使用ZooKeeper在Java中实现高效分布式锁的机制

在Java中使用ZooKeeper实现分布式锁是一种常见且高效的方式，因为ZooKeeper天然就是一个高可用的分布式协调服务，它提供了节点监控、数据发布/订阅等功能，非常适合用于实现分布式锁。

下面是一个基本的步骤和示例代码，说明如何使用ZooKeeper在Java中实现一个分布式锁：

步骤 1: 引入ZooKeeper依赖

首先，你需要在你的Java项目中引入ZooKeeper的客户端库。如果你使用Maven，可以添加以下依赖到你的pom.xml：

<dependency>  
    <groupId>org.apache.zookeeper</groupId>  
    <artifactId>zookeeper</artifactId>  
    <version>3.7.0</version>  
</dependency>

步骤 2: 创建ZooKeeper客户端

在你的Java应用中，你需要创建一个ZooKeeper客户端连接到ZooKeeper集群：

import org.apache.zookeeper.ZooKeeper;  
import java.util.concurrent.CountDownLatch;  
  
public class ZooKeeperClient {  
    private ZooKeeper zk;  
    private CountDownLatch connectedSignal = new CountDownLatch(1);  
  
    public ZooKeeperClient(String connectString, int sessionTimeout) throws Exception {  
        zk = new ZooKeeper(connectString, sessionTimeout, event -> {  
            if (event.getState() == Event.KeeperState.SyncConnected) {  
                connectedSignal.countDown();  
            }  
        });  
        connectedSignal.await(); // 等待连接建立  
    }  
  
    public ZooKeeper getZooKeeper() {  
        return zk;  
    }  
  
    public void close() throws InterruptedException {  
        zk.close();  
    }  
}

步骤 3: 实现分布式锁

接下来，你需要使用ZooKeeper的临时顺序节点来实现锁的逻辑：

import org.apache.zookeeper.CreateMode;  
import org.apache.zookeeper.ZooDefs;  
import org.apache.zookeeper.data.Stat;  
  
public class DistributedLock {  
    private ZooKeeperClient zkClient;  
    private String lockPath = "/locks";  
    private String lockName;  
  
    public DistributedLock(ZooKeeperClient zkClient, String lockName) {  
        this.zkClient = zkClient;  
        this.lockName = lockName;  
    }  
  
    public void lock() throws Exception {  
        String path = zkClient.getZooKeeper().create(lockPath + "/", lockName.getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE,  
                CreateMode.EPHEMERAL_SEQUENTIAL);  
          
        // 找到比自己序号小的最大的节点  
        Stat stat = zkClient.getZooKeeper().exists(lockPath + "/", true);  
        List<String> children = zkClient.getZooKeeper().getChildren(lockPath, true);  
        Collections.sort(children);  
        int index = children.indexOf(path.substring(lockPath.length() + 1));  
  
        String watchPath = index == 0 ? lockPath : lockPath + "/" + children.get(index - 1);  
  
        // 监听前一个节点  
        Stat watchStat = zkClient.getZooKeeper().exists(watchPath, event -> {  
            if (event.getType() == EventType.NodeDeleted) {  
                try {  
                    lock();  
                } catch (Exception e) {  
                    e.printStackTrace();  
                }  
            }  
        });  
  
        // 等待获取锁  
        while (true) {  
            if (zkClient.getZooKeeper().exists(watchPath, false) == null) {  
                // 尝试获取锁  
                zkClient.getZooKeeper().setData(path, lockName.getBytes(), -1);  
                return;  
            }  
            Thread.sleep(100);  
        }  
    }  
  
    public void unlock() throws Exception {  
        zkClient.getZooKeeper().delete(lockPath + "/" + lockName, -1);  
    }  
}