zookeeper可以对某个节点进行监听，监听的事件有以下4种：

• NodeCreated (节点创建)
• NodeDeleted (节点删除)
• NodeDataChanged (节点数据变化)
• NodeChildrenChanged (子节点变化，包括子节点删除、创建和数据变化)
  但每个事件只能触发一次，之后监听就不再生效
  本文介绍怎么实现zookeeper对节点的循环监听，即永久生效

1. 流程

客户端发起对节点的事务操作(以NodeChildrenChanged事件为例)
服务端监听到对应的事件后进行相应的操作

2. 代码实现

(1) Client.java

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.ZooKeeper;

public class Client {

    private static final String CONNECT_STRING = "hadoop01:2181,hadoop02:2181,hadoop03:2181,hadoop04:2181";
    private static final int SESSION_TIMEOUT = 5000;
    private static final String PARENT = "/name";
    private static final String CHILD = "tony";
    
    public static void main(String[] args) throws Exception {
        
        ZooKeeper zk = new ZooKeeper(CONNECT_STRING, SESSION_TIMEOUT, null);
        
        // 客户端创建了一个子节点，会触发NodeChildrenChanged事件
        String path = zk.create(PARENT + "/" + CHILD, CHILD.getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT_SEQUENTIAL);
        System.out.println(path);
        zk.close();
    }
    
}

(2) Server.java

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.ZooKeeper;

/**
 * @Description: 循环监听某节点
 * @author Jed
 * @date 2017年12月19日
 */
public class Server {

    private static ZooKeeper zk;
    private static final String CONNECT_STRING = "hadoop01:2181,hadoop02:2181,hadoop03:2181,hadoop04:2181";
    private static final int SESSION_TIMEOUT = 5000;
    private static final String PARENT = "/name";
    
    public static void main(String[] args) throws Exception {
        zk = new ZooKeeper(CONNECT_STRING, SESSION_TIMEOUT, new Watcher() {
            
            @Override
            public void process(WatchedEvent event) {
                String path = event.getPath();
                EventType type = event.getType();
                KeeperState state = event.getState();
                System.out.println(path + "\t" + type + "\t" + state);
                
                // 循环监听
                try {
                    zk.getChildren(PARENT, true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
        
        // 添加监听
        zk.getChildren(PARENT, true);
        
        // 模拟服务器一直运行
        Thread.sleep(Long.MAX_VALUE);
    }
}

3. 测试

1. 首先运行server.java

控制台输出：

null    None    SyncConnected

这是获取连接时的事件，每次获得连接都会触发

1. 然后运行一次Client.java

Client控制台输出：

/name/tony0000000000

因为Client创建的是PERSISTENT_SEQUENTIAL类型的节点，所以会自动递增的编号
Server控制台的内容实时更新：

null    None    SyncConnected
/name   NodeChildrenChanged SyncConnected

1. 再运行一次Client.java

Client控制台输出：

/name/tony0000000001

Server控制台的内容也会实时更新：

null    None    SyncConnected
/name   NodeChildrenChanged SyncConnected
/name   NodeChildrenChanged SyncConnected

至此，我们已经实现了对某个节点的循环监听！

说明：

1. 监听到对应的事件触发后，我们只是做了打印，具体的行为应该根据业务逻辑来设计
2. 监听某个节点的子节点变化(NodeChildrenChangeds)事件，首先该节点应该存在，本例中监听/name节点的子节点变化，那么/name节点应该提前创建好