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();
}
}
}
输出结果为：
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
通过输出结果，我们能够发现几个问题。
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。