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Spring学习笔记(4)Spring 事件原理及其应用

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在 JDK 中已经提供相应的自定义事件发布功能的基础类:

首先了解几个概念:

img

Spring 事件类结构

img

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。

img

1.1 ApplicationEvent

首先是继承 EventObjectApplicationEvent,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

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 <code>ApplicationContext</code> 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:

img

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<E extends ApplicationEvent> 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<MailSendEvent> {

    @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:

img

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:

img

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。

<bean class="cn.ethan.springframework.test.event.ContextRefreshedEventListener"/>
<bean class="cn.ethan.springframework.test.event.MailSenderListener"/>
<bean class="cn.ethan.springframework.test.event.ContextClosedEventListener"/>

AbstractApplicationContextrefresh()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<ApplicationListener> 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<MailSendEvent> {

    @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<ContextClosedEvent> {

    @Override
    public void onApplicationEvent(ContextClosedEvent event) {
        System.out.println("关闭事件:" + this.getClass().getName());
    }
}
public class ContextRefreshedEventListener implements ApplicationListener<ContextRefreshedEvent> {

    @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:

<bean class="cn.ethan.springframework.test.event.ContextRefreshedEventListener"/>
<bean class="cn.ethan.springframework.test.event.MailSenderListener"/>
<bean class="cn.ethan.springframework.test.event.ContextClosedEventListener"/>

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applicationEventMulticasterApplicationEventMulticaster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<ApplicationListener<ApplicationEvent>> applicationListeners = new LinkedHashSet<>();

    private BeanFactory beanFactory;

    @Override
    public void addApplicationListener(ApplicationListener<?> listener) {
        applicationListeners.add((ApplicationListener<ApplicationEvent>) 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<ApplicationListener> getApplicationListeners(ApplicationEvent event) {
        LinkedList<ApplicationListener> allListeners = new LinkedList<>();
        for (ApplicationListener<ApplicationEvent> listener : allListeners) {
            if (supportsEvent(listener, event)) {
                allListeners.add(listener);
            }
        }
        return allListeners;
    }

    protected boolean supportsEvent(ApplicationListener<ApplicationEvent> applicationListener, ApplicationEvent event) {
        Class<? extends ApplicationListener> listenerClass = applicationListener.getClass();

        /**根据不同实例化类型,判断后获取对应目标 class*/
        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

标签:Spring,void,笔记,public,source,事件,监听器,原理,event
来源: https://www.cnblogs.com/EthanWong/p/16465195.html