Condition 显式条件

显式条件在不同上下文中也可以被称为条件变量、条件队列、或条件

锁用于解决竞态条件问题，条件是线程间的协作机制。显式锁与synchronized相对应，而显式条件与wait/notify相对应。wait/notify与synchronized配合使用，显式条件与显式锁配合使用。条件与锁相关联，创建条件变量需要通过显式锁，Lock接口定义了创建方法：

Condition newCondition();

Condition表示条件变量，是一个接口，它的定义为：

public interface Condition {
  void await() throws InterruptedException;
  void awaitUninterruptibly();
  long awaitNanos(long nanosTimeout) throws InterruptedException;
  boolean await(long time, TimeUnit unit) throws InterruptedException;
  boolean awaitUntil(Date deadline) throws InterruptedException;
  void signal();
  void signalAll();
}

await对应于Object的wait, signal对应于notify, signalAll对应于notifyAll，语义也是一样的。

使用显式条件进行协作的示例:

public class WaitThread extends Thread {
    private volatile boolean fire = false;
    private Lock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();
    @Override
    public void run() {
        try {
            lock.lock();
            try {
                while (!fire) {
                    condition.await();
                }
            } finally {
                lock.unlock();
            }
            System.out.println("fired");
        } catch (InterruptedException e) {
            Thread.interrupted();
        }
    }
    public void fire() {
        lock.lock();
        try {
            this.fire = true;
            condition.signal();
        } finally {
            lock.unlock();
        }
    }
    public static void main(String[] args) throws InterruptedException {
        WaitThread waitThread = new WaitThread();
        waitThread.start();
        Thread.sleep(1000);
        System.out.println("fire");
        waitThread.fire();
    }
}

　　

参考: Java编程的逻辑 16.3 显式条件