SpringBoot使用外置Servlet的原理：
jar包：执行SpringBoot主类的main方法，启动ioc容器，创建嵌入式的Servlet容器；
war包：启动服务器，服务器启动SpringBoot应用，启动ioc容器；
我们可以找到这个类：ServletInitializer
public class ServletInitializer extends SpringBootServletInitializer {
@Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
return application.sources(DemoOgj01Application.class);
}
}
Servlet3.0(Spring注解版)：
规则：
1）、服务器启动（web应用启动）会创建当前web应用里面每个jar包里面ServletContainerInitalizer实例；
2）、ServletContainerInitalizer的实现放在jar包的META-INF/services文件下，有一个名为javax.servlet.ServletContainerInitalizer的文件，内容的内容就是ServletContainerInitalizer的实现类的全类名；
3）、还可以使用@HandlesTyoes，在应用启动的时候加载我们感兴趣的class；
ctrl+H 查看接口下的实现类。
流程：
1）、启动Tomcat
2）、在这个路径下：org\springframework\spring-web\5.2.4.RELEASE\spring-web-5.2.4.RELEASE.jar!\META-INF\services\javax.servlet.ServletContainerInitializer
Spring的web模块中有这个文件：org.springframework.web.SpringServletContainerInitializer
3）、SpringServletContainerInitializer将**@HandlesTypes({WebApplicationInitializer.class})标注的所有这个类型的类传入到onStarup方法的Set**；为这些WebApplicationInitializer类型的类创建实例；
4）、每个WebApplicationInitializer都调用自己的onStarup；
5）、我们又继承了SpringBootServletInitializer，实现了以下函数，相当于我们的SpringBootServletInitializer的类会被创建对象，并执行onStarup方法；
- 下面这个这个函数会告诉SpringBootServletInitializer我们要启动的Spring的IOC容器是哪一个，然后传入到onStarup中。
public class ServletInitializer extends SpringBootServletInitializer {
@Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
return application.sources(DemoOgj01Application.class);
}
//这个函数会告诉SpringBootServletInitializer我们要启动的Spring的IOC容器是哪一个，然后传入到onStarup中。
}
6）、SpringBootServletInitializer的示例执行onStarup的时候会createRootApplicationContext，创建容器：
public void onStartup(ServletContext servletContext) throws ServletException {
this.logger = LogFactory.getLog(this.getClass());
WebApplicationContext rootAppContext = this.createRootApplicationContext(servletContext);
if (rootAppContext != null) {
servletContext.addListener(new ContextLoaderListener(rootAppContext) {
public void contextInitialized(ServletContextEvent event) {
}
});
} else {
this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not return an application context");
}
}
执行createRootApplicationContext
protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {
//1、创建SpringApplicationBuilder
SpringApplicationBuilder builder = this.createSpringApplicationBuilder();
builder.main(this.getClass());
ApplicationContext parent = this.getExistingRootWebApplicationContext(servletContext);
if (parent != null) {
this.logger.info("Root context already created (using as parent).");
servletContext.setAttribute(
WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, (Object)null);
builder.initializers(new ApplicationContextInitializer[]{new ParentContextApplicationContextInitializer(parent)});
}
builder.initializers(new ApplicationContextInitializer[]{new ServletContextApplicationContextInitializer(servletContext)});
builder.contextClass(AnnotationConfigServletWebServerApplicationContext.class);
//2.调用configure方法，子类重写了这个方法，我们调用的是子类方法，将SpringBoot的主程序类传入
builder = this.configure(builder);
builder.listeners(new ApplicationListener[]{new SpringBootServletInitializer.WebEnvironmentPropertySourceInitializer(servletContext)});
//3.使用builder创建一个Spring应用
SpringApplication application = builder.build();
if (application.getAllSources().isEmpty() && MergedAnnotations.from(this.getClass(), SearchStrategy.TYPE_HIERARCHY).isPresent(Configuration.class)) {
application.addPrimarySources(Collections.singleton(this.getClass()));
}
Assert.state(!application.getAllSources().isEmpty(), "No SpringApplication sources have been defined. Either override the configure method or add an @Configuration annotation");
if (this.registerErrorPageFilter) {
application.addPrimarySources(Collections.singleton(ErrorPageFilterConfiguration.class));
}
//4.启动Spring
return this.run(application);
}
7）、Spring的应用就启动了，并且创建IOC容器。
public ConfigurableApplicationContext run(String... args) {
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();
this.configureHeadlessProperty();
SpringApplicationRunListeners listeners = this.getRunListeners(args);
listeners.starting();
Collection exceptionReporters;
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
this.configureIgnoreBeanInfo(environment);
Banner printedBanner = this.printBanner(environment);
context = this.createApplicationContext();
exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
//刷新IOC容器，IOC容器初始化
this.refreshContext(context);
this.afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
(new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
}
listeners.started(context);
this.callRunners(context, applicationArguments);
} catch (Throwable var10) {
this.handleRunFailure(context, var10, exceptionReporters, listeners);
throw new IllegalStateException(var10);
}
try {
listeners.running(context);
return context;
} catch (Throwable var9) {
this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners)null);
throw new IllegalStateException(var9);
}
}
