I am trying to do a test on class A, with a dependency A->B to be spied, and a transitive dependency B->C

This code is typical in a spring application, when A is a high-level service, C is a low-level one (like a Repository), and B is simply a helper service class that delegate most calls to C

To use @Spy on instance of B, requires to mock C, and inject C into B...
a typical test code should be

   @RunWith(MockitoJUnitRunner.class)
   class Test {
        @InjectMocks A a;
	@Spy @InjectMocks  B b; 
        @Spy C c;

       @Test  public void testA() { .. }
}

This generally works fine with standard spring injections for code like

   @Component // or @Service, @Controller, ..
   public class B {
      @Autowired // or @Inject
      private C c;
   }

... BUT fails when using Constructor dependency injection for fields !!!
This is absurd since constructor injection is precisely a best practice for enforcing code stability regarding test

   @Component
   public class B {
      private final C c;
      public B(C c) { this.c = c;  }
   }

There are many strange situations, while testing different combinations of annotations on real class (@Inject, @Autowired..), and @Spy, @InjectMocks in test class

Either
case 1/ spy NOT injected in a.b ... so causing NPE in spy execution
case 2/ spy NOT injected in b.c ... but injected in a.b ... so causing NPE in spy execution
case 3/ MockitoException Unable to initialize @Spy annotated field 'b', Please ensure that the type 'CtorB' has a no-arg constructor

Notice also that the final desired scenario would be to use lombok generated full constructor.. but it also fails !!

	@InjectMocks 
	A a; // ERROR.. a.b == null
	
	@Spy @InjectMocks
	B b; // b instanciated, but not injected into a !!!

	@Spy
	C c;

    // with..
    @AllArgsConstructor(onConstructor_={@Inject}) // equivalent of :  @Inject public A(B b) { this.b = b; }
    public class A {
	private final B b;
    }
   // or also 
   @AllArgsConstructor
   public class A {
	private final B b;
   }

I have joined a full maven test project that reproduce many of these scenarios
test-mockito.zip

You can also browse it (up-to-date) here.