zookeeper应用场景练习（分布式锁）

 在平常的高并发的程序中，为了保证数据的一致性，因此都会用到锁，来对当前的线程进行锁定。在单机操作中，很好做到，比如可以采用Synchronized、Lock或者其他的读写多来锁定当前的线程。但是在分布式的系统中，就很难做到这一点。因此可以采用zookeeper中节点的特性来满足这一点。大致实现的思路如下。

 1.每个客户端都去zookeeper上创建临时的顺序节点

 2.客户端判断当前自己创建的节点是不是最小的

 3.如果是的话，就获得了执行当前任务的锁

 4.如果不是的话，就找到比自己小的节点，然后进行监听，如果被删除的话，就可以获得锁

 上面就是大致的实现思路，下面我们来通过代码来实现一下。

package com.test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CountDownLatch;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.api.CuratorWatcher;
import org.apache.curator.framework.state.ConnectionState;
import org.apache.curator.framework.state.ConnectionStateListener;
import org.apache.curator.retry.RetryNTimes;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher.Event.EventType;
import org.apache.zookeeper.data.Stat;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class DistributedLock {

	private String lockName;
	private final int timeOut = 3000;
	private final String root = "/locks";
	private String myZnode;// 代表当前节点信息
	private String waitZnode;
	private static Logger logger = LoggerFactory
			.getLogger(DistributedLock.class);
	private CuratorFramework client;
	private CountDownLatch latch = new CountDownLatch(1);

	public DistributedLock(String connectString, String lockName) {
		this.lockName = lockName;
		client = CuratorFrameworkFactory.builder().connectionTimeoutMs(timeOut)
				.connectString(connectString)
				.retryPolicy(new RetryNTimes(3, 3000)).build();
		ConnectionStateListener listener = new ConnectionStateListener() {

			public void stateChanged(CuratorFramework client,
					ConnectionState newState) {
				if (newState == ConnectionState.CONNECTED) {
					logger.info("连接成功了");
					latch.countDown();
				}
			}
		};

		client.getConnectionStateListenable().addListener(listener);
		client.start();
		try {
			latch.await();
			createRoot();
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

	}

	/**
	 * @Title: 创建根节点root
	 * @Description: TODO
	 * @param
	 * @return void
	 * @throws
	 */
	private void createRoot() {
		try {
			Stat stat = client.checkExists().forPath(root);
			if (stat != null) {
				logger.info("root has already exists");
			} else {
				// 创建跟节点
				client.create().creatingParentsIfNeeded().forPath(root);

			}
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

	public void getLocks() {

		try {
			myZnode = client.create().withMode(CreateMode.EPHEMERAL_SEQUENTIAL)
					.forPath(root + "/" + lockName);
			logger.info(myZnode + "has created");
			// 取出所有的子节点，然后找出比自己小的节点，进行监听的设置
			List<String> subNodes = client.getChildren().forPath(root);
			// 取出所有带有lockname的节点信息
			List<String> lockObjNodes = new ArrayList<String>();
			for (String node : subNodes) {
				if (node.contains(lockName)) {
					lockObjNodes.add(node);
				}
			}
			// 对当前节点进行排序
			Collections.sort(lockObjNodes);
			// 判断当前的节点是不是最小的节点
			if (myZnode.equals(root + "/" + lockObjNodes.get(0))) {
				doAction();
			} else {
				// 找到比自己节点大一的节点进行监听
				String subMyZone = myZnode
						.substring(myZnode.lastIndexOf("/") + 1);
				waitZnode = lockObjNodes.get(Collections.binarySearch(
						lockObjNodes, subMyZone) - 1);
				// 对节点进行监听
				Stat stat = client.checkExists()
						.usingWatcher(deleteNodeWatcher).forPath("/"+waitZnode);
				if (stat != null) {
					System.out.println(Thread.currentThread().getName()
							+ "处于等待状态");
				} else {
					doAction();
				}
			}
		} catch (Exception e) {
			logger.error(e.getMessage());
		}
	}

	// 删除节点的事件监听
	CuratorWatcher deleteNodeWatcher = new CuratorWatcher() {

		public void process(WatchedEvent event) throws Exception {

			if (event.getType() == EventType.NodeDeleted) {
				doAction();
			}
		}
	};

	private void doAction() {
		System.out.println(Thread.currentThread().getName() + "开始执行");
		client.close();
	}
}

  下面来测试一下

/**     
 * @FileName: TestCurrentZk.java   
 * @Package:com.test   
 * @Description: TODO  
 * @author: LUCKY    
 * @date:2016年2月2日 下午11:36:04   
 * @version V1.0     
 */
package com.test;

/**
 * @ClassName: TestCurrentZk
 * @Description: TODO
 * @author: LUCKY
 * @date:2016年2月2日 下午11:36:04
 */
public class TestCurrentZk {

	public static void main(String[] args) throws Exception {
		Thread threads[] = new Thread[10];
		for (int i = 0; i < threads.length; i++) {
			threads[i] = new Thread(new Runnable() {
				public void run() {
					ClientTest clientTest = new ClientTest(
							"100.66.162.36:2181", "locknametest");
					clientTest.getLocks();
				}
			});

			threads[i].start();

		}
		Thread.sleep(Integer.MAX_VALUE);
	}
}