How to test a class that has private methods, fields or inner classes

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How do I use JUnit to test a class that has internal private methods, fields or nested classes? It seems bad to change the access modifier for a method just to be able to run a test.
  1. Post:397

    Re: How to test a class that has private methods, fields or inner classes

    If you have somewhat of a legacy application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

    Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course you can't change private static final variables through reflection.

    Method method = targetClass.getDeclaredMethod(methodName, argClasses);
    return method.invoke(targetObject, argObjects);
    And for fields:

    Field field = targetClass.getDeclaredField(fieldName);
    field.set(object, value);
    1. targetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for  getDeclaredField.
    2. The setAccessible(true) is required to play around with privates.


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