今天这篇文章从源码去简单理解EventBus的实现。
从源码解读的主要模块有：
入口：
入口函数如下：
EventBus.getDefault()
进入源码我们可以看到EventBus是单例实现的，源码如下：
static volatile EventBus defaultInstance;
/** Convenience singleton for apps using a process-wide EventBus instance. */
public static EventBus getDefault() {
if (defaultInstance == null) {
synchronized (EventBus.class) {
if (defaultInstance == null) {
defaultInstance = new EventBus();
}
}
}
return defaultInstance;
}
这是一个双重检查单例模式（DCL）, 还用到了关键字volatile
volatile 是什么？作用又是什么？
推荐查看这篇文章 Java并发编程：volatile关键字解析
在访问volatile变量时不会执行加锁操作，因此也就不会使执行线程阻塞，volatile 关键字是一种比 sychronized 关键字更轻量级的同步机制。
synchronized 关键字
synchronized是什么？作用是什么？
推荐查看这篇文章 Java synchronized 详解
在构造函数中又调用了一个带参的构造函数，主要是用于用户自定义的配置，如果没配置就使用默认的配置信息，源码如下：
// 默认的builder
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
// 调用下方的构造函数
public EventBus() {
this(DEFAULT_BUILDER);
}
// 初始化操作
EventBus(EventBusBuilder builder) {
logger = builder.getLogger();
subscriptionsByEventType = new HashMap<>();
typesBySubscriber = new HashMap<>();
stickyEvents = new ConcurrentHashMap<>();
mainThreadSupport = builder.getMainThreadSupport();
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
backgroundPoster = new BackgroundPoster(this);
asyncPoster = new AsyncPoster(this);
indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
builder.strictMethodVerification, builder.ignoreGeneratedIndex);
logSubscriberExceptions = builder.logSubscriberExceptions;
logNoSubscriberMessages = builder.logNoSubscriberMessages;
sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
throwSubscriberException = builder.throwSubscriberException;
eventInheritance = builder.eventInheritance;
executorService = builder.executorService;
}
总结：使用单例双重检查锁模式。使用 synchronized和volatila关键字是为了降低EventBus对象在并发操作时可能出现的原子性问题、可见性问题、有序性问题
register 操作
注册观察者
/**
* Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they
* are no longer interested in receiving events.
* <p/>
* Subscribers have event handling methods that must be annotated by {@link Subscribe}.
* The {@link Subscribe} annotation also allows configuration like {@link
* ThreadMode} and priority.
*/
public void register(Object subscriber) {
Class<?> subscriberClass = subscriber.getClass();
// 用 subscriberMethodFinder 提供的方法，找到在 subscriber 这个类里面订阅的内容。
List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
synchronized (this) {
// 为什么加锁？保证代码块的同步性，加锁会安全，但是性能会慢一些
for (SubscriberMethod subscriberMethod : subscriberMethods) {
subscribe(subscriber, subscriberMethod);
}
}
}
在findSubscriberMethods中找到订阅的集合， findSubscriberMethods方法的实现如下：
/**
* 键值对：键是类对象，值是List<SubscriberMethod> 集合对象。SubscriberMethod 是 每个订阅者与回调方法的关系对象，里面封装了包括优先级，是否接收黏性事件和所在线程等信息。
*/
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
// 在缓存中查找 SubscriberMethod 集合
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
//ignoreGeneratedIndex 属性表示是否忽略注解器生成的 MyEventBusIndex，ignoreGeneratedIndex 默认为false
if (ignoreGeneratedIndex) {
// 通过反射查找
subscriberMethods = findUsingReflection(subscriberClass);
} else {
// 查找
subscriberMethods = findUsingInfo(subscriberClass);
}
//在获得subscriberMethods以后，如果订阅者中不存在@Subscribe注解并且为public的订阅方法，则会抛出异常。
if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
// 放入缓存
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}
ignoreGeneratedIndex 默认为``false`，源码如下：
/** Forces the use of reflection even if there's a generated index (default: false). */
public EventBusBuilder ignoreGeneratedIndex(boolean ignoreGeneratedIndex) {
this.ignoreGeneratedIndex = ignoreGeneratedIndex;
return this;
}
所以先查看这个findUsingInfo方法的实现
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
while (findState.clazz != null) {
findState.subscriberInfo = getSubscriberInfo(findState);
if (findState.subscriberInfo != null) {
SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
for (SubscriberMethod subscriberMethod : array) {
if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
findState.subscriberMethods.add(subscriberMethod);
}
}
} else {
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
return getMethodsAndRelease(findState);
}
明天补上。。。。。
总结：
注销
发消息
粘性消息
