大數據教程（3.10）：zookeeper分佈式共享鎖的程序

    本節博主將爲你們提供一個分佈式共享鎖的事例，因爲分佈式共享鎖在不少場景下均須要使用，很是的重要。因此，你們必定要掌握這塊知識點。

    對於還不清楚分佈式共享鎖原理的請先看以前的文章大數據教程（3.3）：zookeeper簡介。

    分佈式共享鎖案例代碼：

package com.empire.zookeeper.test.em_zookeeper.zklock;

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

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

public class DistributedClientLock {
	

	// 會話超時
	private static final int SESSION_TIMEOUT = 2000;
	// zookeeper集羣地址
	private String hosts = "centos-aaron-07:2181,centos-aaron-08:2181,centos-aaron-16:2181";
	private String groupNode = "locks";
	private String subNode = "sub";
	private boolean haveLock = false;
	private CountDownLatch connectedSemaphore = new CountDownLatch(1); 
	private ZooKeeper zk;
	// 記錄本身建立的子節點路徑
	private volatile String thisPath;

	/**
	 * 鏈接zookeeper
	 */
	public void connectZookeeper() throws Exception {
		zk = new ZooKeeper(hosts, SESSION_TIMEOUT, new Watcher() {
			public void process(WatchedEvent event) {
				try {
					connectedSemaphore.countDown(); 
					// 判斷事件類型，此處只處理子節點變化事件
					if (event.getType() == EventType.NodeChildrenChanged && event.getPath().equals("/" + groupNode)) {
						//獲取子節點，並對父節點進行監聽
						List<String> childrenNodes = zk.getChildren("/" + groupNode, true);
						String thisNode = thisPath.substring(("/" + groupNode + "/").length());
						// 去比較是否本身是最小id
						Collections.sort(childrenNodes);
						if (childrenNodes.indexOf(thisNode) == 0) {
							//訪問共享資源處理業務，而且在處理完成以後刪除鎖
							doSomething();
							
							//從新註冊一把新的鎖
							thisPath = zk.create("/" + groupNode + "/" + subNode, null, Ids.OPEN_ACL_UNSAFE,
									CreateMode.EPHEMERAL_SEQUENTIAL);
						}
					}
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});
		connectedSemaphore.await();
		Stat stat = zk.exists("/"+groupNode, false);
		System.out.println(stat==null?"not exist":"exist");
		if(stat==null) {
			String parentNodeCreate = zk.create("/"+groupNode , null, Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
		}
		
		// 一、程序一進來就先註冊一把鎖到zk上
		thisPath = zk.create("/" + groupNode + "/" + subNode, null, Ids.OPEN_ACL_UNSAFE,
				CreateMode.EPHEMERAL_SEQUENTIAL);

		// wait一小會，便於觀察
		Thread.sleep(new Random().nextInt(1000));

		// 從zk的鎖父目錄下，獲取全部子節點，而且註冊對父節點的監聽
		List<String> childrenNodes = zk.getChildren("/" + groupNode, true);

		//若是爭搶資源的程序就只有本身，則能夠直接去訪問共享資源 
		if (childrenNodes.size() == 1) {
			doSomething();
			thisPath = zk.create("/" + groupNode + "/" + subNode, null, Ids.OPEN_ACL_UNSAFE,
					CreateMode.EPHEMERAL_SEQUENTIAL);
		}
	}

	/**
	 * 處理業務邏輯，而且在最後釋放鎖
	 */
	private void doSomething() throws Exception {
		try {
			System.out.println("gain lock: " + thisPath);
			Thread.sleep(2000);
			// do something
		} finally {
			System.out.println("finished: " + thisPath);
			//參數2：指定要刪除的版本，-1表示刪除全部版本
			zk.delete(this.thisPath, -1);
		}
	}

	public static void main(String[] args) throws Exception {
		DistributedClientLock dl = new DistributedClientLock();
		dl.connectZookeeper();
		Thread.sleep(Long.MAX_VALUE);
	}

	
}

 

    最後總結：分佈式共享鎖其實就是利用了zookeeper的短暫有序節點的特性，能夠用序列號的大小來決定當前隨能夠得到鎖從而訪問共享資源。

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