java - Singleton 实例是在什么上同步的？

Question

我读过synchronized void f() { /* body */ }相当于
void f() { synchronized (this) { /* body */ } } 。

所以当我们做 synchronization单例的 get 方法我们正在同步哪个对象。是类还是对象？

public class YourObject {
    private static YourObject instance;

    private YourObject() { }

    public static synchronized YourObject getInstance() {
        if (instance == null) {
            instance = new YourObject();
        }
        return instance;
    }
}

这相当于-

public static YourObject getInstance() {
    synchronized (this) {
        /* body */
    }
}

或

public static YourObject getInstance() {
    synchronized (YourObject.class) {
        /* body */
    }
}

Answer 1

我们来问问the documentation :

You might wonder what happens when a static synchronized method is invoked, since a static method is associated with a class, not an object. In this case, the thread acquires the intrinsic lock for the Class object associated with the class. Thus access to class's static fields is controlled by a lock that's distinct from the lock for any instance of the class.

所以:

public static YourObject getInstance() {
    synchronized (YourObject.class) {
        /* body */
    }
}

相当于你的代码