Accessing private inner class in the same package

问题:

I have two compilation units:

public class OuterClass{

    private static class InnerClass{

        public String test(){
            return "testing123";
        }
    }

    public static void main( String[] args ){
        new CallingClass().test( new InnerClass() );
    }
}


public class CallingClass{

    public void test( Object o ){
        try{
            Method m = o.getClass().getMethod( "test" );
            Object response = m.invoke( o );
            System.out.println( "response: " + response );
        }
        catch( Exception e ){
            e.printStackTrace();
        }
    }
}

If they are in the same package, everything works and "response: testing123" is printed. If they are in separate packages, IllegalAccessException is thrown.

As I understand, exception is thrown because CallingClass cannot invoke private InnerClass methods. But what I do not understand is why is it allowed in the same package? InnerClass is not package protected. Private should not be visible outside OuterClass even if it is in the same package. Do I understand something wrong?

回答1:

The javap signature for an inner class:

class modifiers.OuterClass$InnerClass extends java.lang.Object{
    final modifiers.OuterClass this$0;
    public java.lang.String test();
}

When it comes to bytecode (i.e. runtime) there is no such thing as a private class. This is a fiction maintained by the compiler. To the reflection API, there's a package-accessible type with a public member method.

Actual access modifiers are defined in the JVM spec:

Flag Name      Value   Interpretation
ACC_PUBLIC     0x0001  Declared public; may be accessed from outside its package.
ACC_FINAL      0x0010  Declared final; no subclasses allowed.
ACC_SUPER      0x0020  Treat superclass methods specially when invoked by the
                       invokespecial instruction.
ACC_INTERFACE  0x0200  Is an interface, not a class.
ACC_ABSTRACT   0x0400  Declared abstract; may not be instantiated. 

回答2:

Private access modifier is stronger than package one (i.e. not having an access modifier at all in Java). Hence private elements of class - be it fields, methods or inner classes - are accessible only within that class.