How to mock static member variables

Question

I have a class ClassToTest which has a dependency on ClassToMock.

public class ClassToMock {

  private static final String MEMBER_1 = FileReader.readMemeber1();

  protected void someMethod() {
    ...
  }
}

The unit test case for ClassToTest.

public class ClassToTestTest {
  private ClassToMock _mock;

  @Before
  public void setUp() throws Exception {
     _mock = mock(ClassToMock.class)
  }

}

When mock is called in the setUp() method, FileReader.readMemeber1(); is executed. Is there a way to avoid this? I think one way is to initialize the MEMBER_1 inside a method. Any other alternatives?

Thanks!

Answer 1

Your ClassToMock tightly coupled with FileReader, that's why you are not able to test/mock it. Instead of using tool to hack the byte code so you can mock it. I would suggest you do some simple refactorings to break the dependency.

Step 1. Encapsulate Global References

This technique is also introduced in Michael Feathers's wonderful book : Working Effectively with Legacy Code.

The title pretty much self explained. Instead of directly reference a global variable, you encapsulate it inside a method.

In your case, ClassToMock can be refactored into this :

public class ClassToMock {
  private static final String MEMBER_1 = FileReader.readMemeber1();

  public String getMemberOne() {
    return MEMBER_1;      
  }
}

then you can easily using Mockito to mock getMemberOne().

UPDATED Old Step 1 cannot guarantee Mockito mock safely, if FileReader.readMemeber1() throw exception, then the test will failled miserably. So I suggest add another step to work around it.

Step 1.5. add Setter and Lazy Getter

Since the problem is FileReader.readMember1() will be invoked as soon as ClassToMock is loaded. We have to delay it. So we make the getter call FileReader.readMember1() lazily, and open a setter.

public class ClassToMock {
  private static String MEMBER_1 = null;

  protected String getMemberOne() {
    if (MEMBER_1 == null) {
      MEMBER_1 = FileReader.readMemeber1();
    }
    return MEMBER_1;      
  }

  public void setMemberOne(String memberOne) {
    MEMBER_1 = memberOne;
  }
}

Now, you should able to make a fake ClassToMock even without Mockito. However, this should not be the final state of your code, once you have your test ready, you should continue to Step 2.

Step 2. Dependence Injection

Once you have your test ready, you should refactor it further more. Now Instead of reading the MEMBER_1 by itself. This class should receive the MEMBER_1 from outside world instead. You can either use a setter or constructor to receive it. Below is the code that use setter.

public class ClassToMock {
  private String memberOne;
  public void setMemberOne(String memberOne) {
    this.memberOne = memberOne;
  }

  public String getMemberOne() {
    return memberOne;
  }
}

These two step refactorings are really easy to do, and you can do it even without test at hand. If the code is not that complex, you can just do step 2. Then you can easily test ClassToTest

---

UPDATE 12/8 : answer the comment

See my another answer in this questions.

Answer 2

Powermock core provides a convenient utility method that could be used for this purpose.

Add powermock-core to your project.

testImplementation group: 'org.powermock', name: 'powermock-core', version: '2.0.9'

FileReader fileReader = mock(FileReader.class);
Whitebox.setInternalState(ClassToMock.class, "MEMBER_1", fileReader);

Whitebox.setInternalState is just a convenient method to set the value of a field using reflection. So it could be used along with any Mockito tests.