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EventBus 开源库学习(三)

源码细节阅读

上一节根据EventBus的使用流程把实现源码大体梳理了一遍,因为精力有限,所以看源码都是根据实现过程把基本流程看下,中间实现细节先忽略,否则越看越深不容易把握大体思路,这节把一些细节的部分再看看。

一、注解函数查找源码逻辑

#EventBus   
 public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

在进行注册的时候,我们使用subscriberMethodFinder对象的findSubscriberMethods方法来查找到所有该类中以@Subscribe注解的函数(以下简称为注解函数)。让我们继续看下查找部分的逻辑。

    List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
        if (subscriberMethods != null) {
            return subscriberMethods;
        }

        if (ignoreGeneratedIndex) {
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
            subscriberMethods = findUsingInfo(subscriberClass);
        }
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass
                    + " and its super classes have no public methods with the @Subscribe annotation");
        } else {
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }

METHOD_CACHE是一个ConcurrentHashMap类型的常量,用来保存subscriberClass和对应SubscriberMethod的集合,以提高注册效率,防止重复查找。

如果METHOD_CACHE缓存中不存在,判断变量ignoreGeneratedIndex的值,该值是在EventBusBuilder中初始化的,表示是否忽略注解生成器,默认是false。然后会进入到findUsingInfo函数中进行查找(下面再看),最后将找到的subscriberMethods添加到METHOD_CACHE中。

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        ````
        return getMethodsAndRelease(findState);
    }

先看findUsingInfo前两行,出现一个FindState类,它是SubscriberMethodFinder的内部类,用来辅助查找订阅事件的方法。

    private FindState prepareFindState() {
        synchronized (FIND_STATE_POOL) {
            for (int i = 0; i < POOL_SIZE; i++) {
                FindState state = FIND_STATE_POOL[i];
                if (state != null) {
                    FIND_STATE_POOL[i] = null;
                    return state;
                }
            }
        }
        return new FindState();
    }


    static class FindState {
        ```
        void initForSubscriber(Class<?> subscriberClass) {
            this.subscriberClass = clazz = subscriberClass;
            skipSuperClasses = false;
            subscriberInfo = null;
        }
     }

prepareFindState()方法主要是返回一个FindState对象,先从FIND_STATE_POOL这个缓存池中获取一个现成的,如果没有就新建一个对象。获取对象后调用初始化方法,将subscriberClass赋值给clazz变量,subscriberInfo对象赋值为null

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            findState.subscriberInfo = getSubscriberInfo(findState);
            if (findState.subscriberInfo != null) {
                ```
            } else {
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

再回到findUsingInfo,因为findState.clazz刚赋值过,所以不为空。再来看下getSubscriberInfo方法。

    private SubscriberInfo getSubscriberInfo(FindState findState) {
        if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
            SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
            if (findState.clazz == superclassInfo.getSubscriberClass()) {
                return superclassInfo;
            }
        }
        if (subscriberInfoIndexes != null) {
            for (SubscriberInfoIndex index : subscriberInfoIndexes) {
                SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
                if (info != null) {
                    return info;
                }
            }
        }
        return null;
    }

前面执行initForSubscriber的时候,subscriberInfo赋值为null,因此第一个if不成立。第二个判断是对象subscriberInfoIndexes,该变量也是在EventBusBuilder中初始化的,默认是null,因此该方法返回值null

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            findState.subscriberInfo = getSubscriberInfo(findState);
            if (findState.subscriberInfo != null) {
                ```
            } else {
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

再回到findUsingInfo,由于getSubscriberInfo返回为null,逻辑会走到findUsingReflectionInSingleClass方法中,使用反射来获取所有方法(下面再看)。

获取当前类所有的注解方法后,findState.moveToSuperclass()表示修改findState.clazzsubscriberClass的父类Class,继续遍历父类中的注解方法。

private static final int MODIFIERS_IGNORE = Modifier.ABSTRACT | Modifier.STATIC | BRIDGE | SYNTHETIC;

private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // This is faster than getMethods, especially when subscribers are fat classes like Activities
            methods = findState.clazz.getDeclaredMethods();
        } catch (Throwable th) {
            // Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
            ```
        }
        for (Method method : methods) {
            int modifiers = method.getModifiers();
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class<?>[] parameterTypes = method.getParameterTypes();
                if (parameterTypes.length == 1) {
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        Class<?> eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                    subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                        }
                    }
                } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                    String methodName = method.getDeclaringClass().getName() + "." + method.getName();
                    throw new EventBusException("@Subscribe method " + methodName +
                            "must have exactly 1 parameter but has " + parameterTypes.length);
                }
            } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                String methodName = method.getDeclaringClass().getName() + "." + method.getName();
                throw new EventBusException(methodName +
                        " is a illegal @Subscribe method: must be public, non-static, and non-abstract");
            }
        }
    }

上面代码流程如下:

  • 通过方法getDeclaredMethods()获取该类中所有声明的方法列表;
  • 遍历方法列表,获取方法的修饰符,如果修饰符是public并且不是abstractstatic等进入下一步,MODIFIERS_IGNORE定义如第一行代码;
  • 获取方法的参数列表,如果参数的个数是1,并且使用了Subscribe注解,获取唯一的入参进行下一步;
  • 然后checkAdd()方法用来判断FindState的anyMethodByEventType map是否已经添加过以当前eventTypekey的键值对,没添加过则返回true(就是看看当前类中有没有多个事件相同的注解函数);
  • 通过注解对象subscribeAnnotation获取threadMode,然后新建SubscriberMethod添加到findState.subscriberMethods集合中。

分析完findUsingReflectionInSingleClass这个方法后,我们回到findUsingInfo

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            findState.subscriberInfo = getSubscriberInfo(findState);
            if (findState.subscriberInfo != null) {
                ```
            } else {
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

遍历当前类及其父类,通过findUsingReflectionInSingleClass方法找到所有注解方法后,通过getMethodsAndRelease返回所有相关方法。

    private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
        List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
        findState.recycle();
        synchronized (FIND_STATE_POOL) {
            for (int i = 0; i < POOL_SIZE; i++) {
                if (FIND_STATE_POOL[i] == null) {
                    FIND_STATE_POOL[i] = findState;
                    break;
                }
            }
        }
        return subscriberMethods;
    }

通过上面的解析知道,所有的SubscriberMethod都添加到findState.subscriberMethods集合中,这个方法就是把findState.subscriberMethods集合中的内容复制出来,然后释放findState中的资源,并把findState放到FIND_STATE_POOL缓存中,POOL_SIZE常量是4,这样下次查找的时候可以重复利用无需每次都新建对象。

以上为注解方法查找的过程。在上面分析的过程中出现了两个变量subscriberInfoIndexesignoreGeneratedIndex看样子都是关于索引的,分析的时候都是使用的EventBusBuilder中的默认值。看下subscriberInfoIndexes赋值的地方,看注释意思是:将EventBus注解处理器生成索引添加添加进来。所以注解处理器需要再了解下。

    /** Adds an index generated by EventBus' annotation preprocessor. */
    public EventBusBuilder addIndex(SubscriberInfoIndex index) {
        if (subscriberInfoIndexes == null) {
            subscriberInfoIndexes = new ArrayList<>();
        }
        subscriberInfoIndexes.add(index);
        return this;
    }

二、Subscriber Index注解处理器

通过上面分析可知,EventBus注册事件时,主要是在项目运行时通过反射来查找注解方法信息,如果项目中有大量的注解方法,必然会对项目运行时的性能产生影响。

除了在项目运行时通过反射查找注解方法信息,EventBus还提供了在项目编译时通过注解处理器查找注解方法信息的方式,生成一个辅助的索引类来保存这些信息,这个索引类就是Subscriber Index。

1、Subscriber Index使用方式
首先要在 appbuild.gradle中加入如下配置:

android {
    defaultConfig {
        javaCompileOptions {
            annotationProcessorOptions {
                // 指定辅助索引类的名称和包名,注意,这个类时自动生成的,不需要我们写
                arguments = [ eventBusIndex : 'com.jane.demo.MyEventBusIndex' ]
            }
        }
    }
}
dependencies {
    compile 'org.greenrobot:eventbus:3.3.1'
    // 引入注解处理器
    annotationProcessor 'org.greenrobot:eventbus-annotation-processor:3.3.1'
}

在项目的Application中添加如下配置,以生成一个默认的EventBus 单例:

EventBus.builder().addIndex(new MyEventBusIndex()).installDefaultEventBus();

此时重新编译下项目,会生成一个MyEventBusIndex类(不同的gradle版本生成的位置可能不一样)。

EventBus 开源库学习(三),第1张
MyEventBusIndex.png

生成的MyEventBusIndex代码如下:

public class MyEventBusIndex implements SubscriberInfoIndex {
    private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;

    static {
        SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();

        putIndex(new SimpleSubscriberInfo(EventBusService.class, true, new SubscriberMethodInfo[] {
            new SubscriberMethodInfo("onMsgEventReceived", String.class),
            new SubscriberMethodInfo("onMsgEventReceived", MsgEvent.class),
            new SubscriberMethodInfo("onMsgEventReceived", org.greenrobot.eventbus.NoSubscriberEvent.class),
        }));

        putIndex(new SimpleSubscriberInfo(EventBusActivity.class, true, new SubscriberMethodInfo[] {
             new SubscriberMethodInfo("onMsgEventReceived", Event.class),
        }));
    }

    private static void putIndex(SubscriberInfo info) {
        SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);
    }

    @Override
    public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {
        SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);
        if (info != null) {
            return info;
        } else {
            return null;
        }
    }
}

代码中SUBSCRIBER_INDEX是一个HashMap,保存了当前注册类的Class类型和其中注解方法的信息。

2、Subscriber Index源码

EventBus.builder().addIndex(new MyEventBusIndex()).installDefaultEventBus();

使用的时候,获取EventBusBuilder对象,然后调用addIndex方法,把生成的MyEventBusIndex加入进去。

    /** Adds an index generated by EventBus' annotation preprocessor. */
    public EventBusBuilder addIndex(SubscriberInfoIndex index) {
        if (subscriberInfoIndexes == null) {
            subscriberInfoIndexes = new ArrayList<>();
        }
        subscriberInfoIndexes.add(index);
        return this;
    }

上面的方法是不是比较眼熟,就是上面代码分析的时候出现的变量subscriberInfoIndexes赋值函数。这样,运行的时候就把之前编译好的索引类的实例保存在subscriberInfoIndexes集合中。然后调用installDefaultEventBus方法创建EventBus实例。

    /**
     * Installs the default EventBus returned by {@link EventBus#getDefault()} using this builders' values. Must be
     * done only once before the first usage of the default EventBus.
     *
     * @throws EventBusException if there's already a default EventBus instance in place
     */
    public EventBus installDefaultEventBus() {
        synchronized (EventBus.class) {
            if (EventBus.defaultInstance != null) {
                throw new EventBusException("Default instance already exists." +
                        " It may be only set once before it's used the first time to ensure consistent behavior.");
            }
            EventBus.defaultInstance = build();
            return EventBus.defaultInstance;
        }
    }

    /** Builds an EventBus based on the current configuration. */
    public EventBus build() {
        return new EventBus(this);
    }

上面通过build方法,以当前EventBusBuilder对象作为参数生成EventBus单例对象,并赋值给defaultInstance,这样就把subscriberInfoIndexes信息传递给了EventBus。以后调用EventBusgetDefault()返回的就是这里生成的单例对象,里面包含了subscriberInfoIndexes信息。

而且在Application中生成了EventBus 的默认单例,这样就保证了在项目其它地方执行EventBus.getDefault()就能得到就是上面包含subscriberInfoIndexes信息的单例。

在上面分析查找注解函数的时候,分析过一个函数:

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            findState.subscriberInfo = getSubscriberInfo(findState);
            if (findState.subscriberInfo != null) {
                ····
            } else {
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

如果没有使用注解处理器的话getSubscriberInfo这个方法返回值是null,因此会走到findUsingReflectionInSingleClass,通过反射区获取注解方法。现在使用使用注解处理器的话在重新看下这个函数。

    private SubscriberInfo getSubscriberInfo(FindState findState) {
        if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
            SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
            if (findState.clazz == superclassInfo.getSubscriberClass()) {
                return superclassInfo;
            }
        }
        //不为空
        if (subscriberInfoIndexes != null) {
            for (SubscriberInfoIndex index : subscriberInfoIndexes) {
                SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
                if (info != null) {
                    return info;
                }
            }
        }
        return null;
    }

    @Override
    public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {
        SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);
        if (info != null) {
            return info;
        } else {
            return null;
        }
    }

如果使用了注解处理器,因为subscriberInfoIndexes这个集合不为空,遍历获取当前类对应的注解方法列表,index.getSubscriberInfo这个方法实际调用的就是系统生成的MyEventBusIndex中的getSubscriberInfo方法,这样就获取到了之前编译生成的注解方法。

    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);
    }

通过getSubscriberInfo返回当前类所有注解方法后,遍历所有订阅了事件的方法然后加入到subscriberMethods列表中,其它的和之前的注册流程一样。

参考文章:
EventBus 原理解析


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