Spring Event事件驱动

Spring事件驱动模型,简单来说类似于Message-Queue消息队列中的Pub/Sub发布/订阅模式,也类似于Java设计模式中的观察者模式。

自定义事件

Spring的事件接口位于org.springframework.context.ApplicationEvent,源码如下:

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public abstract class ApplicationEvent extends EventObject {
private static final long serialVersionUID = 7099057708183571937L;
private final long timestamp;
public ApplicationEvent(Object source) {
super(source);
this.timestamp = System.currentTimeMillis();
}
public final long getTimestamp() {
return this.timestamp;
}
}

继承了Java的事件对象EventObject,所以可以使用getSource()方法来获取到事件传播对象。

自定义Spring事件

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public class CustomSpringEvent extends ApplicationEvent {
private String message;

public CustomSpringEvent(Object source, String message) {
super(source);
this.message = message;
}

public String getMessage() {
return message;
}
}

然后定义事件监听器,该监听器实际上等同于消费者,需要交给Spring容器管理。

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@Component
public class CustomSpringEventListener implements ApplicationListener<CustomSpringEvent> {
@Override
public void onApplicationEvent(CustomSpringEvent event) {
System.out.println("Received spring custom event - " + event.getMessage());
}
}

最后定义事件发布者

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@Component
public class CustomSpringEventPublisher {
@Autowired
private ApplicationEventPublisher applicationEventPublisher;

public void doStuffAndPublishAnEvent(final String message) {
System.out.println("Publishing custom event. ");
CustomSpringEvent customSpringEvent = new CustomSpringEvent(this, message);
applicationEventPublisher.publishEvent(customSpringEvent);
}
}

创建测试类

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@RunWith(SpringRunner.class)
@SpringBootTest
public class CustomSpringEventPublisherTest {

@Autowired
private CustomSpringEventPublisher publisher;

@Test
public void publishStringEventTest() {
publisher.doStuffAndPublishAnEvent("111");
}
}

运行测试类,可以看到控制台打印了两条重要信息

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//发布事件
Publishing custom event.
//监听器得到了事件,并相应处理
Received spring custom event - 111

由于Spring事件是发布/订阅的模式,而发布订阅模式有以下三种情况

  1. 1生产者 - 1消费者
  2. 1生产者 - 多消费者
  3. 多生产者 - 多消费者

上面举的例子是第一种情况,我们来试试其他两个情况

继续创建一个事件监听器作为消费者:

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@Component
public class CustomSpringEventListener2 implements ApplicationListener<CustomSpringEvent> {
@Override
public void onApplicationEvent(CustomSpringEvent event) {
System.out.println("CustomSpringEventListener2 Received spring custom event - " + event.getMessage());
}
}

运行测试类后,可以观察到,控制台顺序打印了两条消费信息:

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Publishing custom event. 
CustomSpringEventListener1 Received spring custom event - 111
CustomSpringEventListener2 Received spring custom event - 111

说明,Spring的发布订阅模式是广播模式,所有消费者都能接受到消息,并正常消费

再试试第三种多生产者 - 多消费者的情况

继续创建一个发布者,

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@Component
public class CustomSpringEventPublisher2 {
@Autowired
private ApplicationEventPublisher applicationEventPublisher;

public void doStuffAndPublishAnEvent(final String message) {
System.out.println("CustomSpringEventPublisher2 Publishing custom event. ");
CustomSpringEvent customSpringEvent = new CustomSpringEvent(this, message);
applicationEventPublisher.publishEvent(customSpringEvent);
}
}

控制台输出:

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CustomSpringEventPublisher Publishing custom event. 
CustomSpringEventListener1 Received spring custom event - 111
CustomSpringEventListener2 Received spring custom event - 111
CustomSpringEventPublisher2 Publishing custom event.
CustomSpringEventListener1 Received spring custom event - 222
CustomSpringEventListener2 Received spring custom event - 222

从以上输出内容,我们可以猜测到,Spring的事件发布订阅机制是同步进行的,也就是说,事件必须被所有消费者消费完成之后,发布者的代码才能继续往下走,这显然不是我们想要的效果,那有没有异步执行的事件呢?

Spring中的异步事件

要使用Spring 的异步事件,我们需要自定义异步事件配置类

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@Configuration
public class AsynchronousSpringEventsConfig {
@Bean(name = "applicationEventMulticaster")
public ApplicationEventMulticaster simpleApplicationEventMulticaster() {
SimpleApplicationEventMulticaster eventMulticaster
= new SimpleApplicationEventMulticaster();

eventMulticaster.setTaskExecutor(new SimpleAsyncTaskExecutor());
return eventMulticaster;
}
}

发布和订阅的代码不用变动,直接运行测试类,控制台将打印出:

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CustomSpringEventPublisher Publishing custom event. 
CustomSpringEventPublisher2 Publishing custom event.
CustomSpringEventListener1 Received spring custom event - 111
CustomSpringEventListener2 Received spring custom event - 111
CustomSpringEventListener2 Received spring custom event - 222
CustomSpringEventListener1 Received spring custom event - 222

可以看到,两个发布者几乎同时运行,证明监听器是异步执行的,没有阻塞住发布者的代码。准确的说,监听器将在一个单独的线程中异步处理事件。

Spring自带的事件类型

事件驱动在Spring中是被广泛采用的,我们查看ApplicationEvent的子类可以发现许多Event事件,在此不赘述。

注解驱动的监听器

从Spring 4.2开始,事件监听器不需要是实现ApplicationListener接口的bean,它可以通过@EventListener注解在任何被Spring容器管理的bean的公共方法上。

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@Component
public class AnnotationDrivenContextStartedListener {
@EventListener
public void handleContextStart(CustomSpringEvent cse) {
System.out.println("Handling Custom Spring Event.");
}
}

控制台输出结果:

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CustomSpringEventPublisher Publishing custom event.
Handling Custom Spring Event.
CustomSpringEventPublisher2 Publishing custom event.
Handling Custom Spring Event.

同样的,我们可以看出,这个事件监听器是同步执行的,如果要改为异步监听器,在事件方法上加上@Async,并且在Spring应用中开启异步支持(在SpringBootApplication上添加@EnableAsync)。

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@Component
public class AnnotationDrivenContextStartedListener {
@Async
@EventListener
public void handleContextStart(CustomSpringEvent cse) {
System.out.println("Handling Custom Spring Event.");
}
}

再次运行测试类:

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CustomSpringEventPublisher Publishing custom event. 
CustomSpringEventPublisher2 Publishing custom event.
Handling Custom Spring Event.
Handling Custom Spring Event.

泛型支持

创建一个通用泛型事件模型

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@Data
public class GenericSpringEvent<T> {
private T message;
protected boolean success;

public GenericSpringEvent(T what, boolean success) {
this.message = what;
this.success = success;
}
}

注意GenericSpringEventCustomSpringEvent之间的区别。我们现在可以灵活地发布任何任意事件,并且不再需要从ApplicationEvent扩展。

这样的话,我们无法像之前一样,通过继承ApplicationListener的方式来定义一个监听器,因为ApplicationListener定义了事件必须是ApplicationEvent的子类。所以,我们只能使用注解驱动的监听器。

通过在@EventListener注释上定义布尔SpEL表达式,也可以使事件监听器成为条件。在这种情况下,只会为成功的String的GenericSpringEvent调用事件处理程序:

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@Component
public class AnnotationDrivenEventListener {
@EventListener(condition = "#event.success")
public void handleSuccessful(GenericSpringEvent<String> event) {
System.out.println("Handling generic event (conditional).");
}
}

定义具体类型的事件:

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public class StringGenericSpringEvent extends GenericSpringEvent<String> {
public StringGenericSpringEvent(String message, boolean success) {
super(message, success);
}
}

定义发布者:

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@Component
public class StringGenericSpringEventPublisher {
@Autowired
private ApplicationEventPublisher applicationEventPublisher;

public void doStuffAndPublishAnEvent(final String message, final boolean success) {
System.out.println("CustomSpringEventPublisher Publishing custom event. ");
StringGenericSpringEvent springEvent = new StringGenericSpringEvent(message, success);
applicationEventPublisher.publishEvent(springEvent);
}
}

测试类:

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@RunWith(SpringRunner.class)
@SpringBootTest
public class CustomSpringEventPublisherTest {

@Autowired
private StringGenericSpringEventPublisher publisher;

@Test
public void publishStringEventTest() {
publisher.doStuffAndPublishAnEvent("success", true);
publisher.doStuffAndPublishAnEvent("failed", false);
}
}

运行结果:

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CustomSpringEventPublisher Publishing custom event. 
Handling generic event (conditional) success
CustomSpringEventPublisher Publishing custom event.

监听器只处理了成功的事件,成功忽略掉了失败的事件。这样的好处是,可以为同一个事件定义成功和失败不同的操作。

Spring事件的事务绑定

从Spring 4.2开始,框架提供了一个新的@TransactionalEventListener注解,它是@EventListener的扩展,允许将事件的侦听器绑定到事务的一个阶段。绑定可以进行以下事务阶段:

  • AFTER_COMMIT(默认的):在事务成功后触发
  • AFTER_ROLLBACK:事务回滚时触发
  • AFTER_COMPLETION:事务完成后触发,不论是否成功
  • BEFORE_COMMIT:事务提交之前触发

总结

  1. Spring中处理事件的基础知识:创建一个简单的自定义事件,发布它,然后在监听器中处理它。
  2. 在配置中启用事件的异步处理。
  3. Spring 4.2中引入的改进,例如注释驱动的侦听器,更好的泛型支持以及绑定到事务阶段的事件。

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