从内部类调用外部类的同步方法

Question

I have a program that looks in essence like this

class Outer {
    class Inner implements Runnable {
        public void run() {
            doSomething();
        }
    }

    public synchronized void doSomething() {
        //...
    }
}

Which lock does Inner.run() acquire when it is calling doSomething() ? Is it identical to synchronized(Inner.this) or synchronized(Outer.this) ?

Thanks a lot in advance.

Answer 1

The receiver for the invocation of doSomething() within run() is Outer.this . The synchronized will therefore lock the monitor on the object referenced by that expression.

On computing the target reference in a method invocation expression , the JLS says

Otherwise, let T be the enclosing type declaration of which the method is a member, and let n be an integer such that T is the n'th lexically enclosing type declaration of the class whose declaration immediately contains the method invocation. The target reference is the n'th lexically enclosing instance of this.

T here is Outer , since that's the class that declares it. n is 1, as Outer is the immediately enclosing type declaration of Inner . The target reference is therefore the 1'th lexically enclosing instance of this , ie. Outer.this .

Concerning synchronized methods , the JLS says

For an instance method, the monitor associated with this (the object for which the method was invoked) is used.