基于zookeeper的分布式锁实现（监听临时节点的上一个节点）

    1.父节点：/disLocks1（zk根目录下的disLocks1目录，CreateMode.PERSISTENT类型）

      2.全部须要获取锁的线程，都会在/disLocks1目录下创建一个临时顺序的子节点（CreateMode.EPHEMERAL_SEQUENTIAL类型）

           3.每次都是序号最小的节点获取锁，当最小的节点业务逻辑处理完毕后，断开本次链接（或者删除当前子节点），则临时顺序的节点自动删除，接着让其余没有获取锁的节点去获取锁

          每当一台机器去建立了一个临时顺序节点，去获取锁失败的时候（本节点不是序号最小的节点），则找出当前节点的前一个节点，并在该节点上注册一个监听事件，当前一个节点删除(获取锁成功)，则后一个节点当即去获取锁。

         相比于监听父节点机制的优点：不用全部的子节点都去监听父节点，讲子节点的监听事件均匀的分布在了对应的兄弟子节点上，这样在高并发的状况下，避免了当子节点删除的时候，父节点的监听事件过多，父节点每次须要触发大量的监听事件。

  贴代码：

 

package zoo.com.zk.zoo.lock;

import java.io.IOException;
import java.util.Collections;
import java.util.List;
import java.util.Objects;
import java.util.concurrent.CountDownLatch;

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.Watcher.Event.EventType;
import org.apache.zookeeper.Watcher.Event.KeeperState;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat;


/**
 * 
 * zk分布式锁实现
 * 基于监听父节点下面的所有子节点实现，效率较低
 * */
public class DistributedLock2 implements Watcher{
	
	public static String host="127.0.0.1:2181";
    //缓存时间  
    private static final int TIME_OUT  = 2000; 
    
    private static String FATHER_PATH = "/disLocks1";
    
    private ZooKeeper zk;
    
    private int threadId;
    
    protected  CountDownLatch countDownLatch=new CountDownLatch(1); 
    
    private String lockPath;
    
    public DistributedLock2(int threadId){
    	this.threadId = threadId;
    }
    
    public void getZkClient(String host,int timeout)
	{
    	try {
    		if(null == zk){
        		zk = new ZooKeeper(host, timeout, this);
    		}
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	/**
	 * 
	 * 
	 * */
    public String createNode(){
    	try {
    		//检测节点是否存在
			Stat stat = zk.exists(FATHER_PATH, false);
			//父节点不存在，则建立父节点
			if(Objects.isNull(stat)){
				synchronized (FATHER_PATH) {
					Stat stat2 = zk.exists(FATHER_PATH, false);
					if(Objects.isNull(stat2)){
						//父节点是持久节点
						String path = zk.create(FATHER_PATH, null, Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
						System.out.println("父节点建立成功，返回值【"+path+"】");
					}
				}
			}
			//建立持久性父节点下面的临时顺序子节点,/父节点路径/0000000002
	        String lockPath = zk.create(FATHER_PATH+"/",null, Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);  
        	System.out.println("线程【"+threadId+"】开始执行,子节点建立成功，返回值【"+lockPath+"】");
        	return lockPath;
		} catch (KeeperException e1) {
			e1.printStackTrace();
		} catch (InterruptedException e1) {
			e1.printStackTrace();
		}
		return null;
    }
    
    //校验当前节点是否为序号最小的节点
    public boolean checkLockPath(String lockPath){
    	try {
    		//注册父节点监听事件,当父节点下面的子节点有变化，就会触发Watcher事件
			List<String> nodeList = zk.getChildren(FATHER_PATH, false);
			Collections.sort(nodeList);
			int index = nodeList.indexOf( lockPath.substring(FATHER_PATH.length()+1));  
		     switch (index){  
		         case -1:{  
		        	 System.out.println("本节点已不在了"+lockPath);
		             return false;  
		         }  
		         case 0:{  
		        	 System.out.println("线程【"+threadId+"】获取锁成功，子节点序号【"+lockPath+"】");
		             return true;  
		         }  
		         default:{  
		             String waitPath = nodeList.get(index - 1);  
		             zk.exists(FATHER_PATH+"/"+waitPath, this);
		             System.out.println(waitPath+"在"+nodeList.get(index)+"点前面,须要等待【"+nodeList.get(index)+"】");
		             return false;
		         } 
		     }
		} catch (KeeperException e) {
			e.printStackTrace();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		return false;
    }
    
    public boolean getLock(){
    	//建立获取锁的节点（顺序临时节点）
    	String childPath = createNode();
    	boolean flag = true;
    	if(null != childPath){
    		lockPath = childPath;
			try {
				//轮询等待zk获取锁的通知
				while(flag){
					if(checkLockPath(childPath)){
						//获取锁成功
						return true;
					}else{
						//节点建立成功， 则等待zk通知
						countDownLatch.await();
					}
				}
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
    	}else{
    		System.out.println("节点没有建立成功，获取锁失败");
    	}
    	return false;
    }
    
	public void process(WatchedEvent event) {
		//成功链接zk，状态判断
		if(event.getState() == KeeperState.SyncConnected){
			//子节点有变化
			if(event.getType() == EventType.NodeDeleted){
				System.out.println("临时节点自动删除");
				countDownLatch.countDown();
			}
		}		
	}
	
	public void unlock(){
		try {
			zk.delete(getLockPath(), -1);
			zk.close();
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (KeeperException e) {
			e.printStackTrace();
		}
	}
	
	public String getLockPath() {
		return lockPath;
	}
	public ZooKeeper getZooKeeper(){
		return zk;
	}
	public static void main(String[] args) throws KeeperException, InterruptedException {
		 for(int i=0; i < 10; i++){  
            final int threadId = i+1;  
            new Thread(){  
                @Override  
                public void run() {  
                    try{  
                        DistributedLock2 dis = new DistributedLock2(threadId);  
                        dis.getZkClient(host,TIME_OUT);
                        if(dis.getLock()){
                        	Thread.sleep(200);
                        	dis.unlock();
                        }
                    } catch (Exception e){  
                    	System.out.println("【第"+threadId+"个线程】 抛出的异常：");
                        e.printStackTrace();  
                    }  
                }  
            }.start();  
	     }
	}
}