JMM happens-before原则个人理解

所谓的happens-before原则就是在多线程环境下，比如线程A和线程B，线程A从时间上先执行了一个action1，线程B后 执行了一个action2, 如果要求线程A执行的action1的结果对线程B执行action2的结果可见，就叫action1 happens-before action2。

这样说起来比较抽象，举个例子：

假设某个对象有一个变量x，初始值是5，线程A先调用了该对象的modifyX()方法，将x设成10，然后线程B调用该对象的getX()方法获取x的值。如果没有happens-before原则，那线程B获取到的值可能不是10，而是5。因为线程A对x的修改可能只是在cpu cache中生效，还没有写到内存中。而如果我们使用了锁机制，比如使用synchronize关键字在modifyX和getX方法上，这个时候就保证了modifyX的操作对后来的getX操作都是可见的，这就是happens-beforre原则。

一句话形容happens-before原则就是 - 时间上先发生的操作A的操作结果，对后发生的操作B是可见的。

modifyX(){

  x=10;  

}

getX(){

  return x;

}

符合happens-before原则的例子：

Program order rule. Each action in a thread happens-before every action in that thread that comes later in the program order.

Monitor lock rule. An unlock on a monitor lock happens-before every subsequent lock on that same monitor lock.

Locks and unlocks on explicit Lock objects have the same memory semantics as intrinsic locks.

Volatile variable rule. A write to a volatile field happens-before every subsequent read of that same field.

Reads and writes of atomic variables have the same memory semantics as volatile variables.

Thread start rule. A call to Thread.start on a thread happens-before every action in the started thread.

etc...