java读写锁实现机制

package thread.lock;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
 * @file_name ReadAndWriteLockDemo.java
 * @description 读写锁DEMO
 * @author 张明亮
 * @date 2009.09.27
 */
public class ReadAndWriteLockDemo {
 ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
 // 读锁
 Lock rLock = rwLock.readLock();
 // 写锁
 Lock wLock = rwLock.writeLock();
 // 货架
 List<String> stock = new ArrayList<String>();
    //互斥锁
 ReentrantLock lock = new ReentrantLock();
 /**
  * 商品买卖
  * <br>
  * 当商品数量小于1时，释放读锁并获取写锁 生产商品后，
  * <br>
  * 锁降级在写锁保护下重新获取读锁并消费商品
  */
 public void sell() {
//  lock.lock();
  try {
   rLock.lock();
   if (stock.size() <= 0) {
    rLock.unlock();
    wLock.lock();
    manufacture();
    rLock.lock();
    wLock.unlock();
   }
   consume();
   rLock.unlock();
  } finally {
//   lock.unlock();
  }
 }
 /**
  * 生产商品
  */
 private void manufacture() {
  stock.add("product");
 }
 /**
  * 消费商品
  */
 private void consume() {
  System.err.println(stock.remove(0));
 }
 public static void main(String[] args) {
  final ReadAndWriteLockDemo rwDemo = new ReadAndWriteLockDemo();
  rwDemo.sell();
//  Thread t1 = new Thread("threa--1") {
//   public void run() {
//    rwDemo.sell(