今天继续来聊Spring Boot 2.0的新特性。本文将具体说说2.0版本中的事件模型，尤其是新增的事件：ApplicationStartedEvent。
在Spring Boot 2.0中对事件模型做了一些增强，主要就是增加了ApplicationStartedEvent事件，所以在2.0版本中所有的事件按执行的先后顺序如下：
ApplicationStartingEventApplicationEnvironmentPreparedEventApplicationPreparedEvent-
ApplicationStartedEvent<= 新增的事件 ApplicationReadyEventApplicationFailedEvent
从上面的列表中，我们可以看到ApplicationStartedEvent位于ApplicationPreparedEvent之后，ApplicationReadyEvent之前。
下面我们通过代码的方式来直观的感受这个事件的切入位置，以便与将来我们在这个切入点加入自己需要的逻辑。
第一步：我们可以编写ApplicationPreparedEvent、ApplicationStartedEvent以及ApplicationReadyEvent三个事件的监听器，然后在这三个事件触发的时候打印一些日志来观察它们各自的切入点，比如：
@Slf4j
public class ApplicationPreparedEventListener implements ApplicationListener<ApplicationPreparedEvent> {
@Override
public void onApplicationEvent(ApplicationPreparedEvent event) {
log.info("......ApplicationPreparedEvent......");
}
}
@Slf4j
public class ApplicationStartedEventListener implements ApplicationListener<ApplicationStartedEvent> {
@Override
public void onApplicationEvent(ApplicationStartedEvent event) {
log.info("......ApplicationStartedEvent......");
}
}
@Slf4j
public class ApplicationReadyEventListener implements ApplicationListener<ApplicationReadyEvent> {
@Override
public void onApplicationEvent(ApplicationReadyEvent event) {
log.info("......ApplicationReadyEvent......");
}
}第二步：在/src/main/resources/目录下新建：META-INF/spring.factories配置文件，通过配置org.springframework.context.ApplicationListener来加载上面我们编写的监听器。
org.springframework.context.ApplicationListener=
com.didispace.ApplicationPreparedEventListener,\
com.didispace.ApplicationReadyEventListener,\
com.didispace.ApplicationStartedEventListener此时，我们运行Spring Boot应用可以获得类似如下日志输出：
2018-03-07 18:15:18.591 INFO 83387 --- [ main] com.didispace.Application : Starting Application on zhaiyongchaodeMacBook-Pro.local with PID 83387 (/Users/zhaiyongchao/Documents/git/github/SpringBoot-Learning/Chapter1-2-1/target/classes started by zhaiyongchao in /Users/zhaiyongchao/Documents/git/github/SpringBoot-Learning/Chapter1-2-1)
2018-03-07 18:15:18.591 INFO 83387 --- [ main] com.didispace.Application : No active profile set, falling back to default profiles: default
2018-03-07 18:15:18.658 INFO 83387 --- [ main] c.d.ApplicationPreparedEventListener : ......ApplicationPreparedEvent......
2018-03-07 18:15:18.662 INFO 83387 --- [ main] ConfigServletWebServerApplicationContext : Refreshing org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@20d3d15a: startup date [Wed Mar 07 18:15:18 CST 2018]; root of context hierarchy
2018-03-07 18:15:19.879 INFO 83387 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat initialized with port(s): 8080 (http)
2018-03-07 18:15:19.926 INFO 83387 --- [ main] o.apache.catalina.core.StandardService : Starting service [Tomcat]
2018-03-07 18:15:19.930 INFO 83387 --- [ main] org.apache.catalina.core.StandardEngine : Starting Servlet Engine: Apache Tomcat/8.5.28
2018-03-07 18:15:19.946 INFO 83387 --- [ost-startStop-1] o.a.catalina.core.AprLifecycleListener : The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: [/Users/zhaiyongchao/Library/Java/Extensions:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java:.]
2018-03-07 18:15:20.068 INFO 83387 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext
2018-03-07 18:15:20.068 INFO 83387 --- [ost-startStop-1] o.s.web.context.ContextLoader : Root WebApplicationContext: initialization completed in 1410 ms
2018-03-07 18:15:20.210 INFO 83387 --- [ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean : Servlet dispatcherServlet mapped to [/]
2018-03-07 18:15:20.214 INFO 83387 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'characterEncodingFilter' to: [/*]
2018-03-07 18:15:20.214 INFO 83387 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'hiddenHttpMethodFilter' to: [/*]
2018-03-07 18:15:20.214 INFO 83387 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'httpPutFormContentFilter' to: [/*]
2018-03-07 18:15:20.215 INFO 83387 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'requestContextFilter' to: [/*]
2018-03-07 18:15:20.513 INFO 83387 --- [ main] s.w.s.m.m.a.RequestMappingHandlerAdapter : Looking for @ControllerAdvice: org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@20d3d15a: startup date [Wed Mar 07 18:15:18 CST 2018]; root of context hierarchy
2018-03-07 18:15:20.592 INFO 83387 --- [ main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error]}" onto public org.springframework.http.ResponseEntity<java.util.Map<java.lang.String, java.lang.Object>> org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest)
2018-03-07 18:15:20.593 INFO 83387 --- [ main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error],produces=[text/html]}" onto public org.springframework.web.servlet.ModelAndView org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.errorHtml(javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse)
2018-03-07 18:15:20.623 INFO 83387 --- [ main] o.s.w.s.handler.SimpleUrlHandlerMapping : Mapped URL path [/webjars/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-03-07 18:15:20.623 INFO 83387 --- [ main] o.s.w.s.handler.SimpleUrlHandlerMapping : Mapped URL path [/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-03-07 18:15:20.660 INFO 83387 --- [ main] o.s.w.s.handler.SimpleUrlHandlerMapping : Mapped URL path [/**/favicon.ico] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-03-07 18:15:20.787 INFO 83387 --- [ main] o.s.j.e.a.AnnotationMBeanExporter : Registering beans for JMX exposure on startup
2018-03-07 18:15:20.839 INFO 83387 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat started on port(s): 8080 (http) with context path ''
2018-03-07 18:15:20.843 INFO 83387 --- [ main] com.didispace.Application : Started Application in 2.866 seconds (JVM running for 3.337)
2018-03-07 18:15:20.845 INFO 83387 --- [ main] c.d.ApplicationStartedEventListener : ......ApplicationStartedEvent......
2018-03-07 18:15:20.847 INFO 83387 --- [ main] c.d.ApplicationReadyEventListener : ......ApplicationReadyEvent......
从日志中我们可以看到清晰的看到ApplicationPreparedEvent、ApplicationStartedEvent以及ApplicationReadyEvent三个事件的切入点。通过这个例子可能读者会感到疑问：ApplicationStartedEvent和ApplicationReadyEvent从事件命名和日志输出位置来看，都是应用加载完成之后的事件，它们是否有什么区别呢？
下面可以看看官方文档对ApplicationStartedEvent和ApplicationReadyEvent的解释：
An ApplicationStartedEvent is sent after the context has been refreshed but before any application and command-line runners have been called.An ApplicationReadyEvent is sent after any application and command-line runners have been called. It indicates that the application is ready to service requests
从文档中我们可以知道他们两中间还有一个过程就是command-line runners被调用的内容。所以，为了更准确的感受这两个事件的区别，我们在应用主类中加入CommandLineRunner的实现，比如：
@Slf4j
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
@Bean
public DataLoader dataLoader() {
return new DataLoader();
}
@Slf4j
static class DataLoader implements CommandLineRunner {
@Override
public void run(String... strings) throws Exception {
log.info("Loading data...");
}
}
}最后，我们再运行程序，此时我们可以发现这两个事件中间输出了上面定义的DataLoader的输出内容，具体如下：
2018-03-07 18:15:20.845 INFO 83387 --- [main] c.d.ApplicationStartedEventListener : ......ApplicationStartedEvent......
2018-03-07 18:15:20.846 INFO 83387 --- [main] com.didispace.Application$DataLoader : Loading data...
2018-03-07 18:15:20.847 INFO 83387 --- [main] c.d.ApplicationReadyEventListener : ......ApplicationReadyEvent......代码示例
本文的相关例子可以查看下面仓库中的Chapter1-2-1目录：
- Github：https://github.com/dyc87112/SpringBoot-Learning
- Gitee：https://gitee.com/didispace/SpringBoot-Learning
Spring Booot 2.0 新特性详解正在连载，点击看看都有哪些解读
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