探析Spring容器内部事件发布

其实在 JDK 中已经提供相应的自定义事件发布功能的基础类:

• java.util.EventObject类 ：自定义事件类型
• java.util.EventListener接口：事件的监听器

首先了解几个概念：

Spring 事件类结构

1. 事件类

事件类也就是定义发送的内容，比如可以通过继承ApplicationContextEvent来自定义一个特定事件类。

1.1 ApplicationEvent类

首先是继承 EventObject的ApplicationEvent，通过source来指定事件源：

public abstract class ApplicationEvent extends EventObject {
    /**
     * Constructs a prototypical Event.
     *
     * @param source The object on which the Event initially occurred.
     * @throws IllegalArgumentException if source is null.
     */
    public ApplicationEvent(Object source) {
        super(source);
    }
}

1.2 ApplicationContextEvent类

是主要的容器事件，它有容器启动、刷新、停止以及关闭各种事件的子类。

public class ApplicationContextEvent extends ApplicationEvent {

    /**
     * Constructs a prototypical Event.
     *
     * @param source The object on which the Event initially occurred.
     * @throws IllegalArgumentException if source is null.
     */
    public ApplicationContextEvent(Object source) {
        super(source);
    }

    /**
     * Get the ApplicationContext that the event was raised for.
     */
    public final ApplicationContext getApplicationContext() {
        return (ApplicationContext) getSource();
    }

}

public class ContextClosedEvent extends ApplicationContextEvent{

    /**
     * Constructs a prototypical Event.
     *
     * @param source The object on which the Event initially occurred.
     * @throws IllegalArgumentException if source is null.
     */
    public ContextClosedEvent(Object source) {
        super(source);
    }

}

public class ContextRefreshedEvent extends ApplicationContextEvent{
    /**
     * Constructs a prototypical Event.
     *
     * @param source The object on which the Event initially occurred.
     * @throws IllegalArgumentException if source is null.
     */
    public ContextRefreshedEvent(Object source) {
        super(source);
    }

}

我们可以通过继承该类来实现，特定的事件类型需求，比如要实现一个邮件发送事件。只需要继承ApplicationContextEvent即可：

public class MailSendEvent extends ApplicationContextEvent {
    private String msg;

    public MailSendEvent(Object source, String msg) {
        super(source);
        this.msg = msg;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }
}

同时ApplicationContextEvent也有特定的几个子类，来表示容器启动、刷新、停止以及关闭事件：

2.事件监听器

事件监听器接口中，只定义了一个方法：onApplicationEvent(E event)该方法接收ApplicationEvent事件对象，在该方法中编写事件的响应处理逻辑。

public interface ApplicationListener extends EventListener {

    /**
     * 接收ApplicationEvent 事件对象
     * 在该方法中编写事件的响应处理逻辑
     * @param event
     */
    void onApplicationEvent(E event);
}

我们同样也可以实现该接口来实现特定的事件监听器功能，比如邮件发送的监听器：

public class MailSenderListener implements ApplicationListener {

    @Override
    public void onApplicationEvent(MailSendEvent event) {
        System.out.println("邮件发送器的 resource:" + event.getSource() + "邮件发送器的 msg:" + event.getMsg());
    }
}

3.事件广播器

事件广播器负责将事件通知监听器注册表中的事件监听器，然后再由事件监听器分别对事件进行响应。Spring中定义了如下接口：

public interface ApplicationEventMulticaster {

    /**
     * 添加事件监听器
     * @param listener
     */
    void addApplicationListener(ApplicationListener listener);

    /**
     * 移除事件监听器
     * @param listener
     */
    void removeApplicationListener(ApplicationListener listener);

    /**
     * 广播事件
     * @param event
     */
    void multicastEvent(ApplicationEvent event);
}

及其简单实现类SimpleApplicationEventMulticaster：

public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster{

    public SimpleApplicationEventMulticaster(BeanFactory beanFactory) {
        setBeanFactory(beanFactory);
    }
    /**unchecked 表示告诉编译器忽略指定的警告，不用再编译完成后出现警告信息*/
    @SuppressWarnings("unchecked")
    @Override
    public void multicastEvent(ApplicationEvent event) {
        for (ApplicationListener applicationListener : getApplicationListeners(event)) {
            applicationListener.onApplicationEvent(event);
        }
    }
}

4.事件发布者

它本身作为事件源，会在合适的时点，将相应事件发布给对应的事件监听器：

public interface ApplicationEventPublisher {

    /**
     * 通知监听者并发布事件
     * @param event
     */
    void publishEvent(ApplicationEvent event);
}

在Spring容器事件中，ApplicationContext接口定义继承了ApplicationEventPublisher接口，所以实际上AbstractApplicationContext在事件中承担了事件发布者的角色。
但是在实际上具体实现事件的发布和事件监听器注册方面，将功能转接给ApplicationEventMulticaster接口，最终具体实现则放在AbstractApplicationEventMulticaster的实现类中：

Spring 事件类的应用

那么在Spring中，事件类到底是如何运行的呢？首先我们会在xml配置文件中配置相应的ApplicationListener类型的监听器，因此在容器启动后，这些类型的bean会被ApplicationContext容器所识别，它们负责监听容器内发布的对应的ApplicationEvent类型的事件。

在AbstractApplicationContext的refresh()方法中可以看到自动注册的内容：

public void refresh() throws BeansException {

        // 6. 初始化事件发布者
        initApplicationEventMulticaster();

        // 7. 注册事件监听器
        registerListeners();

        // 9. 发布容器刷新完成事件
        finishRefresh();
}

private void initApplicationEventMulticaster() {
    ConfigurableListableBeanFactory beanFactory = getBeanFactory();
    applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
    beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, applicationEventMulticaster);
}

private void registerListeners() {
    Collection applicationListeners = getBeansOfType(ApplicationListener.class).values();
    for (ApplicationListener listener : applicationListeners) {
        applicationEventMulticaster.addApplicationListener(listener);
    }
}

private void finishRefresh() {
    publishEvent(new ContextRefreshedEvent(this));
}
public void publishEvent(ApplicationEvent event) {
    applicationEventMulticaster.multicastEvent(event);
}

所以在ApplicationContext容器启动时，会自动注册EventListener类型的 Bean，一旦检测到有ApplicationContextEvent类型的事件发布，将通知这些注册到容器的EventListener

应用实例

下面将构建一个发送邮件的Spring事件实例：

1. 邮件发送事件MailSendEvent

public class MailSendEvent extends ApplicationContextEvent {
    private String msg;

    public MailSendEvent(Object source, String msg) {
        super(source);
        this.msg = msg;
    }

    public String getMsg() {
        return msg;
    }
}

2.邮件发送事件监听器MailSendListener(邮件发送事件)、ContextRefreshedEventListener(容器刷新事件) 和 ContextClosedEventListener(容器关闭事件)

public class MailSenderListener implements ApplicationListener {

    @Override
    public void onApplicationEvent(MailSendEvent event) {
        System.out.println("邮件发送器的 resource:" + event.getSource() + "邮件发送器的 msg:" + event.getMsg());
    }
}

public class ContextClosedEventListener implements ApplicationListener {

    @Override
    public void onApplicationEvent(ContextClosedEvent event) {
        System.out.println("关闭事件：" + this.getClass().getName());
    }
}

public class ContextRefreshedEventListener implements ApplicationListener {

    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        System.out.println("刷新/打开事件：" + this.getClass().getName());
    }
}

这时，将监听器们注入xml文件中：

3.邮件发送事件发布者

事件发布者ApplicationEventPublisher，因为前面提到，applicationContext继承了ApplicationEventPublisher，而applicationContext将事件发布功能委托给了ApplicationEventMulticaster，容器在启动开始就会检查是否存在名称为applicationEventMulticaster的 ApplicationEventMulticaster对象实例，如果有就使用提供的实现，没有则默认初始化一个SimpleApplicationEventMulticaster作为将会使用的ApplicationEventMulticaster

/**
 * @description: 实现了事件监听器的管理功能
 * @author: wjw
 * @date: 2022/7/9
 */
public abstract class AbstractApplicationEventMulticaster implements ApplicationEventMulticaster, BeanFactoryAware  {

    public final Set applicationListeners = new LinkedHashSet();

    private BeanFactory beanFactory;

    @Override
    public void addApplicationListener(ApplicationListener listener) {
        applicationListeners.add((ApplicationListener) listener);
    }

    @Override
    public void removeApplicationListener(ApplicationListener listener) {
        applicationListeners.remove(listener);
    }

    @Override
    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
        this.beanFactory = beanFactory;
    }

    /**
     * 获得监听器
     * @param event
     * @return
     */
    protected Collection getApplicationListeners(ApplicationEvent event) {
        LinkedList allListeners = new LinkedList();
        for (ApplicationListener listener : allListeners) {
            if (supportsEvent(listener, event)) {
                allListeners.add(listener);
            }
        }
        return allListeners;
    }

    protected boolean supportsEvent(ApplicationListener applicationListener, ApplicationEvent event) {
        Class targetClass = ClassUtils.isCglibProxyClass(listenerClass) ? listenerClass.getSuperclass() : listenerClass;
        Type genericInterface = targetClass.getGenericInterfaces()[0];

        Type actualTypeArgument = ((ParameterizedType) genericInterface).getActualTypeArguments()[0];
        String className = actualTypeArgument.getTypeName();
        Class eventClassName;
        try {
            eventClassName = Class.forName(className);
        } catch (ClassNotFoundException e) {
            throw new BeansException("wrong event class name: " + className);
        }

        return eventClassName.isAssignableFrom(event.getClass());
    }

}

public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster{

    public SimpleApplicationEventMulticaster(BeanFactory beanFactory) {
        setBeanFactory(beanFactory);
    }
    /**unchecked 表示告诉编译器忽略指定的警告，不用再编译完成后出现警告信息*/
    @SuppressWarnings("unchecked")
    @Override
    public void multicastEvent(ApplicationEvent event) {
        for (ApplicationListener applicationListener : getApplicationListeners(event)) {
            applicationListener.onApplicationEvent(event);
        }
    }
}

4.测试验证

public void test_event() {
    ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath:spring.xml");

    applicationContext.publishEvent(new CustomEvent(applicationContext, 110L, "test！"));

    System.out.println("-----------------------------------------------------------------");
    applicationContext.publishEvent(new MailSendEvent(applicationContext, "邮件发送测试"));
    applicationContext.registerShutdownHook();
}

刷新/打开事件：cn.ethan.springframework.test.event.ContextRefreshedEventListener$$EnhancerByCGLIB$$2e5c458
-----------------------------------------------------------------
邮件发送器的 resource:cn.ethan.springframework.context.support.ClassPathXmlApplicationContext@5f2050f6邮件发送器的 msg:邮件发送测试
关闭事件：cn.ethan.springframework.test.event.ContextClosedEventListener$$EnhancerByCGLIB$$fbc2c978