java多线程编程环境下并发是常见问题，这两天看了锁相关的问题，记录下两个简单的用锁实现等待/唤醒机制的demo。
1.synchronized方式实现等待/唤醒。
public class WaitAndNotify { private static boolean flag = true; private static Object lock = new Object(); public static void main(String[] args) {
Thread waitThread = new Thread(new Wait(), "WaitThread");
waitThread.start(); try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
Thread notifyThread = new Thread(new Notify(), "NotifyThread");
notifyThread.start();
} private static class Wait implements Runnable { @Override
public void run() { synchronized (lock) { while (flag) {
System.out.println(Thread.currentThread() + " flag是true,wait。。" + new SimpleDateFormat("HH:mm:ss").format(new Date())); try {
lock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(Thread.currentThread() + " flag是false，开始继续工作" + new SimpleDateFormat("HH:mm:ss").format(new Date()));
}
}
} private static class Notify implements Runnable { @Override
public void run() { synchronized (lock){
System.out.println(Thread.currentThread() + " 持有锁，发出通知" + new SimpleDateFormat("HH:mm:ss").format(new Date()));
lock.notifyAll();
flag = false; try {
TimeUnit.SECONDS.sleep(5);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
打印结果：
Thread[WaitThread,5,main] flag是true,wait。。18:55:28Thread[NotifyThread,5,main] 持有锁，发出通知18:55:29Thread[WaitThread,5,main] flag是false，开始继续工作18:55:34
分析：根据程序可以看到，大致的过程就是：WaitThread拿到lock对象的锁，然后根据flag标记，自己调用了wait()方法，从而释放锁并进入WAITTING状态。NotifyThread此时获取了lock对象的锁，然后进行notify操作，此时WaitThread被唤醒，但是，它不能立刻执行，因为唤醒线程NotifyThread还持有“该对象的同步锁”。必须等到NotifyThread线程释放了“对象的同步锁”之后，也就是同步代码块执行完以后，即睡眠5秒以后，等待线程WaitThread才能获取到“对象的同步锁”进而继续运行。
2.ReentrantLock方式实现等待/唤醒。
public class ReenterLockCondition { private static ReentrantLock lock = new ReentrantLock(); private static Condition condition = lock.newCondition(); private static Runnable runnable = () -> { try { lock.lock();
System.out.println(Thread.currentThread().getName() + "进入等待。。");
condition.await();
System.out.println(Thread.currentThread().getName() + "继续执行");
} catch (InterruptedException e) {
e.printStackTrace();
} finally { lock.unlock();
}
}; public static void main(String[] args) throws InterruptedException {
Thread thread = new Thread(runnable, "thread--1");
thread.start();
Thread.sleep(2000); lock.lock();
condition.signal();
System.out.println("主线程发出信号"); lock.unlock();
}
}
打印结果：
thread--1进入等待。。主线程发出信号
thread--1继续执行
