今天繼續來聊Spring Boot 2.0的新特性。本文將具體說說2.0版本中的事件模型，尤其是新增的事件：
ApplicationStartedEvent。原文首發：http://blog.didispace.com/Spring-Boot-2-0-feature-2-ApplicationStartedEvent/
在Spring Boot 2.0中對事件模型做了一些增強，主要就是增加了ApplicationStartedEvent事件，所以在2.0版本中所有的事件按執行的先後順序如下：
ApplicationStartingEventApplicationEnvironmentPreparedEventApplicationPreparedEventApplicationStartedEvent<= 新增的事件ApplicationReadyEventApplicationFailedEvent
從上面的列表中，我們可以看到ApplicationStartedEvent位於ApplicationPreparedEvent之後，ApplicationReadyEvent之前。
下面我們通過程式碼的方式來直觀的感受這個事件的切入位置，以便與將來我們在這個切入點加入自己需要的邏輯。
第一步：我們可以編寫ApplicationPreparedEvent、ApplicationStartedEvent以及ApplicationReadyEvent三個事件的監聽器，然後在這三個事件觸發的時候列印一些日誌來觀察它們各自的切入點，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......");
}
}
複製程式碼第二步：在/src/main/resources/目錄下新建：META-INF/spring.factories配置檔案，通過配置org.springframework.context.ApplicationListener來載入上面我們編寫的監聽器。
org.springframework.context.ApplicationListener=
com.didispace.ApplicationPreparedEventListener,\
com.didispace.ApplicationReadyEventListener,\
com.didispace.ApplicationStartedEventListener
複製程式碼此時，我們執行Spring Boot應用可以獲得類似如下日誌輸出：
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......
複製程式碼從日誌中我們可以看到清晰的看到ApplicationPreparedEvent、ApplicationStartedEvent以及ApplicationReadyEvent三個事件的切入點。通過這個例子可能讀者會感到疑問：ApplicationStartedEvent和ApplicationReadyEvent從事件命名和日誌輸出位置來看，都是應用載入完成之後的事件，它們是否有什麼區別呢？
下面可以看看官方文件對ApplicationStartedEvent和ApplicationReadyEvent的解釋：
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
從文件中我們可以知道他們兩中間還有一個過程就是command-line runners被呼叫的內容。所以，為了更準確的感受這兩個事件的區別，我們在應用主類中加入CommandLineRunner的實現，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...");
}
}
}
複製程式碼最後，我們再執行程式，此時我們可以發現這兩個事件中間輸出了上面定義的DataLoader的輸出內容，具體如下：
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......
複製程式碼程式碼示例
本文的相關例子可以檢視下面倉庫中的Chapter1-2-1目錄：
- Github：https://github.com/dyc87112/SpringBoot-Learning
- Gitee：https://gitee.com/didispace/SpringBoot-Learning
Spring Booot 2.0 新特性詳解正在連載，點選看看都有哪些解讀