Java的 wait(), notify() 和 notifyAll()

官方解释

• wait(),notify()和notifyAll()都是java.lang.Object的方法：
• wait(): Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object.
• notify(): Wakes up a single thread that is waiting on this object's monitor.
• notifyAll(): Wakes up all threads that are waiting on this object's monitor.

作用

• 实现线程间阻塞(Blocking)
• 控制进程内调度(inter-process communication)

调用前提

• 必须先获得锁
• 必须锁定该对象

不获得锁会怎样

 public static void main(String[] args) throws InterruptedException {
 Object obj = new Object();
 obj.wait();
 obj.notifyAll();
 }

• 抛出java.lang.IllegalMonitorStateException的异常

不获得该对象的锁会怎样

 public static void main(String[] args) throws InterruptedException {
 Object obj = new Object();
 Object lock = new Object();
 synchronized (lock) {
 obj.wait();
 obj.notifyAll();
 }
}

• 抛出java.lang.IllegalMonitorStateException的异常

为什么必须获得该对象的锁

• 没有锁，wait 和 notify 有可能会产生竞态条件(Race Condition)
• 所以，JVM 通过在执行的时候抛出IllegalMonitorStateException的异常，来确保wait, notify时，获得了对象的锁，从而消除隐藏的Race Condition。

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