Java匿名内部类与回调函数

    之所以将匿名内部类和回调函数两个知识点一起写，是因为最近学习zookeeper的时候正好遇到这么一个例子。详细内容请参考：https://www.w3cschool.cn/zookeeper/zookeeper_api.html

以下是与ZooKeeper集合连接的完整代码。

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 {
	
      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();
   }
}

匿名内部类的创建格式如下：

new 父类构造器（参数列表）|实现接口（）  
    {  
     //匿名内部类的类体部分  
    }

在上面的代码中，connect方法中在实例化ZooKeeper对象时用到了匿名内部类：

zoo = new ZooKeeper(host,5000,new Watcher() {
		
         public void process(WatchedEvent we) {

            if (we.getState() == KeeperState.SyncConnected) {
               connectedSignal.countDown();
            }
         }
      });

这个内部类没有自己的名字，而是用到了Watcher接口，而通常情况下接口是不能用new的，但是在匿名内部类中可以这样。匿名内部类的类体是一个名为process的方法，这个方法就是用来实现Watcher接口中定义的process抽象方法的。

在这个匿名内部类中恰好又运用了回调函数（又叫回调方法）。

public class XyzWatcher implements Watcher {
    @Override
    public void process(WatchedEvent watchedEvent) {
        final CountDownLatch connectedSignal = new CountDownLatch(1);
        if (watchedEvent.getState() == Event.KeeperState.SyncConnected) {
            connectedSignal.countDown();
        }
        try {
            connectedSignal.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class ZooKeeperConnection {

    // declare zookeeper instance to access ZooKeeper ensemble
    private ZooKeeper zoo;
    //public final CountDownLatch connectedSignal = new CountDownLatch(1);

    // Method to connect zookeeper ensemble.
    XyzWatcher xyz = new XyzWatcher();

    public ZooKeeper connect(String host) throws IOException,InterruptedException {
        zoo = new ZooKeeper(host,5000,xyz);
        return zoo;
    }

    // Method to disconnect from zookeeper server
    public void close() throws InterruptedException {
        zoo.close();
    }
}