一、spring boot嵌入式servlet容器运行原理:
1）SpringBoot应用启动时运行主程序的run方法。
2）调用SpringApplication的run()方法，其中调用refreshContext(context);
SpringBoot刷新IOC容器【创建IOC容器对象，并初始化容器，创建容器中的每一个组件】,如果是web应用创建AnnotationConfigEmbeddedWebApplicationContext,否则创建AnnotationConfigApplicationContext;
StopWatch stopWatch = new StopWatch()
stopWatch.start()
ConfigurableApplicationContext context = null
FailureAnalyzers analyzers = null
configureHeadlessProperty()
SpringApplicationRunListeners listeners = getRunListeners(args)
listeners.starting()
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args)
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments)
Banner printedBanner = printBanner(environment)
context = createApplicationContext()
analyzers = new FailureAnalyzers(context)
prepareContext(context, environment, listeners, applicationArguments,
printedBanner)
//调用此方法
refreshContext(context)
afterRefresh(context, applicationArguments)
listeners.finished(context, null)
stopWatch.stop()
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch)
}
return context
}
3）在refreshContext(ConfigurableApplicationContext context)方法中调用refresh(ApplicationContext applicationContext)方法,在refresh(ApplicationContext applicationContext)方法中调用AbstractApplicationContext的refresh()方法刷新刚才创建好的ioc容器；
4）在refresh()方法中，调用onRefresh()方法,子类的EmbeddedWebApplicationContext重写了父类的onRefresh()方法；
synchronized (this.startupShutdownMonitor) {
prepareRefresh();
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
prepareBeanFactory(beanFactory);
try {
postProcessBeanFactory(beanFactory);
invokeBeanFactoryPostProcessors(beanFactory);
registerBeanPostProcessors(beanFactory);
initMessageSource();
initApplicationEventMulticaster();
onRefresh();
registerListeners();
finishBeanFactoryInitialization(beanFactory);
finishRefresh();
}
EmbeddedWebApplicationContext的onRefresh()方法
@Override
protected void onRefresh() {
super.onRefresh();
try {
createEmbeddedServletContainer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start embedded container",
ex);
}
}
5)在子类的onRefresh方法中调用createEmbeddedServletContainer()方法，创建嵌入式servlet容器EmbeddedServletContainer
6)在createEmbeddedServletContainer()方法中获取嵌入式的Servlet容器工厂：
EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory()
从ioc容器中获取EmbeddedServletContainerFactory组件,并创建子类的TomcatEmbeddedServletContainerFactory创建对象，后置处理器一看是这个对象，就获取所有的定制器来先定制Servlet容器的相关配置；
7)使用容器工厂获取嵌入式的Servlet容器:this.embeddedServletContainer = containerFactory.getEmbeddedServletContainer(getSelfInitializer());
private void createEmbeddedServletContainer() {
EmbeddedServletContainer localContainer = this.embeddedServletContainer;
ServletContext localServletContext = getServletContext();
if (localContainer == null && localServletContext == null) {
EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory();
this.embeddedServletContainer = containerFactory
.getEmbeddedServletContainer(getSelfInitializer());
}
else if (localServletContext != null) {
try {
getSelfInitializer().onStartup(localServletContext);
}
catch (ServletException ex) {
throw new ApplicationContextException("Cannot initialize servlet context",
ex);
}
}
initPropertySources();
}
8)子类TomcatEmbeddedServletContainerFactory通过getEmbeddedServletContainerFactory()创建Tomcat对象,通过run()方法启动Servlet容器；先启动嵌入式的Servlet容器，再将ioc容器中剩下没有创建出的对象获取出来；
先创建IOC容器，同时创建嵌入式的Servlet容器，之后在去创建IOC容器的剩下的步骤，将自己写的类扫到容器中
二、spring boot使用外部servlet容器运行原理:
1>servlet3.0之后增加了如下的规则：
1）服务器启动（web应用启动）时会找在当前web应用里面每一个jar包里面找到ServletContainerInitializer实例：
2）ServletContainerInitializer的实现放在jar包的META-INF/services文件夹下，有一个名为javax.servlet.ServletContainerInitializer的文件，内容就是ServletContainerInitializer的实现类的全类名使用@HandlesTypes，在应用启动的时候加载我们感兴趣的类；
2>原理:
1）首先启动tomcat容器
2）在Spring的web模块中有这个文件：
org\springframework\spring-web\4.3.14.RELEASE\spring-web-4.3.14.RELEASE.jar!\METAINF\services\javax.servlet.ServletContainerInitializer：
其中这个文件中有org.springframework.web.SpringServletContainerInitializer这个类
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>；为这些WebApplicationInitializer类型的类创建实例；而WebApplicationInitializer的实现类如下图所示：
4）每一个WebApplicationInitializer都调用自己的onStartup；等同于创建一个SpringBootServletInitializer类的对象，并执行onStartup方法
5）SpringBootServletInitializer实例执行onStartup的时候会调用createRootApplicationContext方法创建容器
protected WebApplicationContext createRootApplicationContext(
ServletContext servletContext) {
SpringApplicationBuilder builder = createSpringApplicationBuilder();
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);
builder = configure(builder);
builder.listeners(new WebEnvironmentPropertySourceInitializer(servletContext));
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);
}
//4、启动Spring应用
return run(application);
}
6）上段代码最后一行调用run()方法，在此方法中再次调用了SpringApplication的run()方法，后续流程，除了不去创建嵌入式tomcat容器外，会创建ioc容器，具体步骤参考上面嵌入式servlet容器运行原理的(2)-(4)，创建过程中不会在调用(4)的onrefresh()方法了。
总结:先启动Servlet容器，再启动SpringBoot应用
