public class ClientZkAgent {//单例模式private static final ClientZkAgent instance = new ClientZkAgent();private ZooKeeper zk; //zk客户端private ClientZkAgent() {connect(); //初始化并连接zk}public static ClientZkAgent getInstance() {return instance;}/*** zk常用模式: 由于zookeeper的连接是异步的，为防止zk对象在建立有效连接之前就返回，* 我们阻塞主线程，并通过zookeeper的EventThread在连接事件中唤醒主线程*/private void connect() {CountDownLatch semaphore = new CountDownLatch(1);zk = new ZooKeeper(zkHost, timeout, watchEvent -> { // #_1switch (e.getState()) {case SyncConnected:semaphore.countDown();break;// 其它逻辑 ....}});semaphore.await(10000, TimeUnit.MILLISECONDS);}
}

上面的代码造成第一次调用ClientZkAgent.getInstance的时候，需耗时10s， 这个时间恰好跟semaphore的超时时间相当. 在此期间，整个世界好像停滞了一样。

"main-EventThread" #13 daemon prio=5 os_prio=0 tid=0x000000001fe36800 nid=0xf0c in Object.wait() [0x000000002032f000]java.lang.Thread.State: RUNNABLEat com.github.dapeng.registry.zookeeper.ClientZkAgent.lambda$connect$0(ClientZkAgent.java:154)at com.github.dapeng.registry.zookeeper.ClientZkAgent$$Lambda$1/116211441.process(Unknown Source)at org.apache.zookeeper.ClientCnxn$EventThread.processEvent(ClientCnxn.java:533)at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:508)Locked ownable synchronizers:- None

final class ClientZkAgent$$Lambda$1 implements Watcher {final ClientZkAgent clientZkAgent;public void process(WatchedEvent event) {clientZkAgent.lambda$connect$0(event);}
}

1. 通过静态方法ClientZkAgent.getInstance()获取实例，第一次访问的时候会触发类ClientZkAgent的装载。

2. 装载过程中，装载静态成员instance，这时候会尝试创建一个ClientZkAgent对象。

3. 在ClientZkAgent的构造函数中连接zk，并通过CountdownLatch进入阻塞状态。注意这时候类装载还没完成。

4. CountdownLatch超时后完成对象的初始化以及整个类的加载

zk事件线程:

public class ClientZkAgent {//单例模式private static final ClientZkAgent instance = new ClientZkAgent();private ZooKeeper zk; //zk客户端private ClientZkAgent() {}public static ClientZkAgent getInstance() {if (instance.zk == null) {synchronized(ClientZkAgent.class) {if (instance.zk == null) {instance.connect();}}}return instance;}

正文结束