【Zookeeper】源码分析之Watcher机制（一）

【Zookeeper】源码分析之Watcher机制（⼀）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 Event {}
说明：可以看到，Event接⼝并没有定义任何属性和⽅法，但是其包含了KeeperState和EventType两个内部枚举类。

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
3.1 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* client connection (the session) is no longer valid.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。

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
6.1 接⼝⽅法　
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
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;
synchronized(dataWatches) { // 同步块
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();
}
}
synchronized(existWatches) { // 同步块
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();
}
}
synchronized(childWatches) { // 同步块
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，可以看到此部分的源码相对简单，之后会分析org.apache.zookeeper.server下的WatchManager和ClientWatchManager所在外部类ZooKeeper，也谢谢各位园友的观看~。