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?
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.
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