springboot应用启动流程分析，嵌入式tomcat

之前我们分析了下springboot自动装载的原理，现在我们看看springboot应用启动的流程： 一般调用如下：

// 应用代码 SpringApplication.run(MiddlewareApplication.class, args); // SpringApplication.java public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) { return run(new Class<?>[] { primarySource }, args); } public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) { return new SpringApplication(primarySources).run(args); } public SpringApplication(Class<?>... primarySources) { this(null, primarySources); } 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(); } 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; } public enum WebApplicationType { /** * The application should not run as a web application and should not start an * embedded web server. */ NONE, /** * The application should run as a servlet-based web application and should start an * embedded servlet web server. */ SERVLET, /** * The application should run as a reactive web application and should start an * embedded reactive web server. */ REACTIVE; private static final String[] SERVLET_INDICATOR_CLASSES = { "javax.servlet.Servlet", "org.springframework.web.context.ConfigurableWebApplicationContext" }; private static final String WEBMVC_INDICATOR_CLASS = "org.springframework." + "web.servlet.DispatcherServlet"; private static final String WEBFLUX_INDICATOR_CLASS = "org." + "springframework.web.reactive.DispatcherHandler"; private static final String JERSEY_INDICATOR_CLASS = "org.glassfish.jersey.servlet.ServletContainer"; private static final String SERVLET_APPLICATION_CONTEXT_CLASS = "org.springframework.web.context.WebApplicationContext"; private static final String REACTIVE_APPLICATION_CONTEXT_CLASS = "org.springframework.boot.web.reactive.context.ReactiveWebApplicationContext"; static WebApplicationType deduceFromClasspath() { if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null) && !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) { return WebApplicationType.REACTIVE; } for (String className : SERVLET_INDICATOR_CLASSES) { if (!ClassUtils.isPresent(className, null)) { return WebApplicationType.NONE; } } return WebApplicationType.SERVLET; } static WebApplicationType deduceFromApplicationContext(Class<?> applicationContextClass) { if (isAssignable(SERVLET_APPLICATION_CONTEXT_CLASS, applicationContextClass)) { return WebApplicationType.SERVLET; } if (isAssignable(REACTIVE_APPLICATION_CONTEXT_CLASS, applicationContextClass)) { return WebApplicationType.REACTIVE; } return WebApplicationType.NONE; } private static boolean isAssignable(String target, Class<?> type) { try { return ClassUtils.resolveClassName(target, null).isAssignableFrom(type); } catch (Throwable ex) { return false; } } }

可以看到，这里首先会判断当前ApplicaitonContext的类型，判断的依据是判断当前环境下是否存在对应的类，返回相应的WebApplicationType，获取到响应WebApplicationType之后会初始化对应的ApplicationContext：

protected ConfigurableApplicationContext createApplicationContext() { Class<?> contextClass = this.applicationContextClass; if (contextClass == null) { try { switch (this.webApplicationType) { case SERVLET: contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS); break; case REACTIVE: contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS); break; default: contextClass = Class.forName(DEFAULT_CONTEXT_CLASS); } } catch (ClassNotFoundException ex) { throw new IllegalStateException( "Unable create a default ApplicationContext, " + "please specify an ApplicationContextClass", ex); } } return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass); }

默认情况下，如果没有web相关类的话，初始化的是： org.springframework.context.annotation.AnnotationConfigApplicationContext,这个我们在讲解springboot自动装配的时候有过说明。 如果是web的话，比如我们默认使用的基于Servlet： org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext

public class AnnotationConfigServletWebServerApplicationContext extends ServletWebServerApplicationContext implements AnnotationConfigRegistry { private final AnnotatedBeanDefinitionReader reader; private final ClassPathBeanDefinitionScanner scanner; private final Set<Class<?>> annotatedClasses = new LinkedHashSet<>(); private String[] basePackages; public AnnotationConfigServletWebServerApplicationContext() { this.reader = new AnnotatedBeanDefinitionReader(this); this.scanner = new ClassPathBeanDefinitionScanner(this); } ... }

可以看到，这里还是和AnnotationConfigApplicationContext一样，使用了AnnotatedBeanDefinitionReader和ClassPathBeanDefinitionScanner.

我们看下，大家有时候在基于springboot构建web应用的时候，只要导入相应的jar包，就可以开启web相关应用，我们看下实现逻辑： 在其父类ServletWebServerApplicationContext我们发现如下方法：

// ServletWebServerApplicationContext.java protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) { beanFactory.addBeanPostProcessor(new WebApplicationContextServletContextAwareProcessor(this)); beanFactory.ignoreDependencyInterface(ServletContextAware.class); registerWebApplicationScopes(); } @Override public final void refresh() throws BeansException, IllegalStateException { try { super.refresh(); } catch (RuntimeException ex) { stopAndReleaseWebServer(); throw ex; } } @Override protected void onRefresh() { super.onRefresh(); try { createWebServer(); } catch (Throwable ex) { throw new ApplicationContextException("Unable to start web server", ex); } } @Override protected void finishRefresh() { super.finishRefresh(); WebServer webServer = startWebServer(); if (webServer != null) { publishEvent(new ServletWebServerInitializedEvent(webServer, this)); } } @Override protected void onClose() { super.onClose(); stopAndReleaseWebServer(); } private void createWebServer() { WebServer webServer = this.webServer; ServletContext servletContext = getServletContext(); if (webServer == null && servletContext == null) { ServletWebServerFactory factory = getWebServerFactory(); this.webServer = factory.getWebServer(getSelfInitializer()); } else if (servletContext != null) { try { getSelfInitializer().onStartup(servletContext); } catch (ServletException ex) { throw new ApplicationContextException("Cannot initialize servlet context", ex); } } initPropertySources(); } protected ServletWebServerFactory getWebServerFactory() { // Use bean names so that we don't consider the hierarchy String[] beanNames = getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class); if (beanNames.length == 0) { throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to missing " + "ServletWebServerFactory bean."); } if (beanNames.length > 1) { throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple " + "ServletWebServerFactory beans : " + StringUtils.arrayToCommaDelimitedString(beanNames)); } return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class); }

可以看到在onFresh的时候，执行了createWebServer，在这里创建了webserver， 而这里的ServletWebServerFactory就是创建webserver的关键

还记之前在研究springboot装载的时候会按照响应条件加载META-INF/spring.factories中相关配置类，其中有一个 org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration这个就是装载webserver类的处理：

@Configuration @AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE) @ConditionalOnClass(ServletRequest.class) @ConditionalOnWebApplication(type = Type.SERVLET) @EnableConfigurationProperties(ServerProperties.class) @Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class, ServletWebServerFactoryConfiguration.EmbeddedTomcat.class, ServletWebServerFactoryConfiguration.EmbeddedJetty.class, ServletWebServerFactoryConfiguration.EmbeddedUndertow.class }) public class ServletWebServerFactoryAutoConfiguration { ... } @Configuration class ServletWebServerFactoryConfiguration { @Configuration @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class }) @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT) static class EmbeddedTomcat { @Bean public TomcatServletWebServerFactory tomcatServletWebServerFactory() { return new TomcatServletWebServerFactory(); } } /** * Nested configuration if Jetty is being used. */ @Configuration @ConditionalOnClass({ Servlet.class, Server.class, Loader.class, WebAppContext.class }) @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT) static class EmbeddedJetty { @Bean public JettyServletWebServerFactory JettyServletWebServerFactory() { return new JettyServletWebServerFactory(); } } /** * Nested configuration if Undertow is being used. */ @Configuration @ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class }) @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT) static class EmbeddedUndertow { @Bean public UndertowServletWebServerFactory undertowServletWebServerFactory() { return new UndertowServletWebServerFactory(); } } }

可以看到只要相应webserver对应的类能够存在导入，则会导入对应的web服务类型，我们以tomcat 为例,，如果存在tomcat服务器相关类型后，则使用TomcatServletWebServerFactory ：

// TomcatServletWebServerFactory.java public WebServer getWebServer(ServletContextInitializer... initializers) { Tomcat tomcat = new Tomcat(); File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat"); tomcat.setBaseDir(baseDir.getAbsolutePath()); Connector connector = new Connector(this.protocol); tomcat.getService().addConnector(connector); customizeConnector(connector); tomcat.setConnector(connector); tomcat.getHost().setAutoDeploy(false); configureEngine(tomcat.getEngine()); for (Connector additionalConnector : this.additionalTomcatConnectors) { tomcat.getService().addConnector(additionalConnector); } prepareContext(tomcat.getHost(), initializers); return getTomcatWebServer(tomcat); } protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) { return new TomcatWebServer(tomcat, getPort() >= 0); } // TomcatWebServer.java public TomcatWebServer(Tomcat tomcat, boolean autoStart) { Assert.notNull(tomcat, "Tomcat Server must not be null"); this.tomcat = tomcat; this.autoStart = autoStart; initialize(); } private void initialize() throws WebServerException { logger.info("Tomcat initialized with port(s): " + getPortsDescription(false)); synchronized (this.monitor) { try { addInstanceIdToEngineName(); Context context = findContext(); context.addLifecycleListener((event) -> { if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) { // Remove service connectors so that protocol binding doesn't // happen when the service is started. removeServiceConnectors(); } }); // Start the server to trigger initialization listeners this.tomcat.start(); // We can re-throw failure exception directly in the main thread rethrowDeferredStartupExceptions(); try { ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader()); } catch (NamingException ex) { // Naming is not enabled. Continue } // Unlike Jetty, all Tomcat threads are daemon threads. We create a // blocking non-daemon to stop immediate shutdown startDaemonAwaitThread(); } catch (Exception ex) { stopSilently(); destroySilently(); throw new WebServerException("Unable to start embedded Tomcat", ex); } } }

这里能够基于注解自动装载tomcat服务器，而不是像一般用的打成war包然后部署到tomcat服务器里面去，tomcat提供了嵌入式tomcat支持，能够像普通写代码一样，在代码里直接生成tomcat web容器。