15. sharding-jdbc源码之EventBus-轻量级进程内事件分发组件

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EventBus来自于google-guava包中。源码注释如下：

Dispatches events to listeners, and provides ways for listeners to register themselves.
The EventBus allows publish-subscribe-style communication between components 
without requiring the components to explicitly register with one another (and thus be 
aware of each other).  It is designed exclusively to replace traditional Java in-process 
event distribution using explicit registration. It is not a general-purpose publish-subscribe
system, nor is it intended for interprocess communication.

翻译：将事件分派给监听器，并为监听器提供注册自己的方法。EventBus允许组件之间的发布 - 订阅式通信，而不需要组件彼此明确注册（并且因此彼此意识到）。 它专门用于使用显式注册替换传统的Java进程内事件分发。 它不是一个通用的发布 - 订阅系统，也不是用于进程间通信。

使用参考

关于EventBus的用例代码提取自sharding-jdbc源码，并结合lombok最大限度的简化：

• EventBusInstance--用于获取EventBus实例（饿汉式单例模式）

@NoArgsConstructor(access = AccessLevel.PRIVATE)
public final class EventBusInstance {

    private static final EventBus INSTANCE = new EventBus();

    public static EventBus getInstance() {
        return INSTANCE;
    }
}

• DMLExecutionEvent--发布订阅事件模型

@Getter
@Setter
public class DMLExecutionEvent {
    private String id;
    private String dataSource;
    private Date sendTime;
}

• DMLExecutionEventListener--事件监听器

public final class DMLExecutionEventListener {

    @Subscribe
    @AllowConcurrentEvents
    public void listener(final DMLExecutionEvent event) {
        System.out.println("监听的DML执行事件: " + JSON.toJSONString(event));
        // do something
    }
}

-- Main--主方法：注册订阅者监听事件，以及发布事件。

/**
 * @author wangzhenfei9
 * @version 1.0.0
 * @since 2018年04月24日
 */
public class Main {

    static{
        System.out.println("register listener...");
        EventBusInstance.getInstance().register(new DMLExecutionEventListener());
    }

    public static void main(String[] args) throws InterruptedException {

        for (int i=0; i<10; i++) {
            pub();
            Thread.sleep(3000);
        }
    }

    private static void pub(){
        DMLExecutionEvent event = new DMLExecutionEvent();
        event.setId(String.valueOf(new Random().nextInt(1000)));
        event.setDataSource("sj_db_1");
        event.setSendTime(new Date());
        System.out.println("发布的DML执行事件: " + JSON.toJSONString(event));
        EventBusInstance.getInstance().post(event);
    }
}

核心方法

EventBus一些重要方法解释如下：

• post(Object)：Posts an event to all registered subscribers. This method will return successfully after the event has been posted to all subscribers, and regardless of any exceptions thrown by subscribers.
• register(Object): Registers all subscriber methods on object to receive events.Subscriber methods are selected and classified using this EventBus's SubscriberFindingStrategy; the default strategy is the AnnotatedSubscriberFinder.
• unregister(Object)：Unregisters all subscriber methods on a registered object.

源码分析

主要分析发布事件以及订阅的核心源码；

发布源码分析

public void post(Object event) {
    // 得到所有该类已经它的所有父类（因为有些注册监听器是监听其父类）
    Set> dispatchTypes = flattenHierarchy(event.getClass());

    boolean dispatched = false;
    // 遍历类本身以及所有父类
    for (Class eventType : dispatchTypes) {
        // 重入读锁先锁住
        subscribersByTypeLock.readLock().lock();
        try {
            // 得到类的所有订阅者，例如DMLExecutionEvent的订阅者就是DMLExecutionEventListener（EventSubscriber有两个属性：重要的属性target和method，target就是监听器即DMLExecutionEventListener，method就是监听器方法即listener；从而知道DMLExecutionEvent这个事件由哪个类的哪个方法监听处理）
            Set wrappers = subscribersByType.get(eventType);

            if (!wrappers.isEmpty()) {
                // 如果有时间订阅者，那么dispatched = true，表示该事件可以分发
                dispatched = true;
                // 遍历所有的时间订阅者，每个订阅者的队列都增加该事件
                for (EventSubscriber wrapper : wrappers) {
                    enqueueEvent(event, wrapper);
                }
            }
        } finally {
            subscribersByTypeLock.readLock().unlock();
        }
    }

    if (!dispatched && !(event instanceof DeadEvent)) {
        post(new DeadEvent(this, event));
    }

    // 分发进入队列的事件
    dispatchQueuedEvents();
}

/** 
 * queues of events for the current thread to dispatch;
 * 核心数据结构为LinkedList，保存的是EventBus.EventWithSubscriber类型数据
 */
private final ThreadLocal> eventsToDispatch =
        new ThreadLocal>() {
            @Override protected Queue initialValue() {
                return new LinkedList();
            }
        };

void enqueueEvent(Object event, EventSubscriber subscriber) {
    // 数据结构为new LinkedList()，EventWithSubscriber就是对event和subscriber的封装，LinkedList数据结构保证进入队列和消费队列顺序一致
    eventsToDispatch.get().offer(new EventBus.EventWithSubscriber(event, subscriber));
}

/**
 * Drain the queue of events to be dispatched. As the queue is being drained,
 * new events may be posted to the end of the queue. 
 * 排干要被分发的事件队列，正在排干的过程中，可能有新的事件被追加到队列尾部
 */
void dispatchQueuedEvents() {
    // don't dispatch if we're already dispatching, that would allow reentrancy
    // and out-of-order events. Instead, leave the events to be dispatched
    // after the in-progress dispatch is complete.
    // 如果正在排干队列，则不分发
    if (isDispatching.get()) {
        return;
    }

    // ThreadLocal设置正在分发即isDispatching为true
    isDispatching.set(true);
    try {
        Queue events = eventsToDispatch.get();
        EventBus.EventWithSubscriber eventWithSubscriber;
        while ((eventWithSubscriber = events.poll()) != null) {
            // 调用订阅者处理事件（method.invoke(target, new Object[] { event });，method和target来自订阅者）
            dispatch(eventWithSubscriber.event, eventWithSubscriber.subscriber);
        }
    } finally {
        // ThreadLocal可能内存泄漏，用完需要remove
        isDispatching.remove();
        // 队列中的事件任务处理完，清空队列，即所谓的排干（Drain）
        eventsToDispatch.remove();
    }
}

订阅源码分析

/**
 * Registers all subscriber methods on {@code object} to receive events.
 * 注册object上所有订阅方法，用来接收事件，上面的使用参考，DMLExecutionEventListener就是这里的object
 */
public void register(Object object) {
    // Multimap是guava自定义数据结构，类似Map>，key就是事件类型，例如DMLExecutionEvent，value就是EventSubscriber即事件订阅者集合（说明，这个的订阅者集合是指object里符合订阅者的所有方法，例如DMLExecutionEventListener.listener()，DMLExecutionEventListener中可以有多个订阅者，注解@Subscribe即可），
    Multimap, EventSubscriber> methodsInListener =
            finder.findAllSubscribers(object);
    // 重入写锁保证线程安全
    subscribersByTypeLock.writeLock().lock();
    try {
        // 把订阅者信息放到map中缓存起来（发布事件post()时就会用到）
        subscribersByType.putAll(methodsInListener);
    } finally {
        subscribersByTypeLock.writeLock().unlock();
    }
}