《Java并发编程的艺术》读书笔记：等待/通知机制

    看这本书之前，对wait和notify认识大概就是，调用wait的线程A阻塞之后，一旦另外有线程调用notify方法，线程A会立刻从wait方法处返回。看完这本书后，发现自己的认识实在太肤浅了。。。。。。

    线程调用wait()后，会释放已经获得的锁。同时进入Waiting状态，而非Blocked状态。只有等待其他的线程调用notify()方法且释放锁之后，当前线程才会从wait()方法处返回。仅仅发出通知是不够用的，还需要发出通知的线程释放锁。然而notify()方法的调用并不意味着锁的释放。

    贴出原书的demo，稍作了些修改。

/** * */package chapter04;import java.text.SimpleDateFormat;import java.util.Date;import java.util.concurrent.TimeUnit;/** * 6-11 */public class WaitNotify {private static final SimpleDateFormat sdf  = new SimpleDateFormat("HH:mm:ss");static               boolean          flag = true;static               Object           lock = new Object();public static void main(String[] args) {Thread waitThread = new Thread(new Wait(), "WaitThread");waitThread.start();sleepSeconds(1);Thread notifyThread = new Thread(new Notify(), "NotifyThread");notifyThread.start();}static class Wait implements Runnable {public void run() {synchronized (lock) {// 当条件不满足时，继续wait，同时释放了lock的锁while (flag) {try {System.out.println(CuttentThreadName() + " flag is true. wait @ " + sdf.format(new Date()));lock.wait();System.out.println(CuttentThreadName() + " gained lock again. wait @ " + sdf.format(new Date()));} catch (InterruptedException e) {e.printStackTrace();}}// 条件满足时，完成工作System.out.println(CuttentThreadName() + " flag is false. running @ " + sdf.format(new Date()));}}}private static String CuttentThreadName() {return Thread.currentThread().getName();}static class Notify implements Runnable {@Overridepublic void run() {// 加锁，拥有lock的Monitorsynchronized (lock) {// 获取lock的锁，然后进行通知，通知时不会释放lock的锁，// 直到当前线程释放了lock后，WaitThread才能从wait方法中返回System.out.println(CuttentThreadName() + " hold lock. notify @ " + sdf.format(new Date()));lock.notifyAll();flag = false;sleepSeconds(5);System.out.println(CuttentThreadName() + " is releasing lock  @ " + sdf.format(new Date()));}// 再次加锁synchronized (lock) {System.out.println(CuttentThreadName() + " hold lock again. sleep @ " + sdf.format(new Date()));sleepSeconds(5);System.out.println(CuttentThreadName() + " is releasing lock  @ " + sdf.format(new Date()));}}}private static void sleepSeconds(int timeout) {try {TimeUnit.SECONDS.sleep(timeout);} catch (InterruptedException e) {e.printStackTrace();}}}

     输出结果为：

WaitThread flag is true. wait @ 20:58:32NotifyThread hold lock. notify @ 20:58:33NotifyThread is releasing lock  @ 20:58:38NotifyThread hold lock again. sleep @ 20:58:38NotifyThread is releasing lock  @ 20:58:43WaitThread gained lock again. wait @ 20:58:43WaitThread flag is false. running @ 20:58:43

    通过输出结果，我们可以发现几个问题。

    1.NotifyThread调用notifyAll()方法后，WaitThread并没有立刻从wait()方法处返回。因为这个时候NotifyThread并没有释放锁。

    2.程序61行，NotifyThread第一次释放锁，然而WaitThread不争气，并没有抢到这把锁，依旧处于Blocked状态。

    3.直到67行，NotifyThread再一次释放锁，WaitThread获得了锁，这才从wait()处返回继续执行。

    4.线程从wait()方法返回的前提是，获得synchronized需要的锁。

    分析完原书的demo之后，是否有跟我一样的感觉，曾经的认识是那么的too young too simple。