why cannot we create spy for Parameterized Constructor using Mockito

Question

I have only parameterized constructor in my code and i need to inject through it.

I want to spy parameterized constructor to inject mock object as dependency for my junit.

public RegDao(){  //original object instantiation here Notification .... EntryService ..... }  public RegDao(Notification notification , EntryService entry) {  // initialize here }  we have something like below :  RegDao dao = Mockito.spy(RegDao.class); 

But do we have something that i can inject mocked object in the Constructor and spy it?.

Answer 1

You can do that by instantiating your main class with parametrized constructor in your junit and then creating a spy from it.

Let's suppose your main class is A. Where B and C are its dependencies

public class A {      private B b;      private C c;      public A(B b,C c)     {         this.b=b;         this.c=c;     }      void method() {         System.out.println("A's method called");         b.method();         c.method();         System.out.println(method2());      }      protected int method2() {         return 10;     } } 

Then you can write junit for this using your parametrized class as below

@RunWith(MockitoJUnitRunner.class) public class ATest {      A a;      @Mock     B b;      @Mock     C c;      @Test     public void test() {         a=new A(b, c);         A spyA=Mockito.spy(a);          doReturn(20).when(spyA).method2();          spyA.method();     } } 

Output of test class

A's method called 20 

1. Here B and C are mocked object that you injected in your class A using parametrized constructor.
2. Then we created a spy of A called spyA.
3. We checked if spy is really working by modifying the return value of a protected method method2 in class A which could not have been possible if spyA was not an actual spy of A.