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

Watcher机制（一）