如何利用Zookeeper實現分布式鎖

ZooKeeper是一個分布式協調服務，它可以用來實現分布式鎖。以下是使用ZooKeeper實現分布式鎖的基本步驟：

1. 創建ZooKeeper客戶端

首先，你需要創建一個ZooKeeper客戶端連接到ZooKeeper集群。

import org.apache.zookeeper.ZooKeeper;

public class ZooKeeperClient {
    private static final String ZK_ADDRESS = "localhost:2181";
    private static final int SESSION_TIMEOUT = 3000;
    private ZooKeeper zk;

    public ZooKeeperClient() throws IOException {
        zk = new ZooKeeper(ZK_ADDRESS, SESSION_TIMEOUT, event -> {
            // 處理連接事件
        });
    }

    public ZooKeeper getZk() {
        return zk;
    }
}

2. 創建鎖節點

在ZooKeeper中創建一個臨時順序節點來表示鎖。

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.data.Stat;

public class DistributedLock {
    private static final String LOCK_ROOT = "/locks";
    private static final String LOCK_NODE = LOCK_ROOT + "/lock_";
    private ZooKeeper zk;
    private String lockPath;

    public DistributedLock(ZooKeeper zk) {
        this.zk = zk;
        try {
            // 創建鎖的根節點
            Stat stat = zk.exists(LOCK_ROOT, false);
            if (stat == null) {
                zk.create(LOCK_ROOT, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void lock() throws Exception {
        lockPath = zk.create(LOCK_NODE, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
        while (true) {
            List<String> children = zk.getChildren(LOCK_ROOT, false);
            Collections.sort(children);
            if (lockPath.endsWith(children.get(0))) {
                // 獲取到鎖
                return;
            } else {
                // 監聽前一個節點的刪除事件
                String previousNode = getPreviousNode(children, lockPath);
                Stat stat = zk.exists(LOCK_ROOT + "/" + previousNode, event -> {
                    if (event.getType() == Watcher.Event.EventType.NodeDeleted) {
                        synchronized (this) {
                            notifyAll();
                        }
                    }
                });
                if (stat != null) {
                    wait();
                }
            }
        }
    }

    private String getPreviousNode(List<String> children, String currentNode) {
        int index = children.indexOf(currentNode.substring(LOCK_ROOT.length() + 1));
        return children.get(index - 1);
    }

    public void unlock() throws Exception {
        if (lockPath != null) {
            zk.delete(lockPath, -1);
            lockPath = null;
        }
    }
}

3. 使用鎖

在你的分布式應用中使用這個鎖來保證操作的原子性。

public class DistributedApplication {
    public static void main(String[] args) {
        try {
            ZooKeeperClient zkClient = new ZooKeeperClient();
            DistributedLock lock = new DistributedLock(zkClient.getZk());

            lock.lock();
            // 執行需要加鎖的操作
            System.out.println("Lock acquired, performing operation...");

            // 模擬操作時間
            Thread.sleep(5000);

            lock.unlock();
            System.out.println("Lock released.");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

注意事項

1. 異常處理：在實際應用中，需要仔細處理各種異常情況，比如連接丟失、節點刪除等。
2. 性能考慮：ZooKeeper的寫操作是相對較慢的，因此在高并發場景下需要謹慎使用。
3. 會話管理：確保ZooKeeper客戶端會話的有效性，避免因為會話過期導致鎖失效。

通過以上步驟，你可以利用ZooKeeper實現一個基本的分布式鎖。根據具體需求，你可能還需要進一步優化和擴展這個實現。