How powermockito intercept new instance? - mockito

Question

I have two classes to try to figure out how whenNew works.
public class RockService {
public RockData serv() {
RockData rockData = new RockData();
rockData.setName("RockService");
rockData.setContent("content from rock service");
return rockData;
} }
And
public class RockData {
String name;
long id;
String content;
// get set method ignored
}
With test code
#RunWith(PowerMockRunner.class)
#PrepareForTest(RockService.class)
public class MockNewInstanceCreation {
#Test
public void mockCreationTest() throws Exception {
RockData rockData = mock(RockData.class);
when(rockData.getName()).thenReturn("this is mock");
whenNew(RockData.class).withNoArguments().thenReturn(rockData);
RockService rockService = new RockService();
RockData servData = rockService.serv();
System.out.println(servData.getName());
System.out.println(servData.getContent());
}
}
So at runtime, if not mock, the output (RockData's getName()) would be "RockService". But with mock, it returns "this is mock". The code works but still I didn't know how exactly Powermock/Mockito did this.
I debugged the code. What confused me is after RockData rockData = new RockData(); executed, what actually created is exactly the instance that created by RockData rockData = mock(RockData.class);. Which means new RockData() doesn't create an new instance at all. It just returned an instance that already created. And when debugging, it jumped to MockGateway.newInstanceCall.
So how does Powermockito intercept new instance?

Answer 1

PowerMockRunner runs tests using special class loader - org.powermock.core.classloader.MockClassLoader.
Instead of loading real class it loads a new one with the same signature. It means that the real constructor won't be invoked.
So the object that is returned by new operator is not a Mock. It is a instance of a different class that could be assigned to the real one and it returns mocked values.
See the code below:
public class RockService {
ClassLoader classLoader = this.getClass().getClassLoader();
System.out.println("Real construct");
//Different class loader
System.out.println(classLoader);
//The same class
System.out.println(this.getClass());
}
public RockData serv() {
RockData rockData = new RockData();
Class<? extends RockData> clazz = rockData.getClass();
//This is a different class
System.out.println("Mocked class: " + clazz.getCanonicalName());
//And different classloader
ClassLoader classLoader = clazz.getClassLoader();
System.out.println(classLoader);
//Mocked class instance could be assigned to real one
System.out.println(RockData.class.isAssignableFrom(clazz));
//it's instance of both RockData.class and mocked class
System.out.println(clazz.isInstance(rockData));
System.out.println(RockData.class.isInstance(rockData));
rockData.setName("RockService");
rockData.setContent("content from rock service");
return rockData;
}