Guava @VisibleForTesting : Help me with a complete example

Question

My intent is to do unit test of private methods and I understand on how to import the @VisibleForTesting and use it for a private method. I have done a quite a bit of search but unable to see a complete example that demonstrates this feature.

For eg:

class MyClass {
    @VisibleForTesting 
    private double[] getWorkArray(double[] values,int length) {
               :
               :
        return <some double array> 
    }
}

Now in JUnit, I must be able to do

@Test
public void testProvateMethod() {
    MyClass object = new MyClass();
    assertNotNull(object.getWorkArray(...);
}

But the difficult part is I am unable to comprehend/do the following a) Snippet of maven compiler plugin for including the relevant annotation processor b) Actually be able to test a private method. (since it throws error related to visibility of method)

I am unable to do it in action while I write a test in JUnit (due to the private access error). For eg: mvn clean test

Please provide a complete example to really all steps involved in getting the JUnit test of private methods done.

Answer 1

Firstly, I do not recommend to test private methods, unit tests should test public methods in most cases. If you have to test private methods, it usually indicates a bad design.

Regarding to @VisibleForTesting , it is used in package-methods in Guava, and not part of JUnit API. The annotation is just a tag to indicate the method can be tested, it even doesn't be loaded in JVM. So if you need to test non-public methods, make the methods package scope which is visible to unit test classes in same package.

Last, by using reflect can access private methods, if you really have to test them.

Answer 2

Testing a private method must be one of the bad patterns. However, there are times when you often feel the urge to test private methods. In this case, I personally use ReflectionTestUtils to test the method. This is because we wanted to keep the original intent of the private method, and only test the method. Below is an example of my sample.

MyClass myClass = new MyClass();  ReflectionTestUtils.invokeMethod(myClass, "getWorkArray", values, length); 

One drawback is the fact that I get the name of the method as a String and it is quite a bit sad except for the fact that refactoring does not convert correctly in IDEA.

I hope it helps.

Thanks.