How to use Mockito when we cannot pass a mock object to an instance of a class

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Fundamentally, you’re trying to replace a private field with an alternative implementation, which means you’d violate encapsulation. Your only other option is to restructure the class or method, to make it better-designed for testing.

There are a lot of short answers in the comments, so I’m aggregating them here (and adding a couple of my own) as Community Wiki. If you have any alternatives, please feel free to add them here.

Restructure the class

  • Create a setter for the field in question, or relax the field’s visibility.

  • Create a dependency-injecting override or static method that takes a DAO, and make the public instance method delegate to it. Test the more-flexible method instead.

    public String myMethod() { return myMethod(dao); }
String myMethod(Dao dao) { /* real implementation here */ }

  • Add a constructor overload or static factory method that replaces private fields for the sake of testing.

  • Fully structure the class for dependency injection. (Sotirios Delimanolis, EJK)

Note that some of these can be package-private for testing, if you put your tests in the same Java package (possibly in a separate source tree). In all cases, good names and documentation are helpful to make your intentions clear.

Violate encapsulation

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