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(); } }