Invoking a private method via JMockit to test result

Question

Am using JMockit 1.1 and all I want to do is invoke a private method and test the return value. However, I am having trouble understanding exactly how to do this from the JMockit De-Encapsulation example.

The method I am trying to test is the private method in this class:

public class StringToTransaction {
   private List<String> parseTransactionString(final String input) {
      // .. processing
      return resultList;
   }
}

And my test code is below.

@Test
public void testParsingForCommas() {
   final StringToTransaction tested = new StringToTransaction();
   final List<String> expected = new ArrayList<String>();
   // Add expected strings list here..
   new Expectations() {
      {
         invoke(tested, "parseTransactionString", "blah blah");
         returns(expected);
      }
   };
}

And the error I am getting is:

java.lang.IllegalStateException: Missing invocation to mocked type at this point; please make sure such invocations appear only after the declaration of a suitable mock field or parameter

Perhaps I have misunderstood the whole API here, because I don't think I want to mock the class.. just test the result of calling the private method.

Answer 1

I think you are making this too complicated. You should not be using the Expectations block at all. All you need to do is something like this:

@Test
public void testParsingForCommas() {
   StringToTransaction tested = new StringToTransaction();
   List<String> expected = new ArrayList<String>();
   // Add expected strings list here..

   List<String> actual = Deencapsulation.invoke(tested, "parseTransactionString", "blah blah");
   assertEquals(expected, actual);
}

Basically, call a private method via Deencapsulation and test that the actual is equal to the expected. Just like you would if the method were public. No mocking is being done, so no Expectations block is needed.

Answer 2

At this point, I don't know if JMockit can or should be used for this. Testing my private method can be done with plain old reflection, although I started this exercise to learn about JMockit (and test my code). In case JMockit cannot be used for this, here is how I can use reflection instead.

@Test
public void testParsingForCommas() throws Exception {
   StringToTransaction tested = new StringToTransaction();
   ArrayList<String> expected = new ArrayList<>();
   expected.add("Test");

   Method declaredMethod =
         tested.getClass().getDeclaredMethod("parseTransactionString",
               String.class);
   declaredMethod.setAccessible(true);
   Object actual = declaredMethod.invoke(tested, "blah blah");
   assertEquals(expected, actual);
}

The call to setAccessible(true) is important here or the invoke will blow up when calling a private method.

declaredMethod.setAccessible(true);

But you want to know what is really cool? If you don't call setAccessible(true), it will blow up with a java.lang.StackOverflowError! :)