Watcher是zookeeper的事件监听机制，今天我们来看看Watcher类的代码都包含了什么内容？

Watcher

Watcher是一个接口，定义了process方法，需要子类实现。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。

abstract public void process(WatchedEvent event);

内部类

1、Event接口

表示事件代表的状态，其包含了KeeperState和EventType两个内部枚举类。

KeeperState

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型值返回对应状态的方法fromInt。

@InterfaceAudience.Public
    public interface Event {
        /**
         * Enumeration of states the ZooKeeper may be at the event
         */
        @InterfaceAudience.Public
        public enum KeeperState {
            /** Unused, this state is never generated by the server */
            //未知状态，服务器不再产生此状态
            @Deprecated
            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 */
            //未同步连接，不再使用，服务器不会产生此状态
            @Deprecated
            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             * client connection (the session) is no longer valid.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),

            /**
             * The client has been closed. This state is never generated by
             * the server, but is generated locally when a client calls
             * {@link ZooKeeper#close()} or {@link ZooKeeper#close(int)}
             */
            //关闭
            Closed (7);

            //代表状态的整型值
            private final int intValue;     // Integer representation of value
                                            // for sending over wire

            KeeperState(int intValue) {
                this.intValue = intValue;
            }

            public int getIntValue() {
                return intValue;
            }
            //从整型构造相应的状态
            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;
                    case   7: return KeeperState.Closed;

                    default:
                        throw new RuntimeException("Invalid integer value for conversion to KeeperState");
                }
            }
        }

EventType

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型值返回对应事件类型的方法fromInt。

 @InterfaceAudience.Public
        public enum EventType {
            //无
            None (-1),
            //结点创建
            NodeCreated (1),
            //结点删除
            NodeDeleted (2),
            //结点数据变化
            NodeDataChanged (3),
            //子结点变化
            NodeChildrenChanged (4),
            //监听移除
            DataWatchRemoved (5),
            //子结点监听移除
            ChildWatchRemoved (6);

            private final int intValue;     // Integer representation of value
                                            // for sending over wire

            EventType(int intValue) {
                this.intValue = intValue;
            }

            public int getIntValue() {
                return intValue;
            }
            //从整型构造相应的事件
            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;
                    case  5: return EventType.DataWatchRemoved;
                    case  6: return EventType.ChildWatchRemoved;

                    default:
                        throw new RuntimeException("Invalid integer value for conversion to EventType");
                }
            }
        }

2、枚举类WatcherType

监听器类型枚举

@InterfaceAudience.Public
    public enum WatcherType {
        //子监听器
        Children(1),
        //数据监听
        Data(2),
        //任意
        Any(3);

        // Integer representation of value
        private final int intValue;

        private WatcherType(int intValue) {
            this.intValue = intValue;
        }

        public int getIntValue() {
            return intValue;
        }
                //整数到类型的转换
        public static WatcherType fromInt(int intValue) {
            switch (intValue) {
            case 1:
                return WatcherType.Children;
            case 2:
                return WatcherType.Data;
            case 3:
                return WatcherType.Any;

            default:
                throw new RuntimeException(
                        "Invalid integer value for conversion to WatcherType");
            }
        }
    }

WatchedEvent

/**
     * Create a WatchedEvent with specified type, state and path
     */
    public WatchedEvent(EventType eventType, KeeperState keeperState, String path) {
        this.keeperState = keeperState;
        this.eventType = eventType;
        this.path = path;
    }

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。

原文地址：https://blog.51cto.com/janephp/2455633