【Springboot总结】21 使用外置的Servlet容器的原理_springbootapplicatoin createrootapplicationcontext-优快云博客

本文详细解析了Spring Boot应用程序的启动流程，从Tomcat启动到SpringServletContainerInitializer的作用，再到WebApplicationInitializer接口及其抽象实现SpringBootServletInitializer。深入探讨了createRootApplicationContext方法中SpringApplicationBuilder的配置过程，以及如何通过构建SpringApplication并运行它来启动Spring Boot应用。

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1、启动Tomcat

2、Spring的web模块里面有这个文件：org.springframework.web.SpringServletContainerInitializer

3、SpringServletContainerInitializer将@HandlesTypes(WebApplicationInitializer.class)标注的所有这个类型的类都传入到onStartup方法的Set<Class<?>>集合里面；为这些WebApplicationInitializer类型的类创建实例；

4、每一个WebApplicationInitializer都调用自己的onStartup

【提示】WebApplicationInitializer是接口，SpringBootServletInitializer是该接口的抽象类

5、相当于我们的SpringBootServletInitializer的类会被创建对象，并执行onStartup方法

6、SpringBootServletInitializer实例执行onStartup的时候会createRootApplicationContext；

protected WebApplicationContext createRootApplicationContext(
      ServletContext servletContext) {
    //1、创建SpringApplicationBuilder
   SpringApplicationBuilder builder = createSpringApplicationBuilder();
   StandardServletEnvironment environment = new StandardServletEnvironment();
   environment.initPropertySources(servletContext, null);
   builder.environment(environment);
   builder.main(getClass());
   ApplicationContext parent = getExistingRootWebApplicationContext(servletContext);
   if (parent != null) {
      this.logger.info("Root context already created (using as parent).");
      servletContext.setAttribute(
            WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, null);
      builder.initializers(new ParentContextApplicationContextInitializer(parent));
   }
   builder.initializers(
         new ServletContextApplicationContextInitializer(servletContext));
   builder.contextClass(AnnotationConfigEmbeddedWebApplicationContext.class);
    
    //调用configure方法，子类重写了这个方法，将SpringBoot的主程序类传入了进来
   builder = configure(builder);
    
    //使用builder创建一个Spring应用
   SpringApplication application = builder.build();
   if (application.getSources().isEmpty() && AnnotationUtils
         .findAnnotation(getClass(), Configuration.class) != null) {
      application.getSources().add(getClass());
   }
   Assert.state(!application.getSources().isEmpty(),
         "No SpringApplication sources have been defined. Either override the "
               + "configure method or add an @Configuration annotation");
   // Ensure error pages are registered
   if (this.registerErrorPageFilter) {
      application.getSources().add(ErrorPageFilterConfiguration.class);
   }
    //启动Spring应用
   return run(application);
}

【提示】createRootApplicationContext会启动springboot的应用

7. 应用启动之后参考：https://blog.youkuaiyun.com/FullStackDeveloper0/article/details/89518313