Zookeeper Watcher详解

Watcher是事件监听器，用户可以在一些节点上创建事件监听，监听的过程叫做watch.
下面用图展示监听的状态和监听的类型分类。

监听的状态分为连接失败、连接成功、认证失败和过期。分别对应图片KeeperState的四种状态。
监听的类型包括：
EventType.NodeCreated : 节点创建事件类型
EventType.NodeDeleted : 节点被删除
EventType.NodeDataChanged : 节点被修改
EventType.None : 客户端与服务器成功建立会话
EventType.NodeChildrenChanged : 子节点列表发生变更
下面用一个例子具体说明每个watch过程的实现。代码借鉴的是尚学堂老师讲课的代码，不过自己有亲自测试过，上传的代码均可测试通过。

需要先在pom.xml中添加zookeeper,我部署zk集群用的是3.4.12版本。

  
            org.apache.zookeeper
            zookeeper
            3.4.12
        

代码测试：

public class ZookeeperWatch implements Watcher {

    /** 定义原子变量
     * 线程安全，使用于高并发
     * */
    AtomicInteger seq = new AtomicInteger();
    /** 定义session失效时间 */
    private static final int SESSION_TIMEOUT = 10000;
    /** zookeeper服务器地址 */
    private static final String CONNECTION_ADDR = "192.168.145.134:2181,192.168.145.135:2181,192.168.145.136:2181";
    /** zk父路径设置 */
    private static final String PARENT_PATH = "/m";
    /** zk子路径设置 */
    private static final String CHILDREN_PATH = "/m/c";
    /** 进入标识 */
    private static final String LOG_PREFIX_OF_MAIN = "【Main】";
    /** zk变量 */
    private ZooKeeper zk = null;
    /**用于等待zookeeper连接建立之后 通知阻塞程序继续向下执行 */
    private CountDownLatch connectedSemaphore = new CountDownLatch(1);
     //创建zk的连接


    /**
     * 创建ZK连接
     * @param connectAddr ZK服务器地址列表
     * @param sessionTimeout Session超时时间
     */
    public void createConnection(String connectAddr, int sessionTimeout) {
        this.releaseConnection();
        try {
            //this表示把当前对象进行传递到其中去（也就是在主函数里实例化的new ZooKeeperWatcher()实例对象）
            zk = new ZooKeeper(connectAddr, sessionTimeout, this);
            System.out.println(LOG_PREFIX_OF_MAIN + "开始连接ZK服务器");
            connectedSemaphore.await();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * 关闭ZK连接
     */
    public void releaseConnection() {
        if (this.zk != null) {
            try {
                this.zk.close();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 创建节点
     * @param path 节点路径
     * @param data 数据内容
     * @return
     */
    public boolean createPath(String path, String data, boolean needWatch) {
        try {
            //设置监控(由于zookeeper的监控都是一次性的所以 每次必须设置监控,若needWatch为false,则不会走process)
            this.zk.exists(path, needWatch);
            System.out.println(LOG_PREFIX_OF_MAIN + "节点创建成功, Path: " +
                    this.zk.create(	/**路径*/
                            path,
                            /**数据*/
                            data.getBytes(),
                            /**所有可见*/
                            ZooDefs.Ids.OPEN_ACL_UNSAFE,
                            /**永久存储*/
                            CreateMode.PERSISTENT ) +
                    ", content: " + data);
        } catch (Exception e) {
            e.printStackTrace();
            return false;
        }
        return true;
    }

    /**
     * 读取指定节点数据内容
     * @param path 节点路径
     * @return
     */
    public String readData(String path, boolean needWatch) {
        try {
            System.out.println("读取数据操作...");
            return new String(this.zk.getData(path, needWatch, null));
        } catch (Exception e) {
            e.printStackTrace();
            return "";
        }
    }

    /**
     * 更新指定节点数据内容
     * @param path 节点路径
     * @param data 数据内容
     * @return
     */
    public boolean writeData(String path, String data) {
        try {
            this.zk.exists(path, true);
            System.out.println(LOG_PREFIX_OF_MAIN + "更新数据成功，path：" + path + ", stat: " +
                    this.zk.setData(path, data.getBytes(), -1));
        } catch (Exception e) {
            e.printStackTrace();
            return false;
        }
        return true;
    }

    /**
     * 删除指定节点
     *
     * @param path
     *            节点path
     */
    public void deleteNode(String path) {
        try {
            this.zk.delete(path, -1);
            System.out.println(LOG_PREFIX_OF_MAIN + "删除节点成功，path：" + path);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * 判断指定节点是否存在
     * @param path 节点路径
     */
    public Stat exists(String path, boolean needWatch) {
        try {
            return this.zk.exists(path, needWatch);
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }

    /**
     * 获取子节点
     * @param path 节点路径
     */
    private List getChildren(String path, boolean needWatch) {
        try {
            System.out.println("读取子节点操作...");
            return this.zk.getChildren(path, needWatch);
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }

    /**
     * 删除所有节点
     */
    public void deleteAllTestPath(boolean needWatch) {
        if(this.exists(CHILDREN_PATH, needWatch) != null){
            this.deleteNode(CHILDREN_PATH);
        }
        if(this.exists(PARENT_PATH, needWatch) != null){
            this.deleteNode(PARENT_PATH);
        }
    }

    /**
     * 收到来自Server的Watcher通知后的处理。
     */
    @Override
    public void process(WatchedEvent event) {

        System.out.println("进入 process 。。。。。event = " + event);

        try {
            Thread.sleep(200);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        if (event == null) {
            return;
        }

        // 连接状态
        KeeperState keeperState = event.getState();
        // 事件类型
        EventType eventType = event.getType();
        // 受影响的path
        String path = event.getPath();
        //原子对象seq 记录进入process的次数
        String logPrefix = "【Watcher-" + this.seq.incrementAndGet() + "】";

        System.out.println(logPrefix + "收到Watcher通知");
        System.out.println(logPrefix + "连接状态:\t" + keeperState.toString());
        System.out.println(logPrefix + "事件类型:\t" + eventType.toString());

        if (KeeperState.SyncConnected == keeperState) {
            // 成功连接上ZK服务器
            if (EventType.None == eventType) {
                System.out.println(logPrefix + "成功连接上ZK服务器");
                connectedSemaphore.countDown();
            }
            //创建节点
            else if (EventType.NodeCreated == eventType) {
                System.out.println(logPrefix + "节点创建");
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //更新节点
            else if (EventType.NodeDataChanged == eventType) {
                System.out.println(logPrefix + "节点数据更新");
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //更新子节点
            else if (EventType.NodeChildrenChanged == eventType) {
                System.out.println(logPrefix + "子节点变更");
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //删除节点
            else if (EventType.NodeDeleted == eventType) {
                System.out.println(logPrefix + "节点 " + path + " 被删除");
            }
            else ;
        }
        else if (KeeperState.Disconnected == keeperState) {
            System.out.println(logPrefix + "与ZK服务器断开连接");
        }
        else if (KeeperState.AuthFailed == keeperState) {
            System.out.println(logPrefix + "权限检查失败");
        }
        else if (KeeperState.Expired == keeperState) {
            System.out.println(logPrefix + "会话失效");
        }
        else ;

        System.out.println("--------------------------------------------");

    }

    /**
     * 方法名称：测试zookeeper监控

     * 概要说明：主要测试watch功能

     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {

        //建立watcher //当前客户端可以称为一个watcher 观察者角色
        ZookeeperWatch zkWatch = new ZookeeperWatch();
        //创建连接
        zkWatch.createConnection(CONNECTION_ADDR, SESSION_TIMEOUT);
        //System.out.println(zkWatch.zk.toString());

        Thread.sleep(1000);

        // 清理节点
     //   zkWatch.deleteAllTestPath(false);

        //-----------------第一步: 创建父节点 /p ------------------------//
        if (zkWatch.createPath(PARENT_PATH, System.currentTimeMillis() + "", false)) {

            Thread.sleep(1000);
            zkWatch.writeData(PARENT_PATH, System.currentTimeMillis() + "");
            zkWatch.createPath(CHILDREN_PATH, System.currentTimeMillis() + "", true);
            zkWatch.getChildren(PARENT_PATH, true);
          /*  //-----------------第二步: 读取节点 /p 和    读取/p节点下的子节点(getChildren)的区别 --------------//
            // 读取数据
            zkWatch.readData(PARENT_PATH, true);

            // 读取子节点(监控childNodeChange事件)


            // 更新数据


            Thread.sleep(1000);
            // 创建子节点
            zkWatch.createPath(CHILDREN_PATH, System.currentTimeMillis() + "", true);


            //-----------------第三步: 建立子节点的触发 --------------//
//			zkWatch.createPath(CHILDREN_PATH + "/c1", System.currentTimeMillis() + "", true);
//			zkWatch.createPath(CHILDREN_PATH + "/c1/c2", System.currentTimeMillis() + "", true);

            //-----------------第四步: 更新子节点数据的触发 --------------//
            //在进行修改之前，我们需要watch一下这个节点：
            Thread.sleep(1000);
            zkWatch.readData(CHILDREN_PATH, true);
            zkWatch.writeData(CHILDREN_PATH, System.currentTimeMillis() + "");*/

        }

      //  Thread.sleep(10000);
        // 清理节点
       // zkWatch.deleteAllTestPath(false);


        Thread.sleep(10000);
        System.out.println("===========关闭zk");
        zkWatch.releaseConnection();

    }

}