Condition对象的await()方法和signal()方法通常用于线程之间的阻塞和唤醒，但是这两个方法必须在Lock里面,比如ReentrantLock，否则会报错，也不能先执行signal()方法后执行await()方法，这样await()方法执行完后就一直阻塞了，下面是例子。

1、正常使用的代码

public class AwaitAndSignal {
    public static void main(String[] args) {
        ReentrantLock lock = new ReentrantLock();
        Condition condition = lock.newCondition();
        new Thread(()->{
                lock.lock();
                try {
                    System.out.println(Thread.currentThread().getName()+"进来啦，然后被await阻塞");
                    condition.await();
                } catch (Exception e) {
                    e.printStackTrace();
                }finally {
                    lock.unlock();
                }
                System.out.println(Thread.currentThread().getName()+"被唤醒啦");
        },"t1").start();
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            lock.lock();
            try {
                System.out.println(Thread.currentThread().getName()+"signal唤醒了t1");
                condition.signal();
            } catch (Exception e) {
                e.printStackTrace();
            }finally {
                lock.unlock();
            }
        },"t2").start();
    }
}

输出结果

t1进来啦，然后被await阻塞
t2signal唤醒了t1
t1被唤醒啦

跟预想的一致。

2、await()和signal()的使用必须要有锁,没有锁的话会报错

public class AwaitAndSignal {
    public static void main(String[] args) {
        ReentrantLock lock = new ReentrantLock();
        Condition condition = lock.newCondition();
        new Thread(()->{
                //lock.lock();
                try {
                    System.out.println(Thread.currentThread().getName()+"进来啦，然后被await阻塞");
                    condition.await();
                } catch (Exception e) {
                    e.printStackTrace();
                }finally {
                    //lock.unlock();
                }
                System.out.println(Thread.currentThread().getName()+"被唤醒啦");
        },"t1").start();
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            //lock.lock();
            try {
                System.out.println(Thread.currentThread().getName()+"signal唤醒了t1");
                condition.signal();
            } catch (Exception e) {
                e.printStackTrace();
            }finally {
                //lock.unlock();
            }
        },"t2").start();
    }
}

t1进来啦，然后被await阻塞
java.lang.IllegalMonitorStateException
t1被唤醒啦
    at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:151)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1261)

3、如果先执行signal()后执行await()则唤醒失败，会阻塞下去

public class AwaitAndSignal {
    public static void main(String[] args) {
        ReentrantLock lock = new ReentrantLock();
        Condition condition = lock.newCondition();
        new Thread(()->{
                try {
                    //这里休息三秒，那么就是她唤醒线程先执行才能到这里的阻塞执行了
                    Thread.sleep(3000);
                } catch (InterruptedException e1) {
                    // TODO Auto-generated catch block
                    e1.printStackTrace();
                }
                lock.lock();
                try {
                    System.out.println(Thread.currentThread().getName()+"进来啦，然后被await阻塞");
                    condition.await();
                } catch (Exception e) {
                    e.printStackTrace();
                }finally {
                    lock.unlock();
                }
                System.out.println(Thread.currentThread().getName()+"被唤醒啦");
        },"t1").start();
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            lock.lock();
            try {
                System.out.println(Thread.currentThread().getName()+"signal唤醒了t1");
                condition.signal();
            } catch (Exception e) {
                e.printStackTrace();
            }finally {
                lock.unlock();
            }
        },"t2").start();
    }
}

t2signal唤醒了t1
t1进来啦，然后被wait阻塞