why cannot we create spy for Parameterized Constructor using Mockito

Pradeep :

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?.

Dhawal Kapil :

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

Here B and C are mocked object that you injected in your class A using parametrized constructor.
Then we created a spy of A called spyA.
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.