Spring Boot与Spring MVC集成启动过程源码分析

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从一个最简单的Spring Boot Web项目聊起

我们知道，用spring-boot写一个web项目非常容易，pom继承spring-boot-parent然后引入依赖spring-boot-starter-web，再写一个这样的主启动类，然后就可以去写Controller了，十分简单，就像这样：

@SpringBootApplication public class SampleApplication { public static void main(String[] args) { SpringApplication.run(SampleApplication.class, args); } } // 然后再写一个Controller声明一个Rest服务 @RestController @RequestMapping("/perf") public class PerfController { @RequestMapping("/trace") public Object trace() { Object result = yourLogic(); return result; } }

聊聊SpringApplication.run

可是我们思考过，这背后spring-boot到底做了什么使我们的工作如此简单，它如何将spring、spring-mvc、tomcat整合到一起的呢？接下来我们以项目启动角度来分析整个初始化过程。

PS：下面代码分析过程中，着重于流程的串接，调用到某个变量时，作者会直接给出这个变量的具体实现，读者也许会产生困惑，但是不要停下来，先想当然的按照作者的思路把流程捋完，后面会针对各个主要的变量初始化及选择实现的过程进行逐个解释。

从SpringApplication.run说起：
方法定义如下

public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) { return new SpringApplication(primarySources).run(args); } 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();//1) exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[] { ConfigurableApplicationContext.class }, context); prepareContext(context, environment, listeners, applicationArguments, printedBanner); refreshContext(context);//2) 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; }

我们来分解下这个run方法
先看1)context = createApplicationContext()
负责创建spring主容器，这个方法内部是根据具体项目运行时依赖的类来动态选择实现的，如果是web项目则会选择AnnotationConfigServletWebServerApplicationContext，至于选择的规则及原因，这里先忽略，后面会专门介绍（时空门：ServletWebServerApplicationContext）。
接下来我们重点看2)refreshContext(context)方法
其方法内部最终调用了((AbstractApplicationContext) applicationContext).refresh()方法，我们把这个方法展开

@Override public void refresh() throws BeansException, IllegalStateException { synchronized (this.startupShutdownMonitor) { prepareRefresh(); ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); prepareBeanFactory(beanFactory); try { postProcessBeanFactory(beanFactory); invokeBeanFactoryPostProcessors(beanFactory); registerBeanPostProcessors(beanFactory); initMessageSource(); initApplicationEventMulticaster(); onRefresh();//3) registerListeners(); finishBeanFactoryInitialization(beanFactory); finishRefresh(); } catch (BeansException ex) { if (logger.isWarnEnabled()) { logger.warn("Exception encountered during context initialization - " + "cancelling refresh attempt: " + ex); } destroyBeans(); cancelRefresh(ex); throw ex; } finally { resetCommonCaches(); } } }

实际上，这里我们的调用已经到了spring-context包，其实跟spring-boot已经没啥关系了，这其实就是一个标准的SpringApplicationContext的标准启动过程中refresh()部分，我们不是对spring启动过程分解，所以我们只关注与tomcat，spring-mvc结合的部分。
直接看3)onRefresh()方法，因为AnnotationConfigServletWebServerApplicationContext是ServletWebServerApplicationContext的子类，所以流程进入ServletWebServerApplicationContext的onRefresh()方法

@Override protected void onRefresh() { super.onRefresh(); try { createWebServer();//4) } catch (Throwable ex) { throw new ApplicationContextException("Unable to start web server", ex); } }

可以看到这个4)createWebServer()，是我们的关键

private void createWebServer() { WebServer webServer = this.webServer; ServletContext servletContext = getServletContext(); if (webServer == null && servletContext == null) { ServletWebServerFactory factory = getWebServerFactory();//5) this.webServer = factory.getWebServer(getSelfInitializer());//6) } else if (servletContext != null) { try { getSelfInitializer().onStartup(servletContext); } catch (ServletException ex) { throw new ApplicationContextException("Cannot initialize servlet context", ex); } } initPropertySources(); }

其中：
5)ServletWebServerFactory factory = getWebServerFactory();
上面这句获取到的具体实现是TomcatServletWebServerFactory（时空门：TomcatServletWebServerFactory）
6)this.webServer = factory.getWebServer(getSelfInitializer());
先看6)中的getSelfInitializer()方法：

private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() { return this::selfInitialize; } private void selfInitialize(ServletContext servletContext) throws ServletException { prepareWebApplicationContext(servletContext); registerApplicationScope(servletContext); WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext); for (ServletContextInitializer beans : getServletContextInitializerBeans()) { beans.onStartup(servletContext); } }

这块有点意思，返回的是一个this::selfInitialize，方法定义是返回org.springframework.boot.web.servlet.ServletContextInitializer，我们看下它是什么定义

@FunctionalInterface public interface ServletContextInitializer { void onStartup(ServletContext servletContext) throws ServletException; }

@FunctionalInterface是java8中lambda支持的一种函数式接口selfInitialize这段逻辑在后面过程当中会被调用。
继续看6)中this.webServer = factory.getWebServer(...)，我们看下实现：

@Override 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);//7) return getTomcatWebServer(tomcat); }

可以看到在里面创建了Tomcat实例作为webServer的内部实现，然后向Tomcat的Service容器注入Connector，然后设置默认Host容器的AutoDeploy属性及其他的Tomcat初始化工作，最重要的一行是7)
我们来看一下：

protected void prepareContext(Host host, ServletContextInitializer[] initializers) { File documentRoot = getValidDocumentRoot(); TomcatEmbeddedContext context = new TomcatEmbeddedContext(); if (documentRoot != null) { context.setResources(new LoaderHidingResourceRoot(context)); } ...//省略我们不关注的部分代码 ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);//8) host.addChild(context);//将context加入host作为host的子容器 configureContext(context, initializersToUse);//9) postProcessContext(context); }

我们可以看到其调用host.addChild(context)将context加入host作为host的子容器，然后
其中8)查找所有ServletContextInitializer实现并合并为一个数组，然后调用9)configureContext方法，我们来看一下：

protected void configureContext(Context context, ServletContextInitializer[] initializers) { TomcatStarter starter = new TomcatStarter(initializers);//10) if (context instanceof TomcatEmbeddedContext) { TomcatEmbeddedContext embeddedContext = (TomcatEmbeddedContext) context; embeddedContext.setStarter(starter); embeddedContext.setFailCtxIfServletStartFails(true); } context.addServletContainerInitializer(starter, NO_CLASSES);//11) ...//忽略 }

10)创建了TomcatStarter对象，并将starter加入context的conainerInitializer列表，见11)，这样在tomcat的容器启动过程中就会调用到这个TomcatStarter实例。
我们来看下TomcatStarter做了什么

class TomcatStarter implements ServletContainerInitializer { ... private final ServletContextInitializer[] initializers; ... TomcatStarter(ServletContextInitializer[] initializers) { this.initializers = initializers; } ... @Override public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException { try { for (ServletContextInitializer initializer : this.initializers) { initializer.onStartup(servletContext); } } catch (Exception ex) { this.startUpException = ex; if (logger.isErrorEnabled()) { logger.error("Error starting Tomcat context. Exception: " + ex.getClass().getName() + ". Message: " + ex.getMessage()); } } } ... }

可以看到TomcatStarter相当于hook了context启动的事件，然后调用所有注入的initializers的onStartup方法，似曾相识是吗？这就是前面说的@FunctionalInterface函数接口，接下来我们就深入看下前面提到的那个initializer的onStartup的具体内容

//ServletWebServerApplicationContext类当中 private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() { return this::selfInitialize; } private void selfInitialize(ServletContext servletContext) throws ServletException { prepareWebApplicationContext(servletContext); registerApplicationScope(servletContext); WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext); for (ServletContextInitializer beans : getServletContextInitializerBeans()) { beans.onStartup(servletContext); } }

可以看到其对getServletContextInitializerBeans()的每个ServletContextInitializer均调用了onStartup方法

protected Collection<ServletContextInitializer> getServletContextInitializerBeans() { return new ServletContextInitializerBeans(getBeanFactory()); }

看看new ServletContextInitializerBeans(getBeanFactory())做了什么

@SafeVarargs public ServletContextInitializerBeans(ListableBeanFactory beanFactory, Class<? extends ServletContextInitializer>... initializerTypes) { this.initializers = new LinkedMultiValueMap<>(); this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes) : Collections.singletonList(ServletContextInitializer.class); addServletContextInitializerBeans(beanFactory); addAdaptableBeans(beanFactory); List<ServletContextInitializer> sortedInitializers = this.initializers.values().stream() .flatMap((value) -> value.stream().sorted(AnnotationAwareOrderComparator.INSTANCE)) .collect(Collectors.toList()); this.sortedList = Collections.unmodifiableList(sortedInitializers); logMappings(this.initializers); }

可以看到其从beanFactory中获取spring容器中所有的ServletContextInitializer实现，这里关于集成的部分在ServletRegistrationBean中，ServletRegistrationBean的注入过程参考：时空门：Dispatcherservletregistrationbean

private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) { for (Class<? extends ServletContextInitializer> initializerType : this.initializerTypes) { for (Entry<String, ? extends ServletContextInitializer> initializerBean : getOrderedBeansOfType(beanFactory, initializerType)) { addServletContextInitializerBean(initializerBean.getKey(), initializerBean.getValue(), beanFactory); } } } private void addServletContextInitializerBean(String beanName, ServletContextInitializer initializer, ListableBeanFactory beanFactory) { if (initializer instanceof ServletRegistrationBean) { Servlet source = ((ServletRegistrationBean<?>) initializer).getServlet(); addServletContextInitializerBean(Servlet.class, beanName, initializer, beanFactory, source); } else if (initializer instanceof FilterRegistrationBean) { Filter source = ((FilterRegistrationBean<?>) initializer).getFilter(); addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source); } else if (initializer instanceof DelegatingFilterProxyRegistrationBean) { String source = ((DelegatingFilterProxyRegistrationBean) initializer).getTargetBeanName(); addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source); } else if (initializer instanceof ServletListenerRegistrationBean) { EventListener source = ((ServletListenerRegistrationBean<?>) initializer).getListener(); addServletContextInitializerBean(EventListener.class, beanName, initializer, beanFactory, source); } else { addServletContextInitializerBean(ServletContextInitializer.class, beanName, initializer, beanFactory, initializer); } }

然后流程就顺了，我们会调用到ServletRegistrationBean的onStartup方法，最终会调用到servletContext.addServlet的Servlet3.0的标准将DispatchServlet注入到servlet容器中拦截所有的请求。
见下面代码：

//RegistrationBean @Override public final void onStartup(ServletContext servletContext) throws ServletException { String description = getDescription(); if (!isEnabled()) { logger.info(StringUtils.capitalize(description) + " was not registered (disabled)"); return; } register(description, servletContext); } //DynamicRegistrationBean @Override protected final void register(String description, ServletContext servletContext) { D registration = addRegistration(description, servletContext); if (registration == null) { logger.info( StringUtils.capitalize(description) + " was not registered " + "(possibly already registered?)"); return; } configure(registration); } //ServletRegistrationBean @Override protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) { String name = getServletName(); return servletContext.addServlet(name, this.servlet); }

至此所有集成完毕，启动过程交给tomcat完成。

没讲完的故事：各个依赖的组件是如何初始化的

TomcatServletWebServerFactory

spring-boot-autoconfigure/META-INF/spring.factories中有一段配置：

... # Auto Configure org.springframework.boot.autoconfigure.EnableAutoConfiguration=\ org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\ ...

然后我们来看下ServletWebServerFactoryAutoConfiguration类

@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 { ... }

其中@Import部分引入了ServletWebServerFactoryConfiguration.EmbeddedTomcat.class，深入看一下

@Configuration class ServletWebServerFactoryConfiguration { @Configuration @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class }) @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT) public static class EmbeddedTomcat { @Bean public TomcatServletWebServerFactory tomcatServletWebServerFactory() { return new TomcatServletWebServerFactory(); } } ... }

这块Spring Boot根据@ConditionalOnClass判断当前运行时环境是否符合条件，即包含了tomcat的jar包，如果满足则创建TomcatServletWebServerFactory的Bean实例加入spring容器管理，后面有用。

ServletWebServerApplicationContext

实际启动时，启动的是其子类AnnotationConfigServletWebServerApplicationContext，我们来看下SpringApplication类，实际上SpringApplication在运行时根据情况决定使用哪种ApplicationContext

查看createApplicationContext()方法

那么这个this.webApplicationType又是哪来的值呢？
我们看下这个构造方法

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(); }

WebApplicationType.deduceFromClasspath()用来自动识别这个值，看下实现：

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; }

可以看到它是通过判断classloader中是否有Servlet相关的class来判断的，所以是运行时判断的。

DispatcherServletRegistrationBean

DispatcherServletRegistrationBean是保证我们的DispatcherServlet被注入到Servlet容器并生效的关键，我们来看下它是如何初始化的
spring-boot-autoconfigure/META-INF/spring.factories中有一段配置：

# Auto Configure org.springframework.boot.autoconfigure.EnableAutoConfiguration=\ org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\

看看实现

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE) @Configuration @ConditionalOnWebApplication(type = Type.SERVLET) @ConditionalOnClass(DispatcherServlet.class) @AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class) public class DispatcherServletAutoConfiguration { @Configuration @Conditional(DispatcherServletRegistrationCondition.class) @ConditionalOnClass(ServletRegistration.class) @EnableConfigurationProperties(WebMvcProperties.class) @Import(DispatcherServletConfiguration.class) protected static class DispatcherServletRegistrationConfiguration { private final WebMvcProperties webMvcProperties; private final MultipartConfigElement multipartConfig; public DispatcherServletRegistrationConfiguration(WebMvcProperties webMvcProperties, ObjectProvider<MultipartConfigElement> multipartConfigProvider) { this.webMvcProperties = webMvcProperties; this.multipartConfig = multipartConfigProvider.getIfAvailable(); } @Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME) @ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME) public DispatcherServletRegistrationBean dispatcherServletRegistration(DispatcherServlet dispatcherServlet) { DispatcherServletRegistrationBean registration = new DispatcherServletRegistrationBean(dispatcherServlet, this.webMvcProperties.getServlet().getPath()); registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME); registration.setLoadOnStartup(this.webMvcProperties.getServlet().getLoadOnStartup()); if (this.multipartConfig != null) { registration.setMultipartConfig(this.multipartConfig); } return registration; } } }

可以看到，其像spring容器注册了DispatcherServletRegistrationBean的Bean实例，看一下它的继承关系：

其父类ServletRegistrationBean类有如下方法：

@Override protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) { String name = getServletName(); return servletContext.addServlet(name, this.servlet); }

其调用了ServletContext.addServlet方法将DispatchServlet加入到Servlet容器，这是Servlet3.0中注册servlet的方法。
那么你也许会问，addRegistration又是什么时机调用的呢？
根据继承关系，查看其父类的父类RegistrationBean，其有一个

@Override public final void onStartup(ServletContext servletContext) throws ServletException { String description = getDescription(); if (!isEnabled()) { logger.info(StringUtils.capitalize(description) + " was not registered (disabled)"); return; } register(description, servletContext); }

register方法是一个模板方法，调用子类DynamicRegistrationBean的实现

@Override protected final void register(String description, ServletContext servletContext) { D registration = addRegistration(description, servletContext); if (registration == null) { logger.info(StringUtils.capitalize(description) + " was not registered " + "(possibly already registered?)"); return; } configure(registration); }

addRegistration方法又是一个模板方法，实现就是前面ServletRegistrationBean的addRegistration实现，而onStartup方法会在SpringApplication.run()方法的流程中被调用到，讲主流程的时候已经讲到，这里不再赘述
这样就将DispatchServlet与Tomcat进行了集成，DispatchServlet使用模板方法设计模式，将具体的请求分配给不同的handler处理，这个后面会讲到，本篇就主要专注在Spring Boot与Spring MVC及Tomcat的集成原理部分。