Friday, 28 August 2015

Testing for Exceptions

The old way

There are a few ways of testing for exceptions. The old way was to just catch the exception and then do any asserts that you need. See below:

  • you can do anything you like
  • boilerplate takes up a large part
  • errorprone, for example forgetting to add the fail, means the testcase will pass if there's no exception.


TestNG provides an extention to the @Test annotation that checks for exceptions. In the example below, it is even possible to check that the appropriate exception message is returned (using a regular expression).

  • it is immediately clear that the test tests an error case
  • not possible to do any checks or asserts after the exception is thrown
I did have a discussion with a colleague of mine, regarding the fact that I wished to verify data as well as exceptions.

His argument is, if your test is both verifying data as well as exceptions, you could pull these two apart into two separate tests. One test that tests for the exception, and one test that verifies the data. I countered that, as in his case, both tests verify the exact same behaviour in the SUT (System Under Test), it should be one test. Let me know what your opinions are.


JUnit, in the new version (since 4.7), has an additional solution based on mocking stuff:

  • asserts after the exception is thrown become possible
  • you can do anything you like
  • a little boilerplate
  • not immediately clear that it's an exceptional testcase
  • I don't much like mocking


StackOverflow - JUnit Testing Exceptions
StackOverflow - JUnit test analysing expected exceptions

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