根据servlet3.0规范：
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；
可以找到全类名为
org.springframework.web.SpringServletContainerInitializer
进入这个类
//这个注解说明了会加载WebApplicationInitializer这个类
@HandlesTypes(WebApplicationInitializer.class) public class SpringServletContainerInitializer implements ServletContainerInitializer { /** * Delegate the {@code ServletContext} to any {@link WebApplicationInitializer} * implementations present on the application classpath. * <p>Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)}, * Servlet 3.0+ containers will automatically scan the classpath for implementations * of Spring's {@code WebApplicationInitializer} interface and provide the set of all * such types to the {@code webAppInitializerClasses} parameter of this method. * <p>If no {@code WebApplicationInitializer} implementations are found on the classpath, * this method is effectively a no-op. An INFO-level log message will be issued notifying * the user that the {@code ServletContainerInitializer} has indeed been invoked but that * no {@code WebApplicationInitializer} implementations were found. * <p>Assuming that one or more {@code WebApplicationInitializer} types are detected, * they will be instantiated (and <em>sorted</em> if the @{@link * org.springframework.core.annotation.Order @Order} annotation is present or * the {@link org.springframework.core.Ordered Ordered} interface has been * implemented). Then the {@link WebApplicationInitializer#onStartup(ServletContext)} * method will be invoked on each instance, delegating the {@code ServletContext} such * that each instance may register and configure servlets such as Spring's * {@code DispatcherServlet}, listeners such as Spring's {@code ContextLoaderListener}, * or any other Servlet API componentry such as filters. * @param webAppInitializerClasses all implementations of * {@link WebApplicationInitializer} found on the application classpath * @param servletContext the servlet context to be initialized * @see WebApplicationInitializer#onStartup(ServletContext) * @see AnnotationAwareOrderComparator */ @Override public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException { List<WebApplicationInitializer> initializers = new LinkedList<>(); if (webAppInitializerClasses != null) { for (Class<?> waiClass : webAppInitializerClasses) { // Be defensive: Some servlet containers provide us with invalid classes, // no matter what @HandlesTypes says... if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && WebApplicationInitializer.class.isAssignableFrom(waiClass)) { try { initializers.add((WebApplicationInitializer) ReflectionUtils.accessibleConstructor(waiClass).newInstance()); } catch (Throwable ex) { throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex); } } } } if (initializers.isEmpty()) { servletContext.log("No Spring WebApplicationInitializer types detected on classpath"); return; } servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath"); AnnotationAwareOrderComparator.sort(initializers); for (WebApplicationInitializer initializer : initializers) {
//执行每一个WebApplicationInitializer的onStartUp方法 initializer.onStartup(servletContext); } } }
而我们的类
public class ServletInitializer extends SpringBootServletInitializer { @Override protected SpringApplicationBuilder configure(SpringApplicationBuilder application) { return application.sources(DemoApplication.class); } }
可以从继承结构看出就是
WebApplicationInitializer
看一下SpringBootServletInitializer 的onStartUp方法
@Override public void onStartup(ServletContext servletContext) throws ServletException { // Logger initialization is deferred in case an ordered // LogServletContextInitializer is being used this.logger = LogFactory.getLog(getClass()); //创建容器，打开详细看一下 WebApplicationContext rootApplicationContext = createRootApplicationContext(servletContext); if (rootApplicationContext != null) { servletContext.addListener(new SpringBootContextLoaderListener(rootApplicationContext, servletContext)); } else { this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not " + "return an application context"); } }
protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) { //这里创建application应用，同嵌入式容器一样执行public //SpringApplication(ResourceLoader resourceLoader, Class<?>... //primarySources) { // this.resourceLoader = resourceLoader; // Assert.notNull(primarySources, "PrimarySources must //not be null"); // this.primarySources = new LinkedHashSet<>//(Arrays.asList(primarySources)); // this.webApplicationType = //WebApplicationType.deduceFromClasspath(); // setInitializers((Collection) //getSpringFactoriesInstances(ApplicationContextInitializer.class))//; // setListeners((Collection) //getSpringFactoriesInstances(ApplicationListener.class)); // this.mainApplicationClass = //deduceMainApplicationClass(); // } 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(AnnotationConfigServletWebServerApplicationContext.class); builder = configure(builder); builder.listeners(new WebEnvironmentPropertySourceInitializer(servletContext));
//得到Spring应用 SpringApplication application = builder.build(); if (application.getAllSources().isEmpty() && MergedAnnotations.from(getClass(), SearchStrategy.TYPE_HIERARCHY).isPresent(Configuration.class)) { application.addPrimarySources(Collections.singleton(getClass())); } Assert.state(!application.getAllSources().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.addPrimarySources(Collections.singleton(ErrorPageFilterConfiguration.class)); } application.setRegisterShutdownHook(false);
//然后执行run方法 return run(application); }
/** * Called to run a fully configured {@link SpringApplication}. * @param application the application to run * @return the {@link WebApplicationContext} */ protected WebApplicationContext run(SpringApplication application) { return (WebApplicationContext) application.run(); } /** * Run the Spring application, creating and refreshing a new * {@link ApplicationContext}. * @param args the application arguments (usually passed from a Java main method) * @return a running {@link ApplicationContext} */ public ConfigurableApplicationContext run(String... args) { StopWatch stopWatch = new StopWatch(); stopWatch.start(); ConfigurableApplicationContext context = null; Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>(); configureHeadlessProperty(); SpringApplicationRunListeners listeners = getRunListeners(args); listeners.starting(); try { ApplicationArguments applicationArguments = new DefaultApplicationArguments(args); ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments); configureIgnoreBeanInfo(environment); Banner printedBanner = printBanner(environment); context = createApplicationContext(); exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[] { ConfigurableApplicationContext.class }, context); prepareContext(context, environment, listeners, applicationArguments, printedBanner); refreshContext(context); afterRefresh(context, applicationArguments); stopWatch.stop(); if (this.logStartupInfo) { new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch); } listeners.started(context); callRunners(context, applicationArguments); } catch (Throwable ex) { handleRunFailure(context, ex, exceptionReporters, listeners); throw new IllegalStateException(ex); } try { listeners.running(context); } catch (Throwable ex) { handleRunFailure(context, ex, exceptionReporters, null); throw new IllegalStateException(ex); } return context; }
执行run以后同嵌入式的一致类，只有一个区别，
refreshContext（context）时不会再创建嵌入式的servlet容器
private void createWebServer() { WebServer webServer = this.webServer; ServletContext servletContext = getServletContext(); if (webServer == null && servletContext == null) { ServletWebServerFactory factory = getWebServerFactory(); this.webServer = factory.getWebServer(getSelfInitializer()); getBeanFactory().registerSingleton("webServerGracefulShutdown", new WebServerGracefulShutdownLifecycle(this.webServer)); getBeanFactory().registerSingleton("webServerStartStop", new WebServerStartStopLifecycle(this, this.webServer)); } else if (servletContext != null) { try { getSelfInitializer().onStartup(servletContext); } catch (ServletException ex) { throw new ApplicationContextException("Cannot initialize servlet context", ex); } } initPropertySources(); }