JUnit4 expected exception

Question

I'm making scenario tests with JUnit4 for a project.

In one of the tests I need to check for an expected exception. With JUnit4 I do this using the annotation.

 @Test(expected=...)

Now the problem is that underneath the code in the test that throws the exception there's some other annotations I need to check which doesn't get excecuted. An example given:

 @Test(expected=NullPointerException.class) public void nullPointerTest() { Object o = null; o.toString(); assertTrue(false); }

This tests passes because it gets the nullpointerexception however there's obviously an assertion error with the asserTrue(false) and thus I want it to fail.

What's the best way to fix this? A solution to this could be the following, but I don't know if this is the correct way to do it.

@Test public void nullPointerTest2() { boolean caught = false; try{ Object o = null; o.toString(); } catch(NullPointerException e) { caught = true; } assertTrue(caught); assertTrue(false); }

This second test fails as predicted.

Why are you doing assertTrue(false)? Or does this stand here for some of your real testing code? Does that "real" code depend on the code before it that throws a NullPointerException? — rgettman
– rgettman, Commented Mar 12, 2013 at 0:19
I'm not sure why you would want to do that. It smells of bad test design. Each test should only test for one thing. — Aurand
– Aurand, Commented Mar 12, 2013 at 0:22

John B · Accepted Answer · 2013-03-12 11:06:18Z

Consider:

@Test(expected=NullPointerException.class) public void nullPointerTest2() { boolean caught = false; try{ Object o = null; o.toString(); } catch(NullPointerException e) { // test other stuff here throw e; } }

This allows for additional checks while still taking full advantage of JUnit's built-in exception checking.

Also, I consider the use of @Rule ExpectedException to be a better option that expected in many cases.

Nathaniel Waisbrot · Accepted Answer · 2013-03-12 00:21:22Z

JUnit4 is behaving as expected: when an exception is thrown, execution does not continue. So the NullPointerException gets thrown, the test method exits, JUnit4 marks it as passing because you expected an exception. The code after the null dereference effectively does not exist.

If you want the behavior of the second test, then what you've written is a good solution. But it's a weird thing to want. It seems to me that you are conflating two different tests. One test should test that an exception is thrown under exceptional circumstances. A second test should test whatever the second assertion checks.

I disagree that the above example is weird. Consider that in the case of an exception being thrown by the class under test, it might also be doing other testable steps prior to throwing the exception (for example releasing resources, logging the error, updating state, etc)
Yes, and if it's doing things prior to the exception, then everything will work. If the early assertions fail, the test will fail. If the later exception isn't thrown, then the test will fail. What's weird is expecting a test to throw an exception and then after that do anything. Exception thrown is the end of the function (otherwise it's not thrown, it's caught).

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