The JUnit exception problem

We wrote a method that is supposed to throw an exception in some cases and we want to test it. We have a big problem!

JUnit to the rescue

JUnit provides a way for you to test exceptional cases your class may encounter.

The syntax is really simple, just add expected and the exception class to the @Test annotation. Like the example below.

@Test(expected=MyException.class)
public void methodToBeTestedTest() {
  testSpecificSetup();
  classToBeTested.methodThatThrowsException();
}

We solved the problem. Problem solved.

But wait. We needed to check whether there was some side effect, or if some mock was called (or not). I am totally freaking out!

Plain old try-catch to the rescue

Not being able to verify mocks and run other asserts is just the first problem with the @Test(expected). Another problem is that a test will pass, when it should fail, if the method testSpecificSetup() throws MyException.

If you think this is unlikely and you would never do something like this, remember, no man is an island. And even the best coder will write something wrong any now and then, no one is perfect.

So how to test it? You can use the plain old try-catch block to catch the expected exception and then you can verify everything you want. You can verify extra attributes in your exception. Just don’t forget to fail the test if the method does not throw the exception.

@Test
public void methodToBeTestedTest() {
  testSpecificSetup();
  try {
    classToBeTested.methodThatThrowsException();
    Assert.fail("Exception was expected in the line above.");
  } catch (MyException exception) {
    // you can verify the exception here
  }
  // and do your other verifications here
}

We solved the problem. Problem solved.