java - "Class"类对象上的锁定实现以进行同步

Question

我正在经历this link 。据此:

Class locks are actually implemented as object locks. When the JVM loads a class file, it creates an instance of class java.lang.Class. When you lock a class, you are actually locking that class's Class object.

但是根据java规范，堆上相同类型(类)的所有对象共享单个Class对象。那么多线程同步访问对象怎么可能是这样呢？

Answer 1

A类锁

synchronized (String.class) {...}

对象锁

//doesn't matter what the lock object is as long as it's not null
private final Object lock = new Object(); 
...
synchronized (lock) {...} // will throw NPE if lock is null

它们都被认为是object锁定是因为String.class返回 Class<String> 的实例。

在类锁中，线程获取Class<String>实例监视器。在对象锁中，线程获取String实例监视器。