【Zookeeper】Java客户端

API介绍

地址：http://zookeeper.apache.org/doc/r3.4.6/api/index.html

org.apache.zookeeper.Zookeeper是客户端入口主类，负责建立与server的会话，它提供了下面几类主要方法

功能	描述
create	在本地目录树中创建一个节点
delete	删除一个节点
exists	测试本地是否存在目标节点
get/setData	从目标节点上读取/写数据
get/setACL	获取/设置目标节点访问控制列表信息
getChildren	获取某一个节点下所有子节点的列表
sync	等待要被传送的数据

示例代码

创建会话

/**
 * 使用SessionId和SessionPasswd创建zookeeper会话
 * @author dj4817
 * @version $Id: ZookeeperWatcher.java, v 0.1 2018/3/12 13:44 dj4817 Exp $$
 */
public class ZooKeeperConstructorUsageWithSIDPASSWD implements Watcher {

    private static CountDownLatch connectedSemaphore = new CountDownLatch(1);

    /**
     * 测试使用SessionId和SessionPasswd创建zookeeper会话
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {

        ZooKeeper zookeeper = new ZooKeeper(Constants.connectString, Constants.sessionTimeout, new ZooKeeperConstructorUsageWithSIDPASSWD());
        connectedSemaphore.await();
        long sessionId = zookeeper.getSessionId();
        byte[] passwd = zookeeper.getSessionPasswd();
        System.out.println("sessionId=" + sessionId + "; passwd=" + passwd.toString());

        //Use illegal sessionId and sessionPassWd
        new ZooKeeper(Constants.connectString, Constants.sessionTimeout, new ZooKeeperConstructorUsageWithSIDPASSWD(), 1L, "test".getBytes());
        //Use correct sessionId and sessionPassWd
        new ZooKeeper(Constants.connectString, Constants.sessionTimeout, new ZooKeeperConstructorUsageWithSIDPASSWD(), sessionId, passwd);
        Thread.sleep(Integer.MAX_VALUE);
    }

    /**
     * 异步通知处理
     * @param event
     */
    @Override
    public void process(WatchedEvent event) {
        System.out.println("Receive watched event：" + event);
        if (KeeperState.SyncConnected == event.getState()) {
            connectedSemaphore.countDown();
        }
    }
}

运行结果：

Receive watched event：WatchedEvent state:SyncConnected type:None path:null
sessionId=27939408385212418; passwd=[B@759ebb3d
Receive watched event：WatchedEvent state:Expired type:None path:null
Receive watched event：WatchedEvent state:SyncConnected type:None path:null

1. 第一次使用错误的sessionId和sessionPasswd来创建Zookeeper客户端的实例，结果客户端接收到了服务端的Expired事件通知；
2. 第二次使用正确的sessionId和sessionPasswd，连接成功。

创建节点

1. Zookeeper不支持递归创建节点，即无法在父节点不存在的情况下创建一个子节点。
2. 如果一个节点已经存在了，那么创建同名节点的时候，会抛出NodeExistsException异常。

/**
 * 使用同步方式创建zookeeper节点
 * @author dj4817
 * @version $Id: ZookeeperWatcher.java, v 0.1 2018/3/12 13:44 dj4817 Exp $$
 */
public class ZooKeeperCreateApiSyncUsage implements Watcher {

    private static CountDownLatch connectedSemaphore = new CountDownLatch(1);

    /**
     * 测试使用同步方式创建zookeeper节点
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {

        ZooKeeper zookeeper = new ZooKeeper(Constants.connectString, Constants.sessionTimeout, new ZooKeeperCreateApiSyncUsage());
        connectedSemaphore.await();
        // Ids.OPEN_ACL_UNSAFE：表明之后这个节点的任何操作都不受权限控制。
        String path1 = zookeeper.create("/zk-test-ephemeral-", "".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
        System.out.println("Success create znode: " + path1);
        String path2 = zookeeper.create("/zk-test-ephemeral-", "".getBytes(), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
        System.out.println("Success create znode: " + path2);
    }

    /**
     * 异步通知处理
     * @param event
     */
    @Override
    public void process(WatchedEvent event) {
        if (KeeperState.SyncConnected == event.getState()) {
            connectedSemaphore.countDown();
        }
    }
}

运行结果

Success create znode: /zk-test-ephemeral-
Success create znode: /zk-test-ephemeral-0000000001

1. 如果创建一个临时节点，那么create方法返回当时传入的path参数。
2. 如果创建一个临时顺序节点，那么Zookeeper会自动在节点后缀加上一个数字，返回create方法返回这个节点的完整路径。