Junit / Mockito如何模拟从类b调用的对象，该对象是从类a调用的

Question

I have a class called a, which mocks a class b.

Class b has a method which is creating an object by calling a data access object (DAO) c. That is, a.method1 -> b.method2 which internally calls c.getdata() .

When I try to create a unit test I get object c is null. How do I resolve this issue?

  class Alpha {
      
    String abc;
    Beta beta = new Beta();
    List<String> seriesOfStgs = new ArrayList<>();
    
    public void alphaMethod() {
        seriesOfStgs.addAll(beta.getStrings());
    }
  }

  class Beta {
      
    StringDao stringDao = new StringDao();
    
    public List<String> getStrings() {
        return stringDao.getListOfStrings();
    }
  }

If you see above, I have 2 separate classes Alpha and Beta . I am writing the test for Alpha . I am able to mock Beta in my JUnit test, TestAlpha.java . I am getting a null pointer exception when I try to get the strings because stringDao is null.

How do I instantiate stringDao in TestAlpha , or how do I pass a reference to StringDao ?

Answer 1

You need to find a way of injecting your Beta mock into the Alpha object that you're testing. Otherwise, your Alpha object will just have the Beta that was created at the line

Beta beta = new Beta();

One way to do that would be to have a setBeta method in your Alpha class, which you could then call in your test.

Another way would be to refactor your Alpha class so that it gets its Beta object from a factory, and then inject the factory when you construct the Alpha object. This is "pattern 2" in my article here

Answer 2

Beta can be also mock object.

for example

@Mock
Alpha alpha;

@Mock
Beta beta;

...

when(alpha.getBeta()).thenReturn(beta);   // now you get mocked beta from alpha
when(beta.getStrings()).thenReturn("mock string");