如何使用 Mockito 在另一个类中模拟依赖项

Question

I am trying to do a JUnit Test. When I do this I get a NullpointerException. Can I solve this problem using Mockito and if so, how?

@Test
public void test() throws Exception {

    Class1 class1 = new Class1(100, "TE", "TEST");
    
    Class2 class2 = new Class2();
    
    //I'm mapping class1 to class3 using class2.map, the usecase here is irrelevant for the problem
    Mapped<Class3> result = class2.map(class1, Class3.class);
    
}

Class2 looks something like this:

public Class class2{
   @Inject
   private EntityMapper entityMapper; //entityMapper: null

   public Mapped<T> map(Class1 class1, Class<T> class3){
      return this.entityMapper.map(class1, class3);
   }
}

When I'm trying to execute my JUnit test, I get a NullpointerException because the entityMapper inside Class class2 is null. So I want to mock the EntityMapper using the Mockito framework but I can't get around this Problem.

Answer 1

Your class2 is currently not properly unit testable. To make it testable, you need to use constructor/method injections so that we can mock and provide dependencies through test cases. See below,

public class Class2{
   
   private EntityMapper entityMapper;

   @Inject
   public Class2(EntityMapper mapper){
       entityMapper = mapper;
   }

   public Mapped<T> map(Class1 class1, Class<T> class3){
      return this.entityMapper.map(class1, class3);
   }
}

Then through your test case you can mock and set dependencies as below,

@Test
public void test() throws Exception {
    EntityMapper mapper = mock(EntityMapper.class);
    Class1 class1 = new Class1(100, "TE", "TEST");
    Class2 class2 = new Class2(mapper);
    
   //TODO: Configure result of mocked functions using mockito when-then methods.
   Mapped<Class3> result = class2.map(class1, Class3.class);
    
}

Answer 2

Instead of creating Class2 class2 = new Class2(); in the test() , you can just use

@Mock
private Class2 class2;

at class level, and if it is a SpringBoot application you can try either @MockBean or @Mock