spring-boot-mvc启动流程

1、前言

在web应用开发中，我们比较常用的是spring boot+spring mvc的组合。该组合的核心点就是内嵌tomcat和mvc的dispatcherServlet，今天，我们就来探究一下其启动流程。

前置知识点：spring boot原理、spring mvc原理

源码版本：spring boot 2.1.7

2、流程

此处先给出结论，抓住主线，便于后面的学习。

核心步骤分四步：

容器类型确定
创建web容器
启动内嵌tomcat
启动DispatcherServlet

大致时序图如下：

3、容器类型

容器类型，由WebApplicationType枚举类定义：

web容器
- mvc容器
- reactive容器
非web容器

SpringApplication的构造方法中设置

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {this.resourceLoader = resourceLoader;。。。。//调用方法设置容器类型this.webApplicationType = WebApplicationType.deduceFromClasspath();。。。。this.mainApplicationClass = deduceMainApplicationClass();
}

WebApplicationType.deduceFromClasspath()获取容器类型

static WebApplicationType deduceFromClasspath() {if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)&& !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {//reactivereturn WebApplicationType.REACTIVE;}for (String className : SERVLET_INDICATOR_CLASSES) {if (!ClassUtils.isPresent(className, null)) {//普通容器return WebApplicationType.NONE;}}//返回servlet容器，即mvcreturn WebApplicationType.SERVLET;
}

WebApplicationType.REACTIVE - 当类路径中存在REACTIVE_WEB_ENVIRONMENT_CLASS并且不存在MVC_WEB_ENVIRONMENT_CLASS时
WebApplicationType.NONE - 也就是非Web型应用(Standard型)，此时类路径中不包含WEB_ENVIRONMENT_CLASSES中定义的任何一个类时
WebApplicationType.SERVLET - 类路径中包含了WEB_ENVIRONMENT_CLASSES中定义的所有类型时

4、创建web容器

在上步，我们已经确定了容器的类型，接下来就是创建容器了。核心步骤：

根据类型获取定义的Class
通过反射创建容器

源码如下：

**SpringApplication.run()方法中的createApplicationContext()**执行

public ConfigurableApplicationContext run(String... args) {StopWatch stopWatch = new StopWatch();stopWatch.start();...try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);//准备环境，确认activefile，确认配置的文件的内容全部可用，此处会准备bootstarpapplication，ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);.    ...//创建容器context = createApplicationContext();...prepareContext(context, environment, listeners, applicationArguments, printedBanner);//刷新容器，关键点，tomcat在这里面的onrefresh中创建refreshContext(context);afterRefresh(context, applicationArguments);stopWatch.stop();...return context;
}

createApplicationContext()方法。根据容器类型创建容器，mvc的容器为ServletWebServerApplicationContext。

protected ConfigurableApplicationContext createApplicationContext() { Class<?> contextClass = this.applicationContextClass;//根据容器type加载classif (contextClass == null) {try {switch (this.webApplicationType) {case SERVLET://需要的servletWeb容器的Class，mvccontextClass = 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) {...}}//根据class创建并返回web容器return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

5、启动tomcat

我们观察项目启动的日志，会有tomcat的相关日志。debug找到相关的类，便于确定调用堆栈。

Debug的调用堆栈如下：

我们跟着调用堆栈走，先看看onRefrensh()方法。

5.1、onRefresh

ServletWebServerApplicationContext实现了onRefresh的方法，该方法在AbstractApplicationContext无实现，交由子类扩展。

实现逻辑如下：

protected void onRefresh() {super.onRefresh();try {//创建webServercreateWebServer();}catch (Throwable ex) {throw new ApplicationContextException("Unable to start web server", ex);}
}

5.2、createWebServer

该方法中通过工厂获取webServer，根据不同的类型，servlet容器是tomcat。

private void createWebServer() {WebServer webServer = this.webServer;ServletContext servletContext = getServletContext();if (webServer == null && servletContext == null) {//通过工厂获取，先加载工厂,此处是tomcatServletWebServerFactoryServletWebServerFactory factory = getWebServerFactory();//工厂生产产品，tomcatServerthis.webServer = factory.getWebServer(getSelfInitializer());}else if (servletContext != null) {try {getSelfInitializer().onStartup(servletContext);}catch (ServletException ex) {throw new ApplicationContextException("Cannot initialize servlet context", ex);}}initPropertySources();
}

5.2、 getWebServerFactory

然后通过容器getBean获取工厂bean，如果还没有，就会创建，进入getBean---->createBean的逻辑。在获取tomcatServletWebServerFactory中，引入了DispatcherServlet

protected ServletWebServerFactory getWebServerFactory() {//名字是tomcatServletWebServerFactoryString[] beanNames = getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);....//先获取工厂，然后通过getBean获取工厂bean,此处是tomcatServletWebServerFactoryreturn getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
}

5.2.1 tomcatServletWebServerFactory注入

上图，获取tomcatServletWebServerFactory工厂，肯定需要该工厂的bean定义，那么定义在哪呢？

是在ServletWebServerFactoryConfiguration.EmbeddedTomcat中注入的，源码如下：

class ServletWebServerFactoryConfiguration {@Configuration(proxyBeanMethods = false)@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)public static class EmbeddedTomcat {//配置TomcatServletWebServerFactory的bean,用于创建tomcatServer@Beanpublic TomcatServletWebServerFactory tomcatServletWebServerFactory(ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,ObjectProvider<TomcatContextCustomizer> contextCustomizers,ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();......return factory;}}
}

5.2.2、ServletWebServerFactoryConfiguration.EmbeddedTomcat

ServletWebServerFactoryConfiguration.EmbeddedTomcat是由ServletWebServerFactoryAutoConfiguration注入的。

@Configuration(proxyBeanMethods = false)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
//是在auto-configuation中注入的该类
@EnableConfigurationProperties(ServerProperties.class)
//注入了tomcat的工厂
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,//tomcat相关ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,ServletWebServerFactoryConfiguration.EmbeddedJetty.class,ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {@Beanpublic ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties) {return new ServletWebServerFactoryCustomizer(serverProperties);}//tomcat的自定义工厂，@Bean@ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(ServerProperties serverProperties) {return new TomcatServletWebServerFactoryCustomizer(serverProperties);}//注册后置处理器public static class BeanPostProcessorsRegistrar implements ImportBeanDefinitionRegistrar, BeanFactoryAware {private ConfigurableListableBeanFactory beanFactory;@Overridepublic void setBeanFactory(BeanFactory beanFactory) throws BeansException {if (beanFactory instanceof ConfigurableListableBeanFactory) {this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;}}//bean的后置处理器，errorPageRegistrarBeanPostProcessor最后挂到了DispatchServlet上@Overridepublic void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,BeanDefinitionRegistry registry) {if (this.beanFactory == null) {return;}registerSyntheticBeanIfMissing(registry, "webServerFactoryCustomizerBeanPostProcessor",WebServerFactoryCustomizerBeanPostProcessor.class);registerSyntheticBeanIfMissing(registry, "errorPageRegistrarBeanPostProcessor",ErrorPageRegistrarBeanPostProcessor.class);}}}

ServletWebServerFactoryAutoConfiguration是在auto-configuration的配置文件中注入的。

可以看见图片中，大多servlet相关的自动注入都在此处。

5.2.3、errorPageRegistrarBeanPostProcessor

创建tomcatServletWebServerFactory后对其进行后置处理时，用到了errorPageRegistrarBeanPostProcessor，该后置处理器初始化时最后调用了DispatchServlet

errorPageRegistrarBeanPostProcessor会获取register

private Collection<ErrorPageRegistrar> getRegistrars() {if (this.registrars == null) {// Look up does not include the parent contextthis.registrars = new ArrayList<>(//引入了ErrorPageRegistrar.class，关键点this.beanFactory.getBeansOfType(ErrorPageRegistrar.class, false, false).values());this.registrars.sort(AnnotationAwareOrderComparator.INSTANCE);this.registrars = Collections.unmodifiableList(this.registrars);}return this.registrars;}

5.2.4、ErrorPageRegistrar

唯一实现ErrorPageCustomizer，构造方法需要DispatcherServletPath的类

private static class ErrorPageCustomizer implements ErrorPageRegistrar, Ordered {private final ServerProperties properties;//DispatcherServletPathprivate final DispatcherServletPath dispatcherServletPath;//引入DispatcherServletPath，关键点protected ErrorPageCustomizer(ServerProperties properties, DispatcherServletPath dispatcherServletPath) {this.properties = properties;this.dispatcherServletPath = dispatcherServletPath;}
}

3.2.5、DispatcherServletPath

DispatcherServletRegistrationBean是DispatcherServletPath的实现，构造方法引入了DispatcherServlet

/*** ServletRegistrationBean for the auto-configured DispatcherServlet. Both registers the servlet and exposes   * DispatcherServletPath information.
*/
public class DispatcherServletRegistrationBean extends ServletRegistrationBean<DispatcherServlet>implements DispatcherServletPath {private final String path;//DispatcherServlet，关键类public DispatcherServletRegistrationBean(DispatcherServlet servlet, String path) {super(servlet);Assert.notNull(path, "Path must not be null");this.path = path;super.addUrlMappings(getServletUrlMapping());}}

5.2.6 DispatcherServlet

DispatcherServlet的由DispatcherServletAutoConfiguration注入如下：

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)
public class DispatcherServletAutoConfiguration {public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration";@Configuration(proxyBeanMethods = false)@Conditional(DefaultDispatcherServletCondition.class)@ConditionalOnClass(ServletRegistration.class)@EnableConfigurationProperties({ HttpProperties.class, WebMvcProperties.class })protected static class DispatcherServletConfiguration {//关键//注入了dispatcherServlet@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)public DispatcherServlet dispatcherServlet(HttpProperties httpProperties, WebMvcProperties webMvcProperties) {DispatcherServlet dispatcherServlet = new DispatcherServlet();......return dispatcherServlet;}}
}

DispatcherServletAutoConfiguration也是由spring.factories的auto-configura注入

5.3、getWebServer

获取webServer，在该方法中，tomcat出场了。

public WebServer getWebServer(ServletContextInitializer... initializers) {if (this.disableMBeanRegistry) {Registry.disableRegistry();}//tomcat出场Tomcat tomcat = new Tomcat();File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");tomcat.setBaseDir(baseDir.getAbsolutePath());//连接器Connector connector = new Connector(this.protocol);connector.setThrowOnFailure(true);//servicetomcat.getService().addConnector(connector);customizeConnector(connector);tomcat.setConnector(connector);tomcat.getHost().setAutoDeploy(false);//engineconfigureEngine(tomcat.getEngine());for (Connector additionalConnector : this.additionalTomcatConnectors) {tomcat.getService().addConnector(additionalConnector);}prepareContext(tomcat.getHost(), initializers);//创建tomcatwebserverreturn getTomcatWebServer(tomcat);
}

5.3.1 getTomcatWebServer

获取TomcatWebServer，原生的tomcat启动有些类似。

public TomcatWebServer(Tomcat tomcat, boolean autoStart) {Assert.notNull(tomcat, "Tomcat Server must not be null");this.tomcat = tomcat;this.autoStart = autoStart;//初始化initialize();
}
//初始化tomcat
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();}});//熟悉的地方// Start the server to trigger initialization listenersthis.tomcat.start();// We can re-throw failure exception directly in the main threadrethrowDeferredStartupExceptions();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 shutdownstartDaemonAwaitThread();}catch (Exception ex) {stopSilently();destroySilently();throw new WebServerException("Unable to start embedded Tomcat", ex);}}
}

6、初始化DispatcherServlet

tomcatServer启动时还没有初始化DispatcherServlet，9大组件都未初始化，等到有请求进来了，再去初始化DispatcherServlet。DispatcherServlet的初始化在onRefresh中：

//刷新
protected void onRefresh(ApplicationContext context) {//初始化initStrategies(context);
}//初始化
protected void initStrategies(ApplicationContext context) {//9大组件initMultipartResolver(context);initLocaleResolver(context);initThemeResolver(context);initHandlerMappings(context);initHandlerAdapters(context);initHandlerExceptionResolvers(context);initRequestToViewNameTranslator(context);initViewResolvers(context);initFlashMapManager(context);
}

6.1、onRefrsh

onRefresh由父类FrameworkServlet的initWebApplicationContext（）调用，初始化逻辑此处不介绍。

protected WebApplicationContext initWebApplicationContext() {//web容器WebApplicationContext rootContext =WebApplicationContextUtils.getWebApplicationContext(getServletContext());WebApplicationContext wac = null;if (this.webApplicationContext != null) {// A context instance was injected at construction time -> use itwac = this.webApplicationContext;....}if (!this.refreshEventReceived) {// Either the context is not a ConfigurableApplicationContext with refresh// support or the context injected at construction time had already been// refreshed -> trigger initial onRefresh manually here.synchronized (this.onRefreshMonitor) {//关键刷新onRefresh(wac);}}if (this.publishContext) {// Publish the context as a servlet context attribute.String attrName = getServletContextAttributeName();getServletContext().setAttribute(attrName, wac);}return wac;
}

6.2、调用栈

具体的调用堆栈如下：

由下至上可以看出:

NioEndpoint接受到请求，交由Processor处理。
不同的协议是不同的Processor处理。此处是Http11Processor，处理请求，封装Request对象，然后交由Engine引擎
Engine解析Host
Host确定容器
容器确定Wrappper，Wrapper就是Servlet的包装。此处就是DispatchServlet
然后DispatchServlet初始化并分发请求。