Class instance obtained through multiple class loader

Question

I was reading this article,and it says that :

If we have two instances of Student class loaded by these CustomClassLoaders say student1 and student2, then student1 and student2 are not type-compatible. In other words,

Student student3 = (Student) student2;

will throw ClassCastException, because the JVM sees these two as separate, distinct class types, since they are defined by different ClassLoader instances.

Now I understand what it wants to convey:that is if we load same class by using two different class loader then then they will not be type compatible.So far so good.

But the example it has used is what confusing me, ie

Student student3 = (Student) student2;

Isn't it that when we declare a reference variable like Student student ,all what compiler looks for is that this Studnet is present at its proper location in compliance with its package structure.Now when at run time we assign this reference variable to a real loaded class,does it really matter which class loader we use to load the said class?I doubt ,because from my understanding,a class loader comes into play only when we do a 'new' or we call a static field on it and not when we simply declare a type of it.

Please clarify me on this and help me rectify my understanding in this regards.

Thanks, Mawia

Answer 1

That is mostly correct but when the assignment happens, the JVM needs to make sure it's correct. So it checks: Can I assign the right hand side (RHS) to the left hand side (LHS).

The rules say that this is allowed when you can downcast the runtime type of the RHS to the type of the LHS.

So at compile time, the code above looks OK. But at runtime, the call isAssignableFrom() returns false because the the two types are different.

The method equals() in Class returns true if the qualified names are equal and when the classloader is the same. That's why you get a ClassCastException in this case.

The methods used are implemented as native code, so you can't see what is happening. But the methods isInstance() and isAssignableFrom() only return true when the classloader for both classes is the same.

Answer 2

Yes, at runtime it really does matter which class loader did load a class. And the class loader is inspected every time a class has to be loaded. That is not only when a new or a static field is encountered. That happens every time there the class name appears in the Java code, or to be more precise, when a class name is referenced in the byte code and the byte code is executed.