在同一个包中访问私有内部类

Question

我有两个编译单元:

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();
        }
    }
}

如果它们在同一个包中,一切正常并且打印出"response:testing123".如果它们位于单独的包中,则抛出IllegalAccessException.

据我所知,抛出异常是因为CallingClass无法调用私有的InnerClass方法.但我不明白为什么在同一个包中允许它？InnerClass不受包保护.即使它在同一个包中,也不应在OuterClass外看到Private.我理解错了吗？

Answer 1

内部类的javap签名:

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

说到字节码(即运行时),就没有私有类这样的东西.这是编译器维护的虚构.对于反射API,有一个包可访问类型,带有公共成员方法.

实际访问修饰符在JVM规范中定义:

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.