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