule that would detect this type of error in unit tests, because it is so easy to forget to call fail(), have the test pass, and move on to the next task.