Exception testing

Expected Exceptions

How do you verify that code throws exceptions as expected? Verifying that code completes normally is important, but making sure the code behaves as expected in exceptional situations is vital too. For example:

    new ArrayList<Object>().get(0);

This code should throw an IndexOutOfBoundsException. The @Test annotation has an optional parameter "expected" that takes as values subclasses of Throwable. If we wanted to verify that ArrayList throws the correct exception, we would write:

    @Test(expected= IndexOutOfBoundsException.class) 
    public void empty() { 
         new ArrayList<Object>().get(0); 
    }

The expected parameter should be used with care. The above test will pass if any code in the method throws IndexOutOfBoundsException. For longer tests, it's recommended to use the ExpectedException rule, which is described below.

Deeper Testing of the Exception

The above approach is useful for simple cases, but it has its limits. For example, you can't test the value of the message in the exception, or the state of a domain object after the exception has been thrown.

Try/Catch Idiom

To address this you can use the try/catch idiom which prevailed in JUnit 3.x:

    @Test
    public void testExceptionMessage() {
        try {
            new ArrayList<Object>().get(0);
            fail("Expected an IndexOutOfBoundsException to be thrown");
        } catch (IndexOutOfBoundsException anIndexOutOfBoundsException) {
            assertThat(anIndexOutOfBoundsException.getMessage(), is("Index: 0, Size: 0"));
        }
    }

ExpectedException Rule

Alternatively, use the ExpectedException rule. This rule lets you indicate not only what exception you are expecting, but also the exception message you are expecting:

    @Rule
    public ExpectedException thrown = ExpectedException.none();

    @Test
    public void shouldTestExceptionMessage() throws IndexOutOfBoundsException {
        List<Object> list = new ArrayList<Object>();

        thrown.expect(IndexOutOfBoundsException.class);
        thrown.expectMessage("Index: 0, Size: 0");
        list.get(0); // execution will never get past this line
    }

The expectMessage also lets you use Matchers, which gives you a bit more flexibility in your tests. An example:

thrown.expectMessage(JUnitMatchers.containsString("Size: 0"));

For an expanded discussion of the ExpectedException rule, see this blog post.