zookeeper事件监听实战

一:事件监听

    原生zookeeper的事件监听采用Watcher实现,不过Watcher监听是一次性的,如果需要继续监听该事件,必须重新注册。Curator中采用cache来封装了对事件的监听,包括监听节点,监听子节点等,下面分别做介绍
1.1 NodeCache
    NodeCache主要用来监听节点本身的变化,当节点的状态发生变更后,回调NodeCachaListener

  public interface NodeCacheListener{
    /**
     * Called when a change has occurred
     */
     public void     nodeChanged() throws Exception;
   }

    看一个例子:

 public class NodeCacheExample {

    private static final String PATH = "/nodeCache";

    private static CountDownLatch latch = new CountDownLatch(1);
   
    static NodeCache nodeCache;

    static CuratorFramework client;

    static {
       
        client = CuratorFrameworkFactory.newClient(
                "host:2181", 5000, 5000, new ExponentialBackoffRetry(
                        1000, 3));
        client.start();
    }

    public static void initCache() throws Exception {
        client.create().forPath(PATH);
        client.setData().forPath(PATH, "节点的初始值".getBytes());
        nodeCache = new NodeCache(client, PATH);
        EnsurePath ensurePath = client.newNamespaceAwareEnsurePath(PATH);
        ensurePath.ensure(client.getZookeeperClient());
        //设置成true,那么nodeCache在第一次启动的时候就会到zookeeper上去获取节点的数据内容，并保存在cache中
        nodeCache.start(true);
        startCache(nodeCache);
    }

    private static void startCache(final NodeCache cache) throws Exception {
       
        ChildData data = cache.getCurrentData();
       
        System.out.println("第一次启动获取到的内容:"  + new String(data.getData()));
       
        cache.getListenable().addListener(new NodeCacheListener() {
            @Override
            public void nodeChanged() throws Exception {
                System.out.println("NodeCache changed,data is "
                        + new String(cache.getCurrentData().getData()));
                latch.countDown();
            }
        });
        Thread.sleep(2000);
        if(client.checkExists().forPath(PATH) != null){
            System.out.println("node is exist,准备给节点设置新的内容");
            client.setData().forPath(PATH, "节点新内容".getBytes());
        }
    }

    public static void main(String[] args) throws Exception {
        initCache();
        latch.await();
    }
} 

   运行结果如下:

 第一次启动获取到的内容:节点的初始值
 node is exist,准备给节点设置新的内容
 NodeCache changed,data is 节点新内容

  1.2 PathChildrenCache
    PathChildrenCache主要用来监听子节点,它有几个构造函数,参数最多的是下面这个

  public PathChildrenCache(CuratorFramework client, String path, boolean cacheData, boolean dataIsCompressed, final ExecutorService executorService)
    {
        this(client, path, cacheData, dataIsCompressed, new CloseableExecutorService(executorService));
    }

    参数解释:
    client：一个Curator客户端实例
    path：监听路径
    cacheData：boolean类型,如果是true,那么curator客户端在请求服务端的时候,会将监听节点的内容保存起来。而zookeeper原生的watcher监听是不会返回节点的内容的,只会返回节点状态,路径等
    dataIsCompressed：表示是否对数据进行压缩
    executorService:说明可以使用线程池来处理事件通知,当子节点数据发生变化时,会回调

  public interface PathChildrenCacheListener
{
    /**
     * Called when a change has occurred
     *
     * @param client the client
     * @param event describes the change
     * @throws Exception errors
     */
    public void     childEvent(CuratorFramework client, PathChildrenCacheEvent event) throws Exception;
}

    并且不会对二级节点进行监听,来看一个例子

import java.util.concurrent.CountDownLatch;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.cache.ChildData;
import org.apache.curator.framework.recipes.cache.PathChildrenCache;
import org.apache.curator.framework.recipes.cache.PathChildrenCacheEvent;
import org.apache.curator.framework.recipes.cache.PathChildrenCacheListener;
import org.apache.curator.retry.ExponentialBackoffRetry;

/**
 *
 * Path Cache用来监控一个ZNode的子节点. 当一个子节点增加， 更新，删除时， Path Cache会改变它的状态，
 *  会包含最新的子节点， 子节点的数据和状态
 *
 * @author tanjie
 *
 */
public class PathCacheExample {

    private static final String PATH = "/cache";

    static PathChildrenCache cache;

    static CuratorFramework client;
   
    static CountDownLatch latch = new CountDownLatch(1);
   
    static CountDownLatch coutCountDownLatch = new CountDownLatch(5);

    static {
        client = CuratorFrameworkFactory.newClient(
                "host:2181", 5000, 5000, new ExponentialBackoffRetry(
                        1000, 3));
        client.start();
    }


    private static void startCache()
            throws Exception {
        cache = new PathChildrenCache(client, PATH, true);
        cache.start();
        // 给当前节点创建子节点
        for (int i = 1; i <= 5; i++) {
            String newPath = PATH + "/child_" + i;
            String childNodeName = "child_" + i;
            if (client.checkExists().forPath(PATH) != null) {
                client.create().creatingParentsIfNeeded().forPath(newPath, childNodeName.getBytes());
            }
            coutCountDownLatch.countDown();
        }
        coutCountDownLatch.await();
        addlistener(cache);
        for (final ChildData childData : cache.getCurrentData()) {
            System.out.println("pathChildCache:" + "路径:" + childData.getPath()
                    + ",内容:" + new String(childData.getData()));
        }
        System.out.println("对父节点进行设置新值不会有通知事件发生........start");
        client.setData().forPath(PATH,"哈哈".getBytes());
        System.out.println("对父节点设置值 end..........");
        //改变子节点的数据
        System.out.println("准备删除子节点........start");
        client.delete().forPath( PATH + "/child_1");
        System.out.println("删除子节点........end,会有节点事件通知返回");
        Thread.sleep(2000);
        //改变二级节点的内容
        for(int j=1;j<=2;j++){
            String newPath = PATH + "/child_2/" + j;
            String childNodeName = "child_2" + j;
            if (client.checkExists().forPath(PATH) != null) {
                client.create().forPath(newPath, childNodeName.getBytes());
            }
        }
        addlistener(cache);
        System.out.println("准备删除二级节点......start");
        client.delete().forPath( PATH + "/child_2/" + 1);
        System.out.println("删除二级节点......end..不会有事件监听返回");
        latch.countDown();
       
    }

    private static void addlistener(final PathChildrenCache cache) {
        final PathChildrenCacheListener pathChildrenCacheListener = new PathChildrenCacheListener() {
            @Override
            public void childEvent(CuratorFramework curatorClient,
                    PathChildrenCacheEvent event) throws Exception {
                System.out.println("childNode path:"
                        + event.getData().getPath() + ",childNode data : "
                        + new String(event.getData().getData()));
            }
        };
        cache.getListenable().addListener(pathChildrenCacheListener);
    }

    public static void main(String[] args) throws Exception {
        startCache();
        latch.await();
    }
}

   运行结果如下:

pathChildCache:路径:/cache/child_1,内容:child_1
pathChildCache:路径:/cache/child_2,内容:child_2
pathChildCache:路径:/cache/child_3,内容:child_3
pathChildCache:路径:/cache/child_4,内容:child_4
pathChildCache:路径:/cache/child_5,内容:child_5
对父节点进行设置新值不会有通知事件发生........start
对父节点设置值 end..........
准备删除子节点........start
childNode path:/cache/child_1,childNode data : child_1
删除子节点........end,会有节点事件通知返回
准备删除二级节点......start
删除二级节点......end..不会有事件监听返回

1.3 TreeCache
    TreeCache即能监听节点也能监听节点的子节点变更

 public class TreeCacheExample {
   
    private static final String PATH = "/treeNodeCache";

    static TreeCache  treeCache;

    static CuratorFramework client;

    static {
        client = CuratorFrameworkFactory.newClient(
                "host:2181", 5000, 5000, new ExponentialBackoffRetry(
                        1000, 3));
        client.start();
    }
   
    public static void  initCache() throws Exception {
        treeCache = new TreeCache(client, PATH);
        client.create().forPath(PATH);
        treeCache.start();
        treeCache.getListenable().addListener(new TreeCacheListener() {
            @Override
            public void childEvent(CuratorFramework client, TreeCacheEvent event)
                    throws Exception {
                if(null != event.getType()){
                    System.out.println("节点状态发送了改变.内容为:" + new String(event.getData().getData()));
                }
            }
        });
       
        //添加子节点
        for(int i=1;i<=5;i++){
            String newPath = PATH + "/child_" + i;
            client.create().forPath(newPath,("child_"+i).getBytes());
        }
        Thread.sleep(3000);
        client.setData().forPath(PATH+ "/child_2", "我是给第二个子节点赋值".getBytes());
        if(null != client.checkExists().forPath(PATH)){
            client.setData().forPath(PATH,"我是来对父节点赋值的".getBytes());
        }
        Thread.sleep(1000);
        client.setData().forPath(PATH,"我是来对父节点第二次赋值的".getBytes());
    }
   
    public static void main(String[] args) throws Exception {
        initCache();
        Thread.sleep(Integer.MAX_VALUE);
    }

}

   运行结果如下:

节点状态发送了改变.内容为:child_1
节点状态发送了改变.内容为:child_2
节点状态发送了改变.内容为:child_3
节点状态发送了改变.内容为:child_4
节点状态发送了改变.内容为:child_5
节点状态发送了改变.内容为:我是给第二个子节点赋值
节点状态发送了改变.内容为:我是来对父节点赋值的
节点状态发送了改变.内容为:我是来对父节点第二次赋值的

    可以看到,只有监听的节点状态发送了变化,监听事件就会执行回调,并且也能监听器父节点且不用反复注册监听