Condition 将 Object 监视器方法(wait、notify 和 notifyAll)分解成截然不同的对象,以便通过将这些对象与任意 Lock 实现组合使用,为每个对象提供多个等待 set(wait-set)。其中,Lock 替代了 synchronized 方法和语句的使用,Condition 替代了 Object 监视器方法的使用。
条件(也称为条件队列 或条件变量)为线程提供了一个含义,以便在某个状态条件现在可能为 true 的另一个线程通知它之前,一直挂起该线程(即让其“等待”)。因为访问此共享状态信息发生在不同的线程中,所以它必须受保护,因此要将某种形式的锁与该条件相关联。等待提供一个条件的主要属性是:以原子方式 释放相关的锁,并挂起当前线程,就像 Object.wait 做的那样。
Condition 实例实质上被绑定到一个锁上。要为特定 Lock 实例获得 Condition 实例,请使用其 newCondition() 方法。
import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; public class ThreeConditionCommunication { public static void main(String[] args) { final Business business = new Business(); final int cycle = 5; new Thread(new Runnable() { @Override public void run() { for (int i = 1; i <= cycle; i++) { business.sub2(i); } } }).start(); new Thread(new Runnable() { @Override public void run() { for (int i = 1; i <= cycle; i++) { business.sub3(i); } } }).start(); for (int i = 1; i <= cycle; i++) { business.main(i); } } static class Business { Lock lock = new ReentrantLock(); Condition condition1 = lock.newCondition(); Condition condition2 = lock.newCondition(); Condition condition3 = lock.newCondition(); private volatile int shouldSub = 1; public void sub2(int i) { lock.lock(); try { while (shouldSub != 2) { try { condition2.await(); } catch (Exception e) { e.printStackTrace(); } } for (int j = 1; j <= 10; j++) { System.out.println("sub2 thread sequence of " + j + ",loop of " + i); } shouldSub = 3; condition3.signal(); } finally { lock.unlock(); } } public void sub3(int i) { lock.lock(); try { while (shouldSub != 3) { try { condition3.await(); } catch (Exception e) { e.printStackTrace(); } } for (int j = 1; j <= 20; j++) { System.out.println("sub3 thread sequence of " + j + ",loop of " + i); } shouldSub = 1; condition1.signal(); } finally { lock.unlock(); } } public void main(int i) { lock.lock(); try { while (shouldSub != 1) { try { condition1.await(); } catch (Exception e) { e.printStackTrace(); } } for (int j = 1; j <= 5; j++) { System.out.println("main thread sequence of " + j + ",loop of " + i); } shouldSub = 2; condition2.signal(); } finally { lock.unlock(); } } } }