Spring执行ApplicationEvent事件顺序ServletWebServerInitializedEvent

测试代码：

package com.github.abel533.event;

import com.github.abel533.C;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.ApplicationListener;
import org.springframework.stereotype.Component;

/**
 * @author liuzh
 */
@Component
public class ApplicationListenerImpl implements ApplicationListener<ApplicationEvent> {

    public ApplicationListenerImpl() {
        C.print("ApplicationListenerImpl#constructor");
    }

    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        C.print("ApplicationListener#" + event.getClass().getSimpleName());
    }
}

package com.github.abel533.event;

import com.github.abel533.lifecycle.BeanLifecycle;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class ListenerApplication {

    public static void main(String[] args) {
        SpringApplication.run(ListenerApplication.class, args).close();
    }

}

用以上代码实现 ApplicationListener 接口，输出所有事件。

当以 @Component 方式配置时
事件触发顺序如下：

ApplicationListener#ContextRefreshedEvent
ApplicationListener#ServletWebServerInitializedEvent
ApplicationListener#ApplicationStartedEvent
ApplicationListener#ApplicationReadyEvent
ApplicationListener#ContextClosedEvent

当通过 /META-INF/spring.factories 配置时
配置内容如下：

org.springframework.context.ApplicationListener=com.github.abel533.event.ApplicationListenerImpl

此时输出的事件顺序如下：

差异

很容易通过对比发现，Event 触发的时间极早，以至于 @Component 方式只能从第 4 个事件才开始获取到。

从这两种方式的加载时机来看这个差异产生的原因。

在 SpringApplication 构造方法中，就调用 getSpringFactoriesInstances 来获取 /META-INF/spring.factories 配置的 ApplicationListener，代码如下：

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    this.resourceLoader = resourceLoader;
    Assert.notNull(primarySources, "PrimarySources must not be null");
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    setInitializers((Collection) getSpringFactoriesInstances(
            ApplicationContextInitializer.class));
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    this.mainApplicationClass = deduceMainApplicationClass();
}

在 SpringFactoriesLoader#loadFactoryNames 实现了从该配置文件获取实现名的方法。从这之后就能收到后续触发的事件。

通过 @Component 方式时，在 SpringApplication#refresh 中调用 registerListeners 获取的所有 ApplicationListener 接口的实现。代码如下：

@Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            try {
                // 注册所有 ApplicationListener 实现
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);

                // 这里会触发 ContextRefreshedEvent
                finishRefresh();
            }
        }
    }

下面先分析前 4 个无法获取的事件顺序。

ApplicationStartingEvent
第 0 个事件是在 EventPublishingRunListener#starting 中发布的，代码如下：

@Override
public void starting() {
    this.initialMulticaster.multicastEvent(
            new ApplicationStartingEvent(this.application, this.args));
}

此时的堆栈调用情况如下：

onApplicationEvent:15, ApplicationListenerImpl (com.github.abel533.event)
doInvokeListener:172, SimpleApplicationEventMulticaster (org.springframework.context.event)
invokeListener:165, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:139, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:127, SimpleApplicationEventMulticaster (org.springframework.context.event)
starting:69, EventPublishingRunListener (org.springframework.boot.context.event)
starting:48, SpringApplicationRunListeners (org.springframework.boot)
run:302, SpringApplication (org.springframework.boot)
run:1260, SpringApplication (org.springframework.boot)
run:1248, SpringApplication (org.springframework.boot)
main:12, ListenerApplication (com.github.abel533.event)

ApplicationEnvironmentPreparedEvent

第 1 个事件是在 EventPublishingRunListener#environmentPrepared 中发布的，代码如下：

@Override
public void environmentPrepared(ConfigurableEnvironment environment) {
    this.initialMulticaster.multicastEvent(new ApplicationEnvironmentPreparedEvent(
            this.application, this.args, environment));
}

此时的堆栈调用情况如下：

onApplicationEvent:15, ApplicationListenerImpl (com.github.abel533.event)
doInvokeListener:172, SimpleApplicationEventMulticaster (org.springframework.context.event)
invokeListener:165, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:139, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:127, SimpleApplicationEventMulticaster (org.springframework.context.event)
environmentPrepared:75, EventPublishingRunListener (org.springframework.boot.context.event)
environmentPrepared:54, SpringApplicationRunListeners (org.springframework.boot)
prepareEnvironment:347, SpringApplication (org.springframework.boot)
run:306, SpringApplication (org.springframework.boot)
run:1260, SpringApplication (org.springframework.boot)
run:1248, SpringApplication (org.springframework.boot)
main:12, ListenerApplication (com.github.abel533.event)

ApplicationContextInitializedEvent

第 2 个事件是在 EventPublishingRunListener#contextPrepared 中发布的，代码如下：

@Override
public void contextPrepared(ConfigurableApplicationContext context) {
    this.initialMulticaster.multicastEvent(new ApplicationContextInitializedEvent(
            this.application, this.args, context));
}

此时的堆栈调用情况如下：

onApplicationEvent:15, ApplicationListenerImpl (com.github.abel533.event)
doInvokeListener:172, SimpleApplicationEventMulticaster (org.springframework.context.event)
invokeListener:165, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:139, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:127, SimpleApplicationEventMulticaster (org.springframework.context.event)
contextPrepared:81, EventPublishingRunListener (org.springframework.boot.context.event)
contextPrepared:60, SpringApplicationRunListeners (org.springframework.boot)
prepareContext:374, SpringApplication (org.springframework.boot)
run:314, SpringApplication (org.springframework.boot)
run:1260, SpringApplication (org.springframework.boot)
run:1248, SpringApplication (org.springframework.boot)
main:12, ListenerApplication (com.github.abel533.event)

ApplicationPreparedEvent

第 3 个事件是在 EventPublishingRunListener#contextLoaded 中发布的，代码如下：

@Override
public void contextLoaded(ConfigurableApplicationContext context) {
    for (ApplicationListener<?> listener : this.application.getListeners()) {
        if (listener instanceof ApplicationContextAware) {
            ((ApplicationContextAware) listener).setApplicationContext(context);
        }
        context.addApplicationListener(listener);
    }
    this.initialMulticaster.multicastEvent(
            new ApplicationPreparedEvent(this.application, this.args, context));
}

此时的堆栈调用情况如下：

onApplicationEvent:15, ApplicationListenerImpl (com.github.abel533.event)
doInvokeListener:172, SimpleApplicationEventMulticaster (org.springframework.context.event)
invokeListener:165, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:139, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:127, SimpleApplicationEventMulticaster (org.springframework.context.event)
contextLoaded:93, EventPublishingRunListener (org.springframework.boot.context.event)
contextLoaded:66, SpringApplicationRunListeners (org.springframework.boot)
prepareContext:393, SpringApplication (org.springframework.boot)
run:314, SpringApplication (org.springframework.boot)
run:1260, SpringApplication (org.springframework.boot)
run:1248, SpringApplication (org.springframework.boot)
main:12, ListenerApplication (com.github.abel533.event)

ContextRefreshedEvent

在上面差异中提到 finishRefresh 会触发 ContextRefreshedEvent，代码如下：

@Override
protected void finishRefresh() {
    super.finishRefresh();
    WebServer webServer = startWebServer();
    if (webServer != null) {
        publishEvent(new ServletWebServerInitializedEvent(webServer, this));
    }
}

注意 super.finishRefresh，代码如下(有删减)：

protected void finishRefresh() {
    // Publish the final event.
    publishEvent(new ContextRefreshedEvent(this));
}

ServletWebServerInitializedEvent

注意前面 finishRefresh 方法，如果存在 webServer != null，就会发布 ServletWebServerInitializedEvent。

ApplicationStartedEvent
在 SpringApplication#run 方法中，执行完成后，就会调用 listeners.started(context); 方法，在这里面会发布 ApplicationStartedEvent。

ApplicationReadyEvent
和上面 ApplicationStartedEvent 一样，如下代码(有删减)：

// ApplicationStartedEvent
listeners.started(context);
callRunners(context, applicationArguments);
// ApplicationReadyEvent
listeners.running(context);

执行完所有 ApplicationRunner 和 CommandLineRunner 接口方法后，就会调用 listeners.running(context)，在这里面就会发布 ApplicationReadyEvent。

在这之后就没有运行期的主要事件了（不考虑 devtools 重启）。在这个事件里，可以请求zookeeper进行服务注册，以便其它服务发现并调用它等相关操作。

ContextClosedEvent
当调用关闭方法的时候，自然就触发了 ContextClosedEvent，调用堆栈如下：

onApplicationEvent:20, ApplicationListenerImpl (com.github.abel533.event)
doInvokeListener:172, SimpleApplicationEventMulticaster (org.springframework.context.event)
invokeListener:165, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:139, SimpleApplicationEventMulticaster (org.springframework.context.event)
publishEvent:398, AbstractApplicationContext (org.springframework.context.support)
publishEvent:355, AbstractApplicationContext (org.springframework.context.support)
doClose:994, AbstractApplicationContext (org.springframework.context.support)
close:961, AbstractApplicationContext (org.springframework.context.support)
main:12, ListenerApplication (com.github.abel533.event)

本文转自：https://blog.csdn.net/isea533/article/details/100146833