理解Condition的用法

Posted 2020-08-21 凌动小生的Blog

这个示例中BoundedBuffer是一个固定长度的集合，这个在其put操作时，如果发现长度已经达到最大长度，那么会等待notFull信号，如果得到notFull信号会像集合中添加元素，并发出notEmpty的信号，而在其take方法中如果发现集合长度为空，那么会等待notEmpty的信号，同时如果拿到一个元素，那么会发出notFull的信号。

package locks;




import java.util.Random;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;




public class AppOfficial {

    /**
     * BoundedBuffer 是一个定长100的集合，当集合中没有元素时，take方法需要等待，直到有元素时才返回元素
     * 当其中的元素数达到最大值时，要等待直到元素被take之后才执行put的操作
     * @author yukaizhao
     *
     */
    static class BoundedBuffer {
        final Lock lock = new ReentrantLock();
        final Condition notFull = lock.newCondition();
        final Condition notEmpty = lock.newCondition();

        final Object[] items = new Object[100];
        int putptr, takeptr, count;

        public void put(Object x) throws InterruptedException {
            System .out.println("put wait lock");
            lock.lock();
            System.out.println("put get lock");
            try {
                while (count == items.length) {
                    System.out.println("buffer full, please wait");
                    notFull.await();
                }
                    
                items[putptr] = x;
                if (++putptr == items.length)
                    putptr = 0;
                ++count;
                notEmpty.signal();
            } finally {
                lock.unlock();
            }
        }




        public Object take() throws InterruptedException {
            System.out.println("take wait lock");
            lock.lock();
            System.out.println("take get lock");
            try {
                while (count == 0) {
                    System.out.println("no elements, please wait");
                    notEmpty.await();
                }
                Object x = items[takeptr];
                if (++takeptr == items.length)
                    takeptr = 0;
                --count;
                notFull.signal();
                return x;
            } finally {
                lock.unlock();
            }
        }
    }
    
    public static void main(String[] args) {
        final BoundedBuffer boundedBuffer = new BoundedBuffer();
        
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("t1 run");
                for (int i=0;i<1000;i++) {
                    try {
                        System.out.println("putting..");
                        boundedBuffer.put(Integer.valueOf(i));
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
            
        }) ;
        
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i=0;i<1000;i++) {
                    try {
                        Object val = boundedBuffer.take();
                        System.out.println(val);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
            
        }) ;
        
        t1.start();
        t2.start();
    }
}