hadoop源码学习(二）之ZooKeeper

　　要能够熟练使用hadoop，就得对其原理和源码有些了解。hadoop中比较重要的概念是NameNode,DataNode,去看这些类时，又会发现其使用了ZooKeeper包，这样就可以将hadoop的分解为若干个部分，理解起来就会稍微简单些了。

　　可以理解zookeeper封装了一个数据树。

　　zookeeper既提供了一个程序可以跑起来，也提供了一个java类库，用户可以调用，使用zookeeper，对于java程序员，这都是非常友好的。

// import java classes
import java.io.IOException;
import java.util.concurrent.CountDownLatch;
 
// import zookeeper classes
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.Watcher.Event.KeeperState;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.AsyncCallback.StatCallback;
import org.apache.zookeeper.KeeperException.Code;
import org.apache.zookeeper.data.Stat;
 
public class ZooKeeperConnection {
 
   // declare zookeeper instance to access ZooKeeper ensemble
   private ZooKeeper zoo;
   final CountDownLatch connectedSignal = new CountDownLatch(1);
 
   // Method to connect zookeeper ensemble.
   public ZooKeeper connect(String host) throws IOException,InterruptedException {
    　　　　// ZooKeeper中Watcher和WatchedEvent的作用。　　// 可以看到这是异步获取zookeeper，需要WatchedEvent触发
      zoo = new ZooKeeper(host,5000,new Watcher() {
         
         public void process(WatchedEvent we) {
 
            if (we.getState() == KeeperState.SyncConnected) {
               connectedSignal.countDown();
            }
         }
      });
         
      connectedSignal.await();
      return zoo;
   }
 
   // Method to disconnect from zookeeper server
   public void close() throws InterruptedException {
      zoo.close();
   }
}