Condition 条件变量条件变量是线程同步对象中的一种，主要用来等待某种条件的发生，条件发生后，可以唤醒等待在该条件上的一个线程，或所有线程。条件变量要与锁一起协同工作。条件变量调用Lock.newCondition()获得一个实例：ReentrantLock lock = new ReentrantLock();Condition condition = lock.newCondition();通常的调用方式如下：// 当条件 con == true 时等待lock.lock();try { if (con == true) { condition.await(); } // do something} finally { lock.unlock();}// 条件变量的唤醒：condition.signal(); //唤醒等待的其中一个线程condition.signalAll(); //唤醒等待的所有线程条件变量类似JDK1.4或以前版本中的 Object.wait(); Object.notify(); Object.notifyAll();值得注意的是当condition.await()时，隐式的将条件变量关联的Lock解锁，而使其他线程有机会获得Lock，而检查条件，并在条件满足时，等待在条件变量上。我们先来看一下没有条件变量时，传统的处理方式：下面这个例子，创建3个等候线程，每个线程循环调用event.wait(), 等到了event后，显示相应的信息。然后创建1个通知线程，每隔1秒调用 event.notify() 通知等待线程。import java.util.Calendar;public class TestWait { // 创建event Object，以使用它的wait(), notify()等方法 private Object event = null; public TestWait() { // 创建event event = new Object(); } public static void main(String[] args) { TestWait tester = new TestWait(); tester.test(); } public Object getEvent() { return event; } public void test() { // 启动3个等候线程 new Thread(new WaitThread(this)).start(); new Thread(new WaitThread(this)).start(); new Thread(new WaitThread(this)).start(); // 启动通知线程 new Thread(new NotifyThread(this)).start(); }}class WaitThread implements Runnable { private TestWait tester = null; public WaitThread(TestWait tester) { this.tester = tester; } public void run() { Calendar now = Calendar.getInstance(); System.out.println(now.getTime() + " W " + Thread.currentThread() + " wait for event."); while (true) { try { // 同步访问 event synchronized (tester.getEvent()) { // 等待在 event 上 tester.getEvent().wait(); } // 等到 event 后，显示信息 "got event" Calendar now1 = Calendar.getInstance(); System.out.println(now1.getTime() + " W " + Thread.currentThread() + " got event."); // do something ... Thread.sleep(500); } catch (Exception e) { e.printStackTrace(); } } }}class NotifyThread implements Runnable { private TestWait tester = null; public NotifyThread(TestWait tester) { this.tester = tester; } public void run() { while (true) { try { // 间隔1秒 Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } // 同步访问 event synchronized (tester.getEvent()) { // 通知等在event上的一个线程 tester.getEvent().notify(); // 通知等在event上的所有线程 // tester.getEvent().notifyAll(); // 打印 "fire event" 信息。 Calendar now = Calendar.getInstance(); System.out.println(now.getTime() + " N " + Thread.currentThread() + " fire event."); } } }}程序运行结果如下：Tue Jan 24 14:42:31 CST 2006 W Thread[Thread-0,5,main] wait for event.Tue Jan 24 14:42:31 CST 2006 W Thread[Thread-2,5,main] wait for event.Tue Jan 24 14:42:31 CST 2006 W Thread[Thread-1,5,main] wait for event.Tue Jan 24 14:42:31 CST 2006 N Thread[Thread-3,5,main] fire event.Tue Jan 24 14:42:31 CST 2006 W Thread[Thread-0,5,main] got event.Tue Jan 24 14:42:32 CST 2006 N Thread[Thread-3,5,main] fire event.Tue Jan 24 14:42:32 CST 2006 W Thread[Thread-2,5,main] got event.Tue Jan 24 14:42:33 CST 2006 N Thread[Thread-3,5,main] fire event.Tue Jan 24 14:42:33 CST 2006 W Thread[Thread-1,5,main] got event.Tue Jan 24 14:42:34 CST 2006 N Thread[Thread-3,5,main] fire event.Tue Jan 24 14:42:34 CST 2006 W Thread[Thread-0,5,main] got event.Tue Jan 24 14:42:36 CST 2006 N Thread[Thread-3,5,main] fire event.Tue Jan 24 14:42:36 CST 2006 W Thread[Thread-2,5,main] got event.前三行启动3个等候线程，线程阻塞在 event.wait()上。第四行通知线程Thread-3，调用event.notify(); 第五行Thread-0线程，got event.一秒钟后，Thread-3,又触发了event.notify();下面将NotifyThread中的 event.notify(); 改为event.notifyAll(); 看一下运行结果：Tue Jan 24 15:25:43 CST 2006 W Thread[Thread-1,5,main] wait for event.Tue Jan 24 15:25:43 CST 2006 W Thread[Thread-2,5,main] wait for event.Tue Jan 24 15:25:43 CST 2006 W Thread[Thread-0,5,main] wait for event.Tue Jan 24 15:25:44 CST 2006 N Thread[Thread-3,5,main] fire event.Tue Jan 24 15:25:44 CST 2006 W Thread[Thread-1,5,main] got event.Tue Jan 24 15:25:44 CST 2006 W Thread[Thread-0,5,main] got event.Tue Jan 24 15:25:44 CST 2006 W Thread[Thread-2,5,main] got event.Tue Jan 24 15:25:45 CST 2006 N Thread[Thread-3,5,main] fire event.Tue Jan 24 15:25:45 CST 2006 W Thread[Thread-0,5,main] got event.Tue Jan 24 15:25:45 CST 2006 W Thread[Thread-1,5,main] got event.Tue Jan 24 15:25:45 CST 2006 W Thread[Thread-2,5,main] got event.Tue Jan 24 15:25:46 CST 2006 N Thread[Thread-3,5,main] fire event.Tue Jan 24 15:25:46 CST 2006 W Thread[Thread-1,5,main] got event.Tue Jan 24 15:25:46 CST 2006 W Thread[Thread-0,5,main] got event.Tue Jan 24 15:25:46 CST 2006 W Thread[Thread-2,5,main] got event.可以看到当Thread-3,event.notifyAll(); 所有的线程都 got event.接下来，我们将这个例子改写为使用条件变量的例子：import java.util.concurrent.locks.Condition;import java.util.concurrent.locks.ReentrantLock;public class TestCondition { private ReentrantLock lock = null; private Condition condition = null; public TestCondition() { lock = new ReentrantLock(); condition = lock.newCondition(); } public static void main(String[] args) { TestCondition tester = new TestCondition(); tester.test(); } public void test() { new Thread(new WaitThread1(this)).start(); new Thread(new WaitThread1(this)).start(); new Thread(new WaitThread1(this)).start(); new Thread(new NotifyThread1(this)).start(); } public ReentrantLock getLock() { return lock; } public Condition getCondition() { return condition; }}class NotifyThread1 implements Runnable { private TestCondition tester = null; public NotifyThread1(TestCondition tester) { this.tester = tester; } public void run() { while (true) { try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } tester.getLock().lock(); tester.getCondition().signal(); System.out.println(Thread.currentThread() + " condition signal."); tester.getLock().unlock(); } }}class WaitThread1 implements Runnable { private TestCondition tester = null; public WaitThread1(TestCondition tester) { this.tester = tester; } public void run() { System.out.println(Thread.currentThread() + " started."); while (true) { tester.getLock().lock(); try { // getCondition().await() 将使getLock()解锁，以便其他线程可以进入 await(); tester.getCondition().await(); System.out.println(Thread.currentThread() + " get condition."); } catch (InterruptedException e) { e.printStackTrace(); } finally { tester.getLock().unlock(); } } }}运行结果如下：Thread[Thread-0,5,main] started.Thread[Thread-1,5,main] started.Thread[Thread-2,5,main] started.Thread[Thread-3,5,main] condition signal.Thread[Thread-0,5,main] get condition.Thread[Thread-3,5,main] condition signal.Thread[Thread-1,5,main] get condition.Thread[Thread-3,5,main] condition signal.Thread[Thread-2,5,main] get condition.Thread[Thread-3,5,main] condition signal.Thread[Thread-0,5,main] get condition.
