等待/通知机制:wait/notify
2017-03-10 11:29
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wait-notify简介:
程序测试:
public class WaitNotify { static boolean flag = true; static Object lock = new Object(); public static void main(String[] args) throws InterruptedException { Thread waitThread = new Thread(new Wait(), "waitThread"); waitThread.start(); try { TimeUnit.SECONDS.sleep(2); } catch (InterruptedException e) { e.printStackTrace(); } Thread notifyThread = new Thread(new Notify(), "notifyThread"); notifyThread.start(); } static class Wait implements Runnable { @Override public void run() { synchronized (lock) { while (flag) { System.out.println(Thread.currentThread() + " flag is true. release lock and wait@ " + LocalDateTime.now()); try { lock.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } System.out.println(Thread.currentThread() + " flag is false. return@ " + LocalDateTime.now()); } } } static class Notify implements Runnable { @Override public void run() { synchronized (lock) { System.out.println(Thread.currentThread() + " hold lock. notify @ " + LocalDateTime.now()); lock.notifyAll(); flag = false; try { TimeUnit.SECONDS.sleep(5); } catch (InterruptedException e) { e.printStackTrace(); } } synchronized (lock) { System.out.println(Thread.currentThread() + " hold lock again. notify @ " + LocalDateTime.now()); try { TimeUnit.SECONDS.sleep(5); } catch (InterruptedException e) { e.printStackTrace(); } } } } }
测试结果:
Thread[waitThread,5,main] flag is true.release lock and wait@2017-03-10T11:05:56.710 Thread[notifyThread,5,main] hold lock. notify @ 2017-03-10T11:05:58.584 Thread[notifyThread,5,main] hold lock again. notify @ 2017-03-10T11:06:03.584 Thread[waitThread,5,main] flag is false. return@ 2017-03-10T11:06:08.584
运行过程原理:
原理说明:
使用wait()、notify()和notifyAll时需要先对对象加锁,否则会报异常:“java.lang.IllegalMonitorStateException”调用wait()方法后,程序状态由running变为waiting,并将当前线程放置到对象的等待队列
notify()或notifyAll()方法返回后,等待线程依旧不会从wait返回,需要调用notfiy()或notifyAll的线程释放锁以后,等待线程才【有机会】从wait返回
notify()将等待队列中的一个线程从“等待队列”中移到“同步队列”中,而notifyAll方法则将等待队列中的所有线程全部移到“同步队列“中,被移动的线程由waiting变为blocked
从wait()方法返回的前提是获得调用对象的锁,由程序的打印结果可以证实
从上述细节可看出,等待/通知依托于同步机制,目的就是确保等待线程从wait()方法返回时,能够感知通知线程对变量做的修改
摘录:《Java并发编程的艺术》
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