zookeeper学习系列：二、api实践

上一章我们知道zookeeper的简介，启动，设置节点以及结构性能。本小节我们来玩玩api，获取下数据。

读一下：http://zookeeper.apache.org/doc/trunk/javaExample.html

然后我说 what the fuck it is？

我就想读个数据，需要这么复杂么。。。

动手改一下

版本1:  只获取数据，不管别的：

import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.ZooKeeper;
import java.io.IOException;
public class ZkReader {
    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
        String hostPort = "192.168.1.2,192.168.1.3,192.168.1.4";
        String znode = "/test";
        ZooKeeper zk = new ZooKeeper(hostPort, 3000, null);
        System.out.println(new String(zk.getData(znode,false,null)));
    }
}

在zkcli上创建 /test 并改变它的值：123，运行，输出：

123

能得到结果，但是报错了：

14/10/17 11:51:58 ERROR zookeeper.ClientCnxn: Error while calling watcher
java.lang.NullPointerException
    at org.apache.zookeeper.ClientCnxn$EventThread.processEvent(ClientCnxn.java:521)
    at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:497)

看下源码，需要注册个watcher，意思是不这样zookeeper就只是个纯配置了？ok

版本2：zk get data＋watcher

import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import java.io.IOException;

public class ZkReader {
    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
        String hostPort = "10.16.73.22,10.16.73.12,10.16.73.13";
        String znode = "/test";
        ZooKeeper zk = new ZooKeeper(hostPort, 3000, new MyWatcher());
        System.out.println(new String(zk.getData(znode,false,null)));
    }
}

class MyWatcher  implements Watcher {

    @Override
    public void process(WatchedEvent event) {
        System.out.println("hello zookeeper");
        System.out.println(String.format("hello event! type=%s, stat=%s, path=%s",event.getType(),event.getState(),event.getPath()));
    }
}

输出却是：

hello zookeeper
123
hello event! type=None, stat=SyncConnected, path=null

data总是在中间？百撕不得姐，在邮件组里咨询下，等答案

这两个版本只是做到了获取数据，如果数据有变动，需要自动更新呢？ok，看第三个版本：