Condition的简单使用

使用Condition实现的有界队列

public class BoundedQueue<T> {
    private Object[] items;
    
    private int addIndex, removeIndex, count;
    
    private Lock lock = new ReentrantLock();
    
    private Condition notFull = lock.newCondition();
    private Condition notEmpty = lock.newCondition();
    
    public BoundedQueue(int capacity){
        this.items = new Object[capacity];
    }
    public void add(T obj){
        lock.lock();
        try{
            while (count == items.length) ////这里必须使用while而不是if
                notFull.await();
            items[addIndex] = obj;
            if(++addIndex == items.length)
                addIndex = 0;
            notEmpty.signal();
            count ++;
        } catch (InterruptedException e) {
            //
        }finally {
            lock.unlock();
        }
    }
    
    public T get(){
        T toReturn = null;
        lock.lock();
        try{
            while (count == 0) //这里必须使用while而不是if
                notEmpty.await();
            toReturn = (T) items[removeIndex];
            if(++removeIndex == items.length)
                removeIndex = 0;
            count --;
            notFull.signal();
        } catch (InterruptedException e) {
            //
        }finally {
            lock.unlock();
        }
        return toReturn;
    }
}

核心方法

void await() throws InterruptedException;//类似于Object对象的wait()方法；可中断；返回时已经获取了对象的锁；
void signal();
void signalAll();//唤醒所有等待在Condition上的线程，能够从等待方法返回的线程必须获得Condition相关联的锁。

await()方法源码分析

 public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();//当前节点加入到该等待队列
            int savedState = fullyRelease(node);//释放锁，唤醒同步队列中的后继节点
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {//如果当前线程不在同步列中，就继续等待（signal）
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)//当前方法如果被signal了，在同步队列中尝试获取同步状态
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

signal 方法

 public final void signal() {
            if (!isHeldExclusively())//前置条件
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignal(first);//唤醒等待队列中的第一个节点，将其移动到同步队列中并使用LockSupport唤醒线程（上面的 while (!isOnSyncQueue(node))循环）去获取同步状态（acquireQueued(node, savedState) ）。
        }

LockSupport工具

park();//当前线程进入阻塞状态
unpark(Thread t);//唤醒线程t