@Test(expected = BadPracticeException.class)

Hello, dear reader.

If you're familiar with JUnit 4 (please be familiar with it) you probably have seen or even used such form of expected exception check @Test(expected = SomeException.class).

Let's see why it is a bad practice.

Initial test
@Test(expected = ValidationFailedException.class)
public void invalidId() {  
  User user = new User();

// code from AndroidDevSummit Architecture Demo app, sorry Yigit 😅 (please consider making User immutable).

Improved version of the test
public void validate_shouldThrowIfUserIdIsInvalid() {  
  User user = new User();

  try {
  } catch (ValidationFailedException expected) {
    assertEquals("invalid user id", expected.getMessage());

Now you can clearly see what method is under the test can throw the exception and actually check the reason via checking the exception message.



#1. @Test(expected = ...) -> try-catch.

We're isolating method under the test via precise try-catch on the required method call. This will also protect us from false-positives if setId() or setName() or even constructor will throw ValidationFailedException.

#2. Explicit fail.

To show our expectations to the readers and to the test runner. You can use even better method failBecauseExceptionWasNotThrown() from AssertJ.

#3. assertEquals("expected exception message", expected.getMessage())

To check that exception was thrown for a right reason. In our case it could also be an exception from the name validation and initial test won't see the difference!

#4. Test name invalidId -> validate_shouldThrowIfUserIdIsInvalid.

Such naming is especially helpful if you write and read a lot of tests. methodName_expectedBehavior or something similar is good approach.

Addition: there is an ExpectedException rule right in JUnit, take a look! Thanks to osi from comments.

@Test(expected = ...) is okay for a test containing only one method call and when you definitely know that there is no sense in checking exception message.

Hope you'll discuss this with your team and prefer more robust approach for checking expected exceptions in your tests!

With ❤️ to testing by Artem Zinnatullin.


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