Spring事件监听机制

前言

spring中的事件机制其实就是设计模式中的观察者模式，主要由以下角色构成：

1. 事件
2. 事件监听器（监听并处理事件）
3. 事件发布者（发布事件）

首先看一下监听器和发布者的接口定义

public interface applicationlistener<e extends applicationevent> extends eventlistener {
      void onapplicationevent(e event);
  }
  
  public interface applicationeventpublisher {
    default void publishevent(applicationevent event) {
        publishevent((object) event);
    }
    void publishevent(object event);

}

事件流转流程

初始化事件广播器

看一下这个方法abstractapplicationcontext.refresh，在ioc源码解析那篇文章已经把这个方法分析完了，所以直接关注事件广播器和事件发布相关的逻辑即可

 public void refresh() throws beansexception, illegalstateexception {
        synchronized (this.startupshutdownmonitor) {
            // prepare this context for refreshing.
            preparerefresh();
 
            // tell the subclass to refresh the internal bean factory.
            configurablelistablebeanfactory beanfactory = obtainfreshbeanfactory();
 
            // prepare the bean factory for use in this context.
            preparebeanfactory(beanfactory);
 
            try {
                // allows post-processing of the bean factory in context subclasses.
                postprocessbeanfactory(beanfactory);
 
                // invoke factory processors registered as beans in the context.
                invokebeanfactorypostprocessors(beanfactory);
 
                // register bean processors that intercept bean creation.
                registerbeanpostprocessors(beanfactory);
 
                // initialize message source for this context.
                initmessagesource();
 
                // 初始化事件广播器
                initapplicationeventmulticaster();
 
                // initialize other special beans in specific context subclasses.
                onrefresh();
 
                // check for listener beans and register them.
                registerlisteners();
 
                // instantiate all remaining (non-lazy-init) singletons.
                finishbeanfactoryinitialization(beanfactory);
 
                // 发布事件
                finishrefresh();
            }
 
            catch (beansexception ex) {
                logger.warn("exception encountered during context initialization - cancelling refresh attempt", ex);
 
                // destroy already created singletons to avoid dangling resources.
                destroybeans();
 
                // reset 'active' flag.
                cancelrefresh(ex);
 
                // propagate exception to caller.
                throw ex;
            }
        }
}

protected void initapplicationeventmulticaster() {
        configurablelistablebeanfactory beanfactory = getbeanfactory();
        if (beanfactory.containslocalbean(application_event_multicaster_bean_name)) {
            this.applicationeventmulticaster =
                    beanfactory.getbean(application_event_multicaster_bean_name, applicationeventmulticaster.class);
            if (logger.isdebugenabled()) {
                logger.debug("using applicationeventmulticaster [" + this.applicationeventmulticaster + "]");
            }
        }
        else {
            this.applicationeventmulticaster = new simpleapplicationeventmulticaster(beanfactory);
            beanfactory.registersingleton(application_event_multicaster_bean_name, this.applicationeventmulticaster);
            if (logger.isdebugenabled()) {
                logger.debug("unable to locate applicationeventmulticaster with name '" +
                        application_event_multicaster_bean_name +
                        "': using default [" + this.applicationeventmulticaster + "]");
            }
        }
    }

可以看到如果没有自定义的事件广播器，默认是使用simpleapplicationeventmulticaster的

发布事件

发布事件是在bean的初始化之后的

    protected void finishrefresh() {
        // initialize lifecycle processor for this context.
        initlifecycleprocessor();
 
        // propagate refresh to lifecycle processor first.
        getlifecycleprocessor().onrefresh();
 
        // 发布事件
        publishevent(new contextrefreshedevent(this));
 
        // participate in livebeansview mbean, if active.
        livebeansview.registerapplicationcontext(this);
    }


public void publishevent(applicationevent event) {
        assert.notnull(event, "event must not be null");
        if (logger.istraceenabled()) {
            logger.trace("publishing event in " + getdisplayname() + ": " + event);
        }
        //1. 获取到事件广播器，发布事件
        getapplicationeventmulticaster().multicastevent(event);
        //2. 如果存在父容器，父容器也将发布事件
        if (this.parent != null) {
            this.parent.publishevent(event);
        }
    }

具体的发布逻辑在multicastevent方法中

public void multicastevent(final applicationevent event) {
        //遍历执行listener，getapplicationlisteners调用abstractapplicationeventmulticaster父类方法
        for (final applicationlistener listener : getapplicationlisteners(event)) {
            executor executor = gettaskexecutor();
            if (executor != null) {
                executor.execute(new runnable() {
                    @override
                    public void run() {
                        listener.onapplicationevent(event);
                    }
                });
            }
            else {
                listener.onapplicationevent(event);
            }
        }
    }

可以看到也没啥特殊的，无非就是起个线程池去调用这些监听器的方法

而监听器的处理就看各个监听器的具体实现了