一、前言
前面已经分析了Zookeeper持久话相关的类，下面接着分析Zookeeper中的Watcher机制所涉及到的类。
二、总体框图
对于Watcher机制而言，主要涉及的类主要如下。
说明：
Watcher |
接口类型，其定义了process方法，需子类实现 |
Event |
接口类型，Watcher的内部类，无任何方法 |
KeeperState |
枚举类型，Event的内部类，表示Zookeeper所处的状态 |
EventType |
枚举类型，Event的内部类，表示Zookeeper中发生的事件类型 |
WatchedEvent |
表示对ZooKeeper上发生变化后的反馈，包含了KeeperState和EventType |
ClientWatchManager |
接口类型，表示客户端的Watcher管理者，其定义了materialized方法，需子类实现 |
ZKWatchManager |
Zookeeper的内部类，继承ClientWatchManager |
MyWatcher |
ZooKeeperMain的内部类，继承Watcher |
ServerCnxn |
接口类型，继承Watcher，表示客户端与服务端的一个连接 |
WatchManager |
管理Watcher |
三、Watcher源码分析
3.1 内部类
Event，接口类型，表示事件代表的状态，除去其内部类，其源码结构如下
public interface Watcher { public interface Event { /** * Enumeration of states the ZooKeeper may be at the event */ public enum KeeperState { @Deprecated Unknown (-1), Disconnected (0), @Deprecated NoSyncConnected (1), SyncConnected (3), AuthFailed (4), ConnectedReadOnly (5), SaslAuthenticated(6), Expired (-112); private final int intValue; KeeperState(int intValue) { this.intValue = intValue; } public int getIntValue() { return intValue; } public static KeeperState fromInt(int intValue) { switch(intValue) { case -1: return KeeperState.Unknown; case 0: return KeeperState.Disconnected; case 1: return KeeperState.NoSyncConnected; case 3: return KeeperState.SyncConnected; case 4: return KeeperState.AuthFailed; case 5: return KeeperState.ConnectedReadOnly; case 6: return KeeperState.SaslAuthenticated; case -112: return KeeperState.Expired; default: throw new RuntimeException("Invalid integer value for conversion to KeeperState"); } } } /** * Enumeration of types of events that may occur on the ZooKeeper */ public enum EventType { None (-1), NodeCreated (1), NodeDeleted (2), NodeDataChanged (3), NodeChildrenChanged (4); private final int intValue; EventType(int intValue) { this.intValue = intValue; } public int getIntValue() { return intValue; } public static EventType fromInt(int intValue) { switch(intValue) { case -1: return EventType.None; case 1: return EventType.NodeCreated; case 2: return EventType.NodeDeleted; case 3: return EventType.NodeDataChanged; case 4: return EventType.NodeChildrenChanged; default: throw new RuntimeException("Invalid integer value for conversion to EventType"); } } } } }
说明：可以看到，Event接口并没有定义任何属性和方法，但其包含了KeeperState和EventType两个内部枚举类。
可以简化成：
public interface Event {}
3.2 接口方法
abstract public void process(WatchedEvent event);
说明：其代表了实现Watcher接口时必须实现的的方法，即定义进行处理，WatchedEvent表示观察的事件。
四、Event源码分析(即3.1内部类)
4.1 内部类
1. KeeperState
public enum KeeperState { // 事件发生时Zookeeper的状态 /** Unused, this state is never generated by the server */ @Deprecated // 未知状态，不再使用，服务器不会产生此状态 Unknown (-1), /** The client is in the disconnected state - it is not connected * to any server in the ensemble. */ // 断开 Disconnected (0), /** Unused, this state is never generated by the server */ @Deprecated // 未同步连接，不再使用，服务器不会产生此状态 NoSyncConnected (1), /** The client is in the connected state - it is connected * to a server in the ensemble (one of the servers specified * in the host connection parameter during ZooKeeper client * creation). */ // 同步连接状态 SyncConnected (3), /** * Auth failed state */ // 认证失败状态 AuthFailed (4), /** * The client is connected to a read-only server, that is the * server which is not currently connected to the majority. * The only operations allowed after receiving this state is * read operations. * This state is generated for read-only clients only since * read/write clients aren't allowed to connect to r/o servers. */ // 只读连接状态 ConnectedReadOnly (5), /** * SaslAuthenticated: used to notify clients that they are SASL-authenticated, * so that they can perform Zookeeper actions with their SASL-authorized permissions. */ // SASL认证通过状态 SaslAuthenticated(6), /** The serving cluster has expired this session. The ZooKeeper * client connection (the session) is no longer valid. You must * create a new client connection (instantiate a new ZooKeeper * instance) if you with to access the ensemble. */ // 过期状态 Expired (-112); // 代表状态的整形值 private final int intValue; // Integer representation of value // for sending over wire // 构造函数 KeeperState(int intValue) { this.intValue = intValue; } // 返回整形值 public int getIntValue() { return intValue; } // 从整形值构造相应的状态 public static KeeperState fromInt(int intValue) { switch(intValue) { case -1: return KeeperState.Unknown; case 0: return KeeperState.Disconnected; case 1: return KeeperState.NoSyncConnected; case 3: return KeeperState.SyncConnected; case 4: return KeeperState.AuthFailed; case 5: return KeeperState.ConnectedReadOnly; case 6: return KeeperState.SaslAuthenticated; case -112: return KeeperState.Expired; default: throw new RuntimeException("Invalid integer value for conversion to KeeperState"); } } }
说明：KeeperState是一个枚举类，其定义了在事件发生时Zookeeper所处的各种状态，其还定义了一个从整形值返回对应状态的方法fromInt。
2. EventType
public enum EventType { // 事件类型 // 无 None (-1), // 结点创建 NodeCreated (1), // 结点删除 NodeDeleted (2), // 结点数据变化 NodeDataChanged (3), // 结点子节点变化 NodeChildrenChanged (4); // 代表事件类型的整形 private final int intValue; // Integer representation of value // for sending over wire // 构造函数 EventType(int intValue) { this.intValue = intValue; } // 返回整形 public int getIntValue() { return intValue; } // 从整形构造相应的事件 public static EventType fromInt(int intValue) { switch(intValue) { case -1: return EventType.None; case 1: return EventType.NodeCreated; case 2: return EventType.NodeDeleted; case 3: return EventType.NodeDataChanged; case 4: return EventType.NodeChildrenChanged; default: throw new RuntimeException("Invalid integer value for conversion to EventType"); } } }
说明：EventType是一个枚举类，其定义了事件的类型（如创建节点、删除节点等事件），同时，其还定义了一个从整形值返回对应事件类型的方法fromInt。
五、WatchedEvent
5.1 类的属性
public class WatchedEvent { // Zookeeper的状态 final private KeeperState keeperState; // 事件类型 final private EventType eventType; // 事件所涉及节点的路径 private String path; }
说明：WatchedEvent类包含了三个属性，分别代表事件发生时Zookeeper的状态、事件类型和发生事件所涉及的节点路径。
5.2 构造函数
1. public WatchedEvent(EventType eventType, KeeperState keeperState, String path)型构造函数
public WatchedEvent(EventType eventType, KeeperState keeperState, String path) { // 初始化属性 this.keeperState = keeperState; this.eventType = eventType; this.path = path; }
说明：构造函数传入了三个参数，然后分别对属性进行赋值操作。
2. public WatchedEvent(WatcherEvent eventMessage)型构造函数
public WatchedEvent(WatcherEvent eventMessage) { // 从eventMessage中取出相应属性进行赋值 keeperState = KeeperState.fromInt(eventMessage.getState()); eventType = EventType.fromInt(eventMessage.getType()); path = eventMessage.getPath(); }
说明：构造函数传入了WatcherEvent参数，之后直接从该参数中取出相应属性进行赋值操作。
五总结：对于WatchedEvent类的方法而言，相对简单，包含了几个getXXX方法，用于获取相应的属性值。
六、ClientWatchManager
public Set<Watcher> materialize(Watcher.Event.KeeperState state, Watcher.Event.EventType type, String path);
说明：该方法表示事件发生时，返回需要被通知的Watcher集合，可能为空集合。
七、ZKWatchManager(zookeeper内)
7.1 类的属性
private static class ZKWatchManager implements ClientWatchManager { // 数据变化的Watchers private final Map<String, Set<Watcher>> dataWatches = new HashMap<String, Set<Watcher>>(); // 节点存在与否的Watchers private final Map<String, Set<Watcher>> existWatches = new HashMap<String, Set<Watcher>>(); // 子节点变化的Watchers private final Map<String, Set<Watcher>> childWatches = new HashMap<String, Set<Watcher>>(); }
说明：ZKWatchManager实现了ClientWatchManager，并定义了三个Map键值对，键为节点路径，值为Watcher。分别对应数据变化的Watcher、节点是否存在的Watcher、子节点变化的Watcher。
7.2 核心方法分析
1. materialize方法
public Set<Watcher> materialize(Watcher.Event.KeeperState state, Watcher.Event.EventType type, String clientPath) { // 新生成结果Watcher集合 Set<Watcher> result = new HashSet<Watcher>(); switch (type) { // 确定事件类型 case None: // 无类型 // 添加默认Watcher result.add(defaultWatcher); // 是否需要清空(提取对zookeeper.disableAutoWatchReset字段进行配置的值、 // Zookeeper的状态是否为同步连接) boolean clear = ClientCnxn.getDisableAutoResetWatch() && state != Watcher.Event.KeeperState.SyncConnected; // 同步块 synchronized(dataWatches) { for(Set<Watcher> ws: dataWatches.values()) { // 添加至结果集合 result.addAll(ws); } if (clear) { // 是否需要清空 dataWatches.clear(); } } // 同步块 synchronized(existWatches) { for(Set<Watcher> ws: existWatches.values()) { // 添加至结果集合 result.addAll(ws); } if (clear) { // 是否需要清空 existWatches.clear(); } } // 同步块 synchronized(childWatches) { for(Set<Watcher> ws: childWatches.values()) { // 添加至结果集合 result.addAll(ws); } if (clear) { // 是否需要清空 childWatches.clear(); } } // 返回结果 return result; case NodeDataChanged: // 节点数据变化 case NodeCreated: // 创建节点 synchronized (dataWatches) { // 同步块 // 移除clientPath对应的Watcher后全部添加至结果集合 addTo(dataWatches.remove(clientPath), result); } synchronized (existWatches) { // 移除clientPath对应的Watcher后全部添加至结果集合 addTo(existWatches.remove(clientPath), result); } break; case NodeChildrenChanged: // 节点子节点变化 synchronized (childWatches) { // 移除clientPath对应的Watcher后全部添加至结果集合 addTo(childWatches.remove(clientPath), result); } break; case NodeDeleted: // 删除节点 synchronized (dataWatches) { // 移除clientPath对应的Watcher后全部添加至结果集合 addTo(dataWatches.remove(clientPath), result); } // XXX This shouldn't be needed, but just in case synchronized (existWatches) { // 移除clientPath对应的Watcher Set<Watcher> list = existWatches.remove(clientPath); if (list != null) { // 移除clientPath对应的Watcher后全部添加至结果集合 addTo(existWatches.remove(clientPath), result); LOG.warn("We are triggering an exists watch for delete! Shouldn't happen!"); } } synchronized (childWatches) { // 移除clientPath对应的Watcher后全部添加至结果集合 addTo(childWatches.remove(clientPath), result); } break; default: // 缺省处理 String msg = "Unhandled watch event type " + type + " with state " + state + " on path " + clientPath; LOG.error(msg); throw new RuntimeException(msg); } // 返回结果集合 return result; }
说明：该方法在事件发生后，返回需要被通知的Watcher集合。在该方法中，首先会根据EventType类型确定相应的事件类型，然后根据事件类型的不同做出相应的操作：
如针对None类型，即无任何事件，则首先会从三个键值对中删除clientPath对应的Watcher，然后将剩余的Watcher集合添加至结果集合；
针对NodeDataChanged和NodeCreated事件而言，其会从dataWatches和existWatches中删除clientPath对应的Watcher，然后将剩余的Watcher集合添加至结果集合。
八、总结
针对Watcher机制的第一部分的源码分析就已经完成，本章节需重点关注：
- 事件的变化，状态的定义依赖于Event内部类的两组枚举值
- 上下游调用关系图需记忆一下，为加强记忆，再最后再贴一下
