通過zk實作分布式共享鎖原理

思路直接上圖

實作核心兩點：

1.zookeeper的WatchedEvent監聽事件

2.zookeeper儲存節點資料的兩個特點。有序和臨時

基本核心代碼（簡單粗暴，很多細節沒有考慮）：

public class DistributedClientLock {
	

	// 會話逾時
	private static final int SESSION_TIMEOUT = 2000;
	// zookeeper叢集位址
	private String hosts = "mini1:2181,mini2:2181,mini3:2181";
	private String groupNode = "locks";
	private String subNode = "sub";
	private boolean haveLock = false;

	private ZooKeeper zk;
	// 記錄自己建立的子節點路徑
	private volatile String thisPath;

	/**
	 * 連接配接zookeeper
	 */
	public void connectZookeeper() throws Exception {
		zk = new ZooKeeper(hosts, SESSION_TIMEOUT, new Watcher() {
		public void process(WatchedEvent event) {
		    try {

			// 判斷事件類型，此處隻處理子節點變化事件
			if (event.getType() == EventType.NodeChildrenChanged &&                    event.getPath().equals("/" + groupNode)) {
			//擷取子節點，并對父節點進行監聽
			List<String> childrenNodes = zk.getChildren("/" + groupNode, true);
			String thisNode = thisPath.substring(("/" + groupNode + "/").length());
			// 去比較是否自己是最小id
			Collections.sort(childrenNodes);
			if (childrenNodes.indexOf(thisNode) == 0) {
			//通路共享資源處理業務，并且在處理完成之後删除鎖
			doSomething();
							
			//重新注冊一把新的鎖
		    thisPath = zk.create("/" + groupNode + "/" + subNode, null, Ids.OPEN_ACL_UNSAFE,CreateMode.EPHEMERAL_SEQUENTIAL);
			}
		}
	} catch (Exception e) {
		e.printStackTrace();
	}
}});

		// 1、程式一進來就先注冊一把鎖到zk上
		thisPath = zk.create("/" + groupNode + "/" + subNode, null, Ids.OPEN_ACL_UNSAFE,
				CreateMode.EPHEMERAL_SEQUENTIAL);

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

		// 從zk的鎖父目錄下，擷取所有子節點，并且注冊對父節點的監聽
		List<String> childrenNodes = zk.getChildren("/" + groupNode, true);

		//如果争搶資源的程式就隻有自己，則可以直接去通路共享資源 
		if (childrenNodes.size() == 1) {
			doSomething();
			thisPath = zk.create("/" + groupNode + "/" + subNode, null, Ids.OPEN_ACL_UNSAFE,
					CreateMode.EPHEMERAL_SEQUENTIAL);
		}
	}

	/**
	 * 處理業務邏輯，并且在最後釋放鎖
	 */
	private void doSomething() throws Exception {
		try {
			System.out.println("gain lock: " + thisPath);
			Thread.sleep(2000);
			// do something
		} finally {
			System.out.println("finished: " + thisPath);
			//業務執行完後，删除目前節點，釋放鎖
			zk.delete(this.thisPath, -1);
		}
	}

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

	
}