《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堵塞之后，一旦另外有线程调用notify方法。线程A会立马从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。

同一时候进入Waiting状态，而非Blocked状态。唯独等待其它的线程调用notify()方法且释放锁之后。当前线程才会从wait()方法处返回。只发出通知是不够用的。还须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，稍作了些改动。

/**
 *
 */
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，同一时候释放了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。完毕工作
				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 {
		@Override
		public void run() {
			// 加锁，拥有lock的Monitor
			synchronized (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才干从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();
		}
	}
}

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

    1.NotifyThread调用notifyAll()方法后，WaitThread并没有立马从wait()方法处返回。

由于这个时候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。依然处于Blocked状态。

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

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

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