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【SpringBootServletInitializer】，启动ioc容器；
servlet3.0规范：
8.2.4 Shared libraries / runtimes pluggability
In addition to supporting fragments and use of annotations one of the requirements
is that not only we be able to plug-in things that are bundled in the WEB-INF/lib
but also plugin shared copies of frameworks - including being able to plug-in to the
web container things like JAX-WS, JAX-RS and JSF that build on top of the web
container. The ServletContainerInitializer allows handling such a use case
as described below.
The ServletContainerInitializer class is looked up via the jar services API.
For each application, an instance of the ServletContainerInitializer is
created by the container at application startup time. The framework providing an
implementation of the ServletContainerInitializer MUST bundle in the
META-INF/services directory of the jar file a file called
javax.servlet.ServletContainerInitializer, as per the jar services API,
that points to the implementation class of the ServletContainerInitializer.
In addition to the ServletContainerInitializer we also have an annotation -
HandlesTypes. The HandlesTypes annotation on the implementation of the
ServletContainerInitializer is used to express interest in classes that may
have annotations (type, method or field level annotations) specified in the value of
the HandlesTypes or if it extends / implements one those classes anywhere in the
class’ super types. The HandlesTypes annotation is applied irrespective of the
setting of metadata-complete.
When examining the classes of an application to see if they match any of the criteria
specified by the HandlesTypes annotation of a ServletContainerInitializer,
the container may run into class loading problems if one or more of the application's
optional JAR files are missing. Since the container is not in a position to decide
whether these types of class loading failures will prevent the application from
working correctly, it must ignore them, while at the same time providing a
configuration option that would log them.
If an implementation of ServletContainerInitializer does not have the
@HandlesTypes annotation, or if there are no matches to any of the HandlesType
specified, then it will get invoked once for every application with null as the value
of the Set. This will allow for the initializer to determine based on the resources
available in the application whether it needs to initialize a servlet / filter or not.
The onStartup method of the ServletContainerInitializer will be invoked
when the application is coming up before any of the servlet listener events are fired.
The onStartup method of the ServletContainerInitializer is called with a
Set of Classes that either extend / implement the classes that the initializer
expressed interest in or if it is annotated with any of the classes specified via the
@HandlesTypes annotation.
A concrete example below showcases how this would work
servlet3.0规范有几点规则：
1）、服务器启动（web应用启动）会创建当前web应用里面每一个jar包里面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
3）、还可以使用@HandlesTypes，在应用启动的时候加载我们感兴趣的类；
流程：
1）、启动Tomcat
2）、org\springframework\spring-web\4.3.14.RELEASE\spring-web-4.3.14.RELEASE.jar!\METAINF\services\javax.servlet.ServletContainerInitializer：
SpringServletContainerInitializer 源码如下:
@HandlesTypes({WebApplicationInitializer.class})
public class SpringServletContainerInitializer implements ServletContainerInitializer {
public SpringServletContainerInitializer() {
}
public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException {
List<WebApplicationInitializer> initializers = new LinkedList();
Iterator var4;
if (webAppInitializerClasses != null) {
var4 = webAppInitializerClasses.iterator();
while(var4.hasNext()) {
Class<?> waiClass = (Class)var4.next();
if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
try {
initializers.add((WebApplicationInitializer)ReflectionUtils.accessibleConstructor(waiClass, new Class[0]).newInstance());
} catch (Throwable var7) {
throw new ServletException("Failed to instantiate WebApplicationInitializer class", var7);
}
}
}
}
if (initializers.isEmpty()) {
servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
} else {
servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
AnnotationAwareOrderComparator.sort(initializers);
var4 = initializers.iterator();
while(var4.hasNext()) {
WebApplicationInitializer initializer = (WebApplicationInitializer)var4.next();
initializer.onStartup(servletContext);
}
}
}
}
3）、SpringServletContainerInitializer 将 @HandlesTypes({WebApplicationInitializer.class}) 标注的所有的类都传入到 onStartup 的 Set<Class<?>> 中，
为 Set<Class<?>> 中为 WebApplicationInitializer类型的类创建实例
4）、WebApplicationInitializer
5）、相当于我们自己的的 ServletInitializer的类会被创建对象，并执行onStartup方法，ServletInitializer 继承于SpringBootServletInitializer
SpringBootServletInitializer源码如下：
public abstract class SpringBootServletInitializer implements WebApplicationInitializer {
protected Log logger;
private boolean registerErrorPageFilter = true;
public SpringBootServletInitializer() {
}
protected final void setRegisterErrorPageFilter(boolean registerErrorPageFilter) {
this.registerErrorPageFilter = registerErrorPageFilter;
}
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");
}
}
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);
//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 = this.configure(builder);
builder.listeners(new ApplicationListener[]{new SpringBootServletInitializer.WebEnvironmentPropertySourceInitializer(servletContext, null)});
//使用builder创建一个Spring应用
SpringApplication application = builder.build();
if (application.getAllSources().isEmpty() && AnnotationUtils.findAnnotation(this.getClass(), Configuration.class) != null) {
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));
}
//启动Spring应用
return this.run(application);
}
protected SpringApplicationBuilder createSpringApplicationBuilder() {
return new SpringApplicationBuilder(new Class[0]);
}
protected WebApplicationContext run(SpringApplication application) {
return (WebApplicationContext)application.run(new String[0]);
}
private ApplicationContext getExistingRootWebApplicationContext(ServletContext servletContext) {
Object context = servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
return context instanceof ApplicationContext ? (ApplicationContext)context : null;
}
protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
return builder;
}
private static final class WebEnvironmentPropertySourceInitializer implements ApplicationListener<ApplicationEnvironmentPreparedEvent>, Ordered {
private final ServletContext servletContext;
private WebEnvironmentPropertySourceInitializer(ServletContext servletContext) {
this.servletContext = servletContext;
}
public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {
ConfigurableEnvironment environment = event.getEnvironment();
if (environment instanceof ConfigurableWebEnvironment) {
((ConfigurableWebEnvironment)environment).initPropertySources(this.servletContext, (ServletConfig)null);
}
}
public int getOrder() {
return -2147483648;
}
}
}
6）、SpringBootServletInitializer实例执行onStartup的时候会createRootApplicationContext；创建容器
7）、Spring的应用就启动并且创建IOC容器
run();源码如下：
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
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);
}
}