前言
最近在做公司的业务需要用到事件通知，比如启动成功打印日志，通知其他业务做相应的操作，就用到了Spring的事件通知机制。
1. Spring的事件通知
Spring的事件通知本质上就是发布-订阅，即生产者-消费者；体现了观察者设计模式或者回调通知，那么Spring的事件是如何使用的？有3要素：发布者-->事件-->监听者
2. Spring事件通知使用
Spring的事件必须依赖Spring容器，所以我在写demo的时候，需要Spring-boot-starter。
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter</artifactId>
<version>2.1.1.RELEASE</version>
</dependency>
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.18.4</version>
</dependency>
</dependencies>2.1 Spring的事件
Spring的事件定义在ApplicationEvent类中，我们可以根据自己的需要自定义事件实体javabean。
@Data
public class TestSpringEvent extends ApplicationEvent {
/**
* Create a new ApplicationEvent.
*
* @param source the object on which the event initially occurred (never {@code null})
*/
public TestSpringEvent(Object source) {
super(source);
}
private Integer code;
private String message;
}跟踪源码：
public abstract class ApplicationEvent extends EventObject {
/** use serialVersionUID from Spring 1.2 for interoperability. */
private static final long serialVersionUID = 7099057708183571937L;
/** System time when the event happened. */
private final long timestamp;
/**
* Create a new ApplicationEvent.
* @param source the object on which the event initially occurred (never {@code null})
*/
public ApplicationEvent(Object source) {
super(source);
this.timestamp = System.currentTimeMillis();
}
/**
* Return the system time in milliseconds when the event happened.
*/
public final long getTimestamp() {
return this.timestamp;
}
}可以看出ApplicationEvent有属性时间戳，即事件的产生的时间，并有Object source属性，进一步跟踪
public class EventObject implements java.io.Serializable {
private static final long serialVersionUID = 5516075349620653480L;
/**
* The object on which the Event initially occurred.
*/
protected transient Object source;
/**
* Constructs a prototypical Event.
*
* @param source The object on which the Event initially occurred.
* @exception IllegalArgumentException if source is null.
*/
public EventObject(Object source) {
if (source == null)
throw new IllegalArgumentException("null source");
this.source = source;
}
/**
* The object on which the Event initially occurred.
*
* @return The object on which the Event initially occurred.
*/
public Object getSource() {
return source;
}
/**
* Returns a String representation of this EventObject.
*
* @return A a String representation of this EventObject.
*/
public String toString() {
return getClass().getName() + "[source=" + source + "]";
}
}定义了一个事件发生的不能序列化的对象
2.2 事件监听
Spring事件的监听由ApplicationListener定义
@Component
public class TestApplicationListener implements ApplicationListener<TestSpringEvent> {
@Async
public void onApplicationEvent(TestSpringEvent event) {
System.out.println("code is:\t" + event.getCode() + ",\tmessage is:\t" + event.getMessage());
}
}跟踪代码，可以看出ApplicationListener是一个函数式接口
@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
/**
* Handle an application event.
* @param event the event to respond to
*/
void onApplicationEvent(E event);
}Spring事件监听器的默认是单线程同步执行，异步执行需要加上Spring的@Async注解，或者自定义线程池实现
2.3 事件发布
Spring的事件的发布定义在ApplicationEventPublisher类中，而ApplicationContext继承了这个类，因此ApplicationContext具有发布事件的能力，而且已经注入Spring的bean容器中，可直接获取。
public class SpringEventMain {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
context.scan(SpringEventMain.class.getCanonicalName());
context.refresh();
TestSpringEvent testSpringEvent = new TestSpringEvent("Spring test event");
testSpringEvent.setCode(10000);
testSpringEvent.setMessage("---------My Spring task event------------");
context.publishEvent(testSpringEvent);
}
}运行后打印如下日志：
/software/jdk1.8.0_191/bin/java -javaagent:/software/ideaIde/lib/idea_rt.jar=44651:/software/ideaIde/bin -Dfile.encoding=UTF-8 -classpath /software/jdk1.8.0_191/jre/lib/charsets.jar:/software/jdk1.8.0_191/jre/lib/deploy.jar:/software/jdk1.8.0_191/jre/lib/ext/cldrdata.jar:/software/jdk1.8.0_191/jre/lib/ext/dnsns.jar:/software/jdk1.8.0_191/jre/lib/ext/jaccess.jar:/software/jdk1.8.0_191/jre/lib/ext/jfxrt.jar:/software/jdk1.8.0_191/jre/lib/ext/localedata.jar:/software/jdk1.8.0_191/jre/lib/ext/nashorn.jar:/software/jdk1.8.0_191/jre/lib/ext/sunec.jar:/software/jdk1.8.0_191/jre/lib/ext/sunjce_provider.jar:/software/jdk1.8.0_191/jre/lib/ext/sunpkcs11.jar:/software/jdk1.8.0_191/jre/lib/ext/zipfs.jar:/software/jdk1.8.0_191/jre/lib/javaws.jar:/software/jdk1.8.0_191/jre/lib/jce.jar:/software/jdk1.8.0_191/jre/lib/jfr.jar:/software/jdk1.8.0_191/jre/lib/jfxswt.jar:/software/jdk1.8.0_191/jre/lib/jsse.jar:/software/jdk1.8.0_191/jre/lib/management-agent.jar:/software/jdk1.8.0_191/jre/lib/plugin.jar:/software/jdk1.8.0_191/jre/lib/resources.jar:/software/jdk1.8.0_191/jre/lib/rt.jar:/home/huahua/IdeaProjects/feng/spring-event/target/classes:/home/huahua/.m2/repository/org/springframework/boot/spring-boot-starter/2.1.1.RELEASE/spring-boot-starter-2.1.1.RELEASE.jar:/home/huahua/.m2/repository/org/springframework/boot/spring-boot/2.1.1.RELEASE/spring-boot-2.1.1.RELEASE.jar:/home/huahua/.m2/repository/org/springframework/spring-context/5.1.3.RELEASE/spring-context-5.1.3.RELEASE.jar:/home/huahua/.m2/repository/org/springframework/spring-aop/5.1.3.RELEASE/spring-aop-5.1.3.RELEASE.jar:/home/huahua/.m2/repository/org/springframework/spring-beans/5.1.3.RELEASE/spring-beans-5.1.3.RELEASE.jar:/home/huahua/.m2/repository/org/springframework/spring-expression/5.1.3.RELEASE/spring-expression-5.1.3.RELEASE.jar:/home/huahua/.m2/repository/org/springframework/boot/spring-boot-autoconfigure/2.1.1.RELEASE/spring-boot-autoconfigure-2.1.1.RELEASE.jar:/home/huahua/.m2/repository/org/springframework/boot/spring-boot-starter-logging/2.1.1.RELEASE/spring-boot-starter-logging-2.1.1.RELEASE.jar:/home/huahua/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar:/home/huahua/.m2/repository/ch/qos/logback/logback-core/1.2.3/logback-core-1.2.3.jar:/home/huahua/.m2/repository/org/slf4j/slf4j-api/1.7.25/slf4j-api-1.7.25.jar:/home/huahua/.m2/repository/org/apache/logging/log4j/log4j-to-slf4j/2.11.1/log4j-to-slf4j-2.11.1.jar:/home/huahua/.m2/repository/org/apache/logging/log4j/log4j-api/2.11.1/log4j-api-2.11.1.jar:/home/huahua/.m2/repository/org/slf4j/jul-to-slf4j/1.7.25/jul-to-slf4j-1.7.25.jar:/home/huahua/.m2/repository/javax/annotation/javax.annotation-api/1.3.2/javax.annotation-api-1.3.2.jar:/home/huahua/.m2/repository/org/springframework/spring-core/5.1.3.RELEASE/spring-core-5.1.3.RELEASE.jar:/home/huahua/.m2/repository/org/springframework/spring-jcl/5.1.3.RELEASE/spring-jcl-5.1.3.RELEASE.jar:/home/huahua/.m2/repository/org/yaml/snakeyaml/1.23/snakeyaml-1.23.jar:/home/huahua/.m2/repository/org/projectlombok/lombok/1.18.4/lombok-1.18.4.jar com.feng.spring.SpringEventMain
21:00:10.351 [main] DEBUG org.springframework.context.annotation.ClassPathBeanDefinitionScanner - Identified candidate component class: file [/home/huahua/IdeaProjects/feng/spring-event/target/classes/com/feng/spring/listener/TestApplicationListener.class]
21:00:10.360 [main] DEBUG org.springframework.context.annotation.AnnotationConfigApplicationContext - Refreshing org.spring[email protected]64b8f8f4
21:00:10.388 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalConfigurationAnnotationProcessor'
21:00:10.451 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.event.internalEventListenerProcessor'
21:00:10.453 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.event.internalEventListenerFactory'
21:00:10.456 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalAutowiredAnnotationProcessor'
21:00:10.456 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalCommonAnnotationProcessor'
21:00:10.463 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'testApplicationListener'
code is: 10000, message is: ---------My Spring task event------------说明Spring事件发布订阅成功。
3. 总结
Spring的事件通知demo已经完成，如果需要详细的Spring事件从发布到订阅的过程，需要跟踪Spring的ApplicationContext的publishEvent的过程，其实Spring的ApplicationContext的启动的过程中也定义了很详细的事件，使用intellij idea分析，如下：
Spring事件通知的本质：生产者-消费者，体现了设计模式的观察者模式。