Spring Boot 启动原理

// 一直点run方法，进入这个方法publicConfigurableApplicationContextrun(String... args) {
StopWatchstopWatch=newStopWatch();
stopWatch.start();
DefaultBootstrapContextbootstrapContext=this.createBootstrapContext();
ConfigurableApplicationContextcontext=null;
this.configureHeadlessProperty();
SpringApplicationRunListenerslisteners=this.getRunListeners(args);
listeners.starting(bootstrapContext, this.mainApplicationClass);
try {
ApplicationArgumentsapplicationArguments=newDefaultApplicationArguments(args);
// ① 环境准备工作，如读取application.properties(yml)文件ConfigurableEnvironmentenvironment=this.prepareEnvironment(listeners, bootstrapContext, applicationArguments);
this.configureIgnoreBeanInfo(environment);
// 这个地方就是执行打印banner的方法，如何替换banner可以尝试一下BannerprintedBanner=this.printBanner(environment);
// ② 选择创建一个对应类型的应用上下文context=this.createApplicationContext();
context.setApplicationStartup(this.applicationStartup);
this.prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
// ④ 点进去或者debug跟着最后就会到Spring的refresh方法this.refreshContext(context);
this.afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
            (newStartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
        }
listeners.started(context);
this.callRunners(context, applicationArguments);
    } catch (Throwablevar10) {
this.handleRunFailure(context, var10, listeners);
thrownewIllegalStateException(var10);
    }
// 省略...}
// ③ 这个是一个函数式接口写法（当前SpringBoot版本是2.5.4）对比下面ApplicationContextFactoryDEFAULT= (webApplicationType) -> {
try {
switch(webApplicationType) {
caseSERVLET:
// 默认启动情况下创建当前类型的上下文 （直接new）returnnewAnnotationConfigServletWebServerApplicationContext();
caseREACTIVE:
returnnewAnnotationConfigReactiveWebServerApplicationContext();
default:
returnnewAnnotationConfigApplicationContext();
        }
    } catch (Exceptionvar2) {
thrownewIllegalStateException("Unable create a default ApplicationContext instance, you may need a custom ApplicationContextFactory", var2);
    }
};
// 这个是SpringBoot 2.3.4版本的 利用的是反射创建对应类型的上下文protectedConfigurableApplicationContextcreateApplicationContext() {
Class<?>contextClass=this.applicationContextClass;
if (contextClass==null) {
try {
switch(this.webApplicationType) {
caseSERVLET:
contextClass=Class.forName("org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext");
break;
caseREACTIVE:
contextClass=Class.forName("org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext");
break;
default:
contextClass=Class.forName("org.springframework.context.annotation.AnnotationConfigApplicationContext");
            }
        } catch (ClassNotFoundExceptionvar3) {
thrownewIllegalStateException("Unable create a default ApplicationContext, please specify an ApplicationContextClass", var3);
        }
    }
// 通过反射方式return (ConfigurableApplicationContext)BeanUtils.instantiateClass(contextClass);
}

protectedvoidonRefresh() {
super.onRefresh();
try {
// 创建web serverthis.createWebServer();
    } catch (Throwablevar2) {
thrownewApplicationContextException("Unable to start web server", var2);
    }
}

// 创建 web serverprivatevoidcreateWebServer() {
WebServerwebServer=this.webServer;
ServletContextservletContext=this.getServletContext();
// 判断一下是否存在（存在时，可能是使用的外部web server 就不创建内嵌的web server）if (webServer==null&&servletContext==null) {
StartupStepcreateWebServer=this.getApplicationStartup().start("spring.boot.webserver.create");
// ① 获取 web server factoryServletWebServerFactoryfactory=this.getWebServerFactory();
createWebServer.tag("factory", factory.getClass().toString());
// ③ 接下就是new Tomcat 设置对应的参数，一个内嵌web server创建完成this.webServer=factory.getWebServer(newServletContextInitializer[]{this.getSelfInitializer()});
createWebServer.end();
this.getBeanFactory().registerSingleton("webServerGracefulShutdown", newWebServerGracefulShutdownLifecycle(this.webServer));
this.getBeanFactory().registerSingleton("webServerStartStop", newWebServerStartStopLifecycle(this, this.webServer));
    } elseif (servletContext!=null) {
try {
this.getSelfInitializer().onStartup(servletContext);
        } catch (ServletExceptionvar5) {
thrownewApplicationContextException("Cannot initialize servlet context", var5);
        }
    }
this.initPropertySources();
}
// ②  获取 web server factoryprotectedServletWebServerFactorygetWebServerFactory() {
// debug 可以看到获取到beanNames[0]="tomcatServletWebServerFactory"// 是因为SpringBoot 默认使用的Tomcat (还支持Jetty和Undertow)// org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryConfigurationString[] beanNames=this.getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);
if (beanNames.length==0) {
thrownewApplicationContextException("Unable to start ServletWebServerApplicationContext due to missing ServletWebServerFactory bean.");
    } elseif (beanNames.length>1) {
thrownewApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple ServletWebServerFactory beans : "+StringUtils.arrayToCommaDelimitedString(beanNames));
    } else {
return (ServletWebServerFactory)this.getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
    }
}