Mocking a dependency on a property file

Question

I'm trying to write a unit test that involves using legacy code. Problem is, from what I can tell, the legacy code uses a key/value pair from a property file to initialize one of its final static private members, and I haven't the slightest as to where that property file might be (the entire application is quite huge).

So, in my test, I want to do something like this (using Mockito):

LegacyClass legacyClass = mock(LegacyClass.class);

I end up getting a ExceptionInInitializationError which indicates it can't find a certain property key.

In LegacyClass.java, there's:

private static final int LEGACY_PROPERTY = 
    Integer.parseInt(LegacyPropertyManager.getProp("legacy.property.key"));

Is there a way to write a test that uses this legacy class, even if the property key it's looking for doesn't exist? Can it be mocked somehow?

Add your own properties file for teh unit test like [here](http://stackoverflow.com/questions/1557562/does-junit-support-properties-files-for-tests) — Rob Garwood, Aug 01 '13 at 15:48
@RobGarwood But the problem is that it's the legacy class that uses the property - how would make LegacyClass use my own properties file upon instantiation instead of the one looked up by LegacyPropertyManager? — dashik, Aug 01 '13 at 16:30
@JeffBowman Clever indeed; though I'm not too sure how to use `mockStatic` to mock `getProp` so that it uses my property file. In my unit test I tried mocking `LegacyPropertyManager` and then setting up a `when(...).thenReturn(...)` like in the example you linked to, but it looks like LegacyClass still uses the original. Was there an extra step I need to do? — dashik, Aug 01 '13 at 18:39

score 2 · Accepted Answer · answered Aug 01 '13 at 19:03

2

You might not get very far without a clever library like PowerMock. Note that your LegacyClass.java initializes this property in a static final field, which means that the initializer will run as soon as it's loaded. PowerMock uses deeper magic (read: bytecode manipulation) to allow you to mock the static getProp method you cited above.

You'll need to do the following to get started with PowerMockito:

@RunWith(PowerMockRunner.class)
@PrepareForTest(LegacyPropertyManager.class)
public class YourClass {

  @Before public void stubLegacyPropertyManager() {
    Mockito.when(LegacyPropertyManager.getProp("legacy.property.key"))
        .thenReturn("42");
  }

  @Test public void yourTest() {
    // ...
  }
}

Note the class-level annotations, which respectively allow for PowerMock initialization and register the correct class for static-level mocking.

answered Aug 01 '13 at 19:03

Jeff Bowman

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Here in this case I am not getting the `getProp`. Getting error. In eclipse recomndation it is asking to do `getProperties.` But test cases are failing. – David Jul 12 '18 at 04:23
@David It sounds like you might have a slightly different problem. The comments aren't a good place to solve it, as the comment box is small and formatting-restricted; please consider asking a new top-level question. To show the research/attempts, you can link to this answer and describe what parts about it are inapplicable for you. – Jeff Bowman Jul 12 '18 at 04:42
i add a question here. Please have a look. https://stackoverflow.com/questions/51297854/mock-test-is-getting-failed-because-of-properties-file – David Jul 12 '18 at 05:23

score 1 · Answer 2 · edited May 23 '17 at 12:33

1

An example of mocking a properties file, completely ignoring where it is would be the following. You can create your own set of key/value pairs right in the mocked properties object.

Properties mockProperties = mock(Properties.class);
when((mockProperties.getProperty("keyName"))).thenReturn("value");

Getting this object to be used by your legacy class may involve code changes to the legacy class such as changing the private properties object to protected or creating a set method.

When unable to enhance the legacy code, I know you can venture into the area of partial mocks/spies (Mockito 1.8) such as described in the following post .. Mockito bypass static method for testing and its link to Effective Mockito. I haven't used them so I cannot offer more help. Good luck.

edited May 23 '17 at 12:33

Community

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answered Aug 01 '13 at 16:25

Steven Mai

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Useful to know, but unfortunately I'm not allowed to touch the legacy class code. – dashik Aug 01 '13 at 16:50
If you're not allowed to touch the class code, then what will you do if your testing uncovers a bug in that code? – Dawood ibn Kareem Aug 01 '13 at 20:02
Ah, just to clear things up, I'm not testing the legacy code. It's been in use since the late 90's (albeit, poorly written by today's standards), so we know it's functional. I just needed to write a class (and unit test) which uses that legacy code, but its bad design gave me a hard time mocking it. I put in a request with my boss about gaining permission for refactoring, and he said he would try to do something about it. – dashik Aug 02 '13 at 01:18

Mocking a dependency on a property file

2 Answers2