Test the error code of a custom exception with JUnit 4

Question

I would like to test the return code of an exception. Here is my production code:

class A {
  try {
    something...
  }
  catch (Exception e)
  {
    throw new MyExceptionClass(INTERNAL_ERROR_CODE, e);
  }
}

And the corresponding exception:

class MyExceptionClass extends ... {
  private errorCode;

  public MyExceptionClass(int errorCode){
    this.errorCode = errorCode;
  }

  public getErrorCode(){ 
    return this.errorCode;
  }
}

My unit test:

public class AUnitTests{
  @Rule
  public ExpectedException thrown= ExpectedException.none();

  @Test (expected = MyExceptionClass.class, 
  public void whenRunningSomething_shouldThrowMyExceptionWithInternalErrorCode() throws Exception {
      thrown.expect(MyExceptionClass.class);
      ??? expected return code INTERNAL_ERROR_CODE ???

      something();
  }
}

Answer 1

Simple:

 @Test 
 public void whenSerialNumberIsEmpty_shouldThrowSerialNumberInvalid() throws Exception {
  try{
     whenRunningSomething_shouldThrowMyExceptionWithInternalErrorCode();     
     fail("should have thrown");
  }
  catch (MyExceptionClass e){
     assertThat(e.getCode(), is(MyExceptionClass.INTERNAL_ERROR_CODE));
  }

That is all you need here:

• you don't want to expect that specific exception, as you want to check some properties of it
• you know that you want to enter that specific catch block; thus you simply fail when the call doesn't throw
• you don't need any other checking - when the method throws any other exception, JUnit will report that as error anyway

Answer 2

You can check for it using hamcres matchers as long as thrown.expect is overload to receive Matcher

thrown.expect(CombinableMatcher.both(
           CoreMatchers.is(CoreMatchers.instanceOf(MyExceptionClass.class)))
           .and(Matchers.hasProperty("errorCode", CoreMatchers.is(123))));

Note that you will need to add hamcrest matcher to your dependencies. Core matched that are included in JUnit is not enough.

Or if you don't want to use CombinableMatcher:

thrown.expect(CoreMatchers.instanceOf(MyExceptionClass.class));
thrown.expect(Matchers.hasProperty("errorCode", CoreMatchers.is(123));

Also, you don't need (expected = MyExceptionClass.class) declaration for @Test annotation