【Spring源码】Spring Event事件

/**
 * 这里定义事件DamIllegalDataEvent。
 */
public class DamIllegalDataEvent extends ApplicationEvent {
    // 声明构造函数，接收DamIllegalDataDto集合传递到事件中
    public DamIllegalDataEvent(List<DamIllegalDataDto> list) {
        super(list);
    }
}

3.2、发布事件

发布事件时可以注入Appli cationEventPublisher，也可以获取到Appli cationContext，然后调用 publisherEvent()方法推送事件。

@RestController
@RequestMapping("anno/dam")
public class DamTestController {
    @Autowired
    private ApplicationEventPublisher applicationPushBuilder;
    
    @GetMapping("test_audit")
    public String test_audit(){
        DamIllegalDataDto build = DamIllegalDataDto.builder().illegalData("11111").source("2222").functionDesc("数据清理中错误了").functionName("333").build();
        // 注入applicationPushBuilder
        applicationPushBuilder.publishEvent(new DamIllegalDataEvent(Collections.singletonList(build)));
        // 这里也可以直接使用hutool工具类直接发布
        SpringUtil.publishEvent(new DamIllegalDataEvent(Collections.singletonList(build)));
        return "ok";
    }
}

3.3、监听事件

监听事件也可称为订阅事件，即当事件发布了之后，需要监听该事件并进行消费。Spring里面提供了两种事件订阅的方式：

继承Appli cationListener，并实现onAppli cationEvent方法。
使用@EventListener注解方法。

3.3.1、继承Appli cationListener

创建一个监听器DamIllegalDataEventListener继承ApplicationListener，通过泛型指定需要监听的事件类。如：

@Slf4j
@Component
public class DamIllegalDataEventListener implements ApplicationListener<DamIllegalDataEvent> {

    @Autowired
    private DamIllegalDataAuditService damIllegalDataAuditService;

    @Override
    public void onApplicationEvent(DamIllegalDataEvent event) {
        LOGGER.info("异常数据审计事件开始执行...");
        List<DamIllegalDataDto> damIllegalDataDtos = (List<DamIllegalDataDto>) event.getSource();
        // todo......
        doSomething();
    }
}

3.3.2、使用@EventListener注解

使用@EventListener注解方法，将其包装为事件处理器。它适用于：1. 不想为每个事件处理都创建一个Applica tionListener实现类；2. 希望支持更复杂的事件条件过滤。@EventListener的c lasses属性可以过滤事件类型，而condi tion属性可以根据事件对象是否满足条件表达式来过滤事件。

@Slf4j
@Component
public class DamIllegalDataEventListener {

    /**
     * EventListener注解定义事件处理器，并指定监听事件为DamIllegalDataEvent。
     * condition声明只有事件的code==200时，才进入该事件
     */
    @EventListener(classes = {DamIllegalDataEvent.class}, condition="#event.code==200")
    public void onApplicationEvent(DamIllegalDataEvent event) {
        LOGGER.info("异常数据审计事件开始执行...");
        List<DamIllegalDataDto> damIllegalDataDtos = (List<DamIllegalDataDto>) event.getSource();
        // todo......
        doSomething();
    }
}

4、Spring Event是同步 还是 异步？

默认情况下 Spring Event是同步执行的。你怎么这么确定？我们先来演示下上面的demo。先实现一个测试接口，该接口发布了一个事件，发布完后打印一行日志：

@GetMapping("test_audit")
public String test_audit(){
    DamIllegalDataDto build = DamIllegalDataDto.builder().illegalData("11111").source("2222").functionDesc("数据清理中错误了").functionName("333").build();
    SpringUtil.publishEvent(new DamIllegalDataEvent(Collections.singletonList(build)));
    System.out.println("接口请求完成......");
    return "ok";
}

事件监听中打印一行日志，并睡眠5s：

@Slf4j
@Component
public class DamIllegalDataEventListener implements ApplicationListener<DamIllegalDataEvent> {

    @Override
    public void onApplicationEvent(DamIllegalDataEvent event) {
        LOGGER.info("异常数据审计事件开始执行...");
        ThreadUtil.sleep(5000);
    }
}

执行查看结果，可以发现不管如何请求，日志打印总是按顺序执行，并且会间隔5S。

4.1、源码 实现

如果还是不信？那我们来看源码：org.springframework.context.ApplicationEventPublisher#publishEvent(java.lang.Object)，断点跟进到org.springframework.context.support.AbstractApplicationContext#publishEvent(java.lang.Object, org.springframework.core.ResolvableTy pe)。

protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
    // 包装ApplicationEvent
    ApplicationEvent applicationEvent;
    if (event instanceof ApplicationEvent) {
        applicationEvent = (ApplicationEvent) event;
    }
    else {
        applicationEvent = new PayloadApplicationEvent<>(this, event);
        if (eventType == null) {
            eventType = ((PayloadApplicationEvent<?>) applicationEvent).getResolvableType();
        }
    }
    // 考虑到部分事件在Listener注册之前就发布了，因此先保存起来
    if (this.earlyApplicationEvents != null) {
        this.earlyApplicationEvents.add(applicationEvent);
    }
    else {
        // 重点是这里
        // 铜通过getApplicationEventMulticaster()获取事件发布器；
        // 调用multicastEvent方法发布事件
        getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
    }
    // 同时给父容器发布事件
    if (this.parent != null) {
        if (this.parent instanceof AbstractApplicationContext) {
            ((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
        }
        else {
            this.parent.publishEvent(event);
        }
    }
}

跟进multi castEvent()方法，org.springframework.context.event.SimpleApplicationEventMulticaster#multi castEvent(org.springframework.context.ApplicationEvent, org.springframework.core.ResolvableType)：

@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
    ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
    Executor executor = getTaskExecutor();
    for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
        // 这里可以看出，如果有指定任务执行器，那么就异步执行；否则直接调用，也就是同步执行。
       if (executor != null) {
          executor.execute(() -> invokeListener(listener, event));
       }
       else {
          invokeListener(listener, event);
       }
    }
}

4.2、如何实现异步

实现异步方式，可以有3中实现：

使用@Async 注解
手动实现异步线程池
自定义ApplicationEventMulticaster

4.2.1、使用@Async注解

使用这个很简单，只要在事件监听方法上添加@Async注解即可，springboot的启动器需要开启异步@EnableAsync。

@Async
@Override
public void onApplicationEvent(DamIllegalDataEvent event) {
    LOGGER.info("异常数据审计事件开始执行...");
    ThreadUtil.sleep(5000);
}

注意：

使用@Async时，最好自己配置相应的线程池核心数以及延迟队列等等。由于Spring中使用@Async异步线程每次都会创建一个新线程执行，如果滥用它，可能会有内存问题。

4.2.2、手动实现异步 线程池

顾名思义就是手动创建一个线程池执行，与@Async类似。

@Slf4j
@Component
public class DamIllegalDataEventListener implements ApplicationListener<DamIllegalDataEvent> {

    @Override
    public void onApplicationEvent(DamIllegalDataEvent event) {
        ThreadUtil.execAsync(() -> {
            LOGGER.info("异常数据审计事件开始执行...");
            ThreadUtil.sleep(5000);
        });
    }
}

4.2.3、自定义ApplicationEventMulticaster

由于Spring容器会优先使用beanName为applicationEventMulticater 的bean作为事件转发处理器，如果不存在则默认使用SimpleApplicationEventMulticaster作为事件转发处理器，它默认是同步执行的。但它支持设置Executor，那么我们可以将自定义的线程池处理器作为Executor，以此来支持异步执行。

@Configuration
public class DamEventConfig {

    @Bean(AbstractApplicationContext.APPLICATION_EVENT_MULTICASTER_BEAN_NAME)
    public SimpleApplicationEventMulticaster eventMulticaster(){
        SimpleApplicationEventMulticaster simpleApplicationEventMulticaster = new SimpleApplicationEventMulticaster();
        simpleApplicationEventMulticaster.setTaskExecutor(taskExecutor());
        return simpleApplicationEventMulticaster;
    }

    /**
     * 目前服务器为8c，默认给他4个，一般事件推送的情况不会多。如果多的话，请检查一下业务使用
     * @return
     */
    @Bean
    public TaskExecutor taskExecutor(){
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(4);
        return executor;
    }
}

配置完之后，事件监听那边都无需修改。

注意：

这种方式的配置是全局性的，一旦配置了之后，所有的事件都是异步的形式处理。如果需要个别业务是同步的，那么此种方式要特别注意。

5、@TransactionalEventListener

提到事件，这里再提一个注解@TransactionalEventListener，也即感知事务，基于事件形式与事务的某个阶段进行绑定。比如在事务提交之前或之后进行一些业务的处理，如短信提醒等等。@TransactionEventListener允许事件处理方法感知事务。它的phase属性，表示希望在事务的哪个阶段执行事件处理。

@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@EventListener
public @interface TransactionalEventListener {

    /**
     * Phase to bind the handling of an event to.
     * <p>The default phase is {@link TransactionPhase#AFTER_COMMIT}.
     * <p>If no transaction is in progress, the event is not processed at
     * all unless {@link #fallbackExecution} has been enabled explicitly.
     */
    TransactionPhase phase() default TransactionPhase.AFTER_COMMIT;

    /**
     * Whether the event should be handled if no transaction is running.
     */
    boolean fallbackExecution() default false;

    /**
     * Alias for {@link #classes}.
     */
    @AliasFor(annotation = EventListener.class, attribute = "classes")
    Class<?>[] value() default {};

    /**
     * The event classes that this listener handles.
     * <p>If this attribute is specified with a single value, the annotated
     * method may optionally accept a single parameter. However, if this
     * attribute is specified with multiple values, the annotated method
     * must <em>not</em> declare any parameters.
     */
    @AliasFor(annotation = EventListener.class, attribute = "classes")
    Class<?>[] classes() default {};

    /**
     * Spring Expression Language (SpEL) attribute used for making the event
     * handling conditional.
     * <p>The default is {@code ""}, meaning the event is always handled.
     * @see EventListener#condition
     */
    @AliasFor(annotation = EventListener.class, attribute = "condition")
    String condition() default "";

    /**
     * An optional identifier for the listener, defaulting to the fully-qualified
     * signature of the declaring method (e.g. "mypackage.MyClass.myMethod()").
     * @since 5.3
     * @see EventListener#id
     * @see TransactionalApplicationListener#getListenerId()
     */
    @AliasFor(annotation = EventListener.class, attribute = "id")
    String id() default "";

}

TransactionPhase枚举声明了事务提交的各个阶段：

public enum TransactionPhase {

    /**
     * Handle the event before transaction commit.
     * @see TransactionSynchronization#beforeCommit(boolean)
     */
    BEFORE_COMMIT,

    /**
     * Handle the event after the commit has completed successfully.
     * <p>Note: This is a specialization of {@link #AFTER_COMPLETION} and therefore
     * executes in the same sequence of events as {@code AFTER_COMPLETION}
     * (and not in {@link TransactionSynchronization#afterCommit()}).
     * <p>Interactions with the underlying transactional resource will not be
     * committed in this phase. See
     * {@link TransactionSynchronization#afterCompletion(int)} for details.
     * @see TransactionSynchronization#afterCompletion(int)
     * @see TransactionSynchronization#STATUS_COMMITTED
     */
    AFTER_COMMIT,

    /**
     * Handle the event if the transaction has rolled back.
     * <p>Note: This is a specialization of {@link #AFTER_COMPLETION} and therefore
     * executes in the same sequence of events as {@code AFTER_COMPLETION}.
     * <p>Interactions with the underlying transactional resource will not be
     * committed in this phase. See
     * {@link TransactionSynchronization#afterCompletion(int)} for details.
     * @see TransactionSynchronization#afterCompletion(int)
     * @see TransactionSynchronization#STATUS_ROLLED_BACK
     */
    AFTER_ROLLBACK,

    /**
     * Handle the event after the transaction has completed.
     * <p>For more fine-grained events, use {@link #AFTER_COMMIT} or
     * {@link #AFTER_ROLLBACK} to intercept transaction commit
     * or rollback, respectively.
     * <p>Interactions with the underlying transactional resource will not be
     * committed in this phase. See
     * {@link TransactionSynchronization#afterCompletion(int)} for details.
     * @see TransactionSynchronization#afterCompletion(int)
     */
    AFTER_COMPLETION
}

5.1、基本使用

在含有事务的方法里发布事件：

@Transactional(rollbackFor = Exception.class)
public void test(){
    DamIllegalDataAudit audit = new DamIllegalDataAudit();
    audit.setId("1726931543097610240");
    audit.setRemark("xxx");

    this.baseMapper.updateById(audit);
    DamIllegalDataDto build = DamIllegalDataDto.builder().illegalData("11111").source("2222").functionDesc("数据清理中错误了").functionName("333").build();
    applicationEventPublisher.publishEvent(new DamIllegalDataEvent(Collections.singletonList(build)));
}

定义感知事务监听：

@Component
public class TransactionalEventProcess {

    @TransactionalEventListener(phase = TransactionPhase.AFTER_COMMIT)
    public void afterCommit(DamIllegalDataEvent event){
        System.out.println("事务提交后事件处理");
    }

    @TransactionalEventListener(phase = TransactionPhase.AFTER_ROLLBACK)
    public void afterRollback(DamIllegalDataEvent event){
        System.out.println("事务回滚后事件处理");
    }
}