SpringBoot内嵌并加载WebServer容器源码解析

先搭建一个SpringBoot的web工程，引入jar包，简单的一个启动类执行main方法即可启动一个web服务了。

引入一个springboot基础依赖包，一个web相关的包即可。

<parent> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-parent</artifactId> <version>2.3.1.RELEASE</version> </parent> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency>

启动类

@SpringBootApplication public class DemoApplication { public static void main(String[] args) { SpringApplication.run(DemoApplication.class, args); } }

本文将重点关注与嵌入WebServer容器相关的代码部分，但是这个流程又是包含在整个spring加载的主流程中，所以也会涉及一点点spring相关的知识，如有不理解只需跟着文章看就可以，重在理解大致的脉络，之后再回过头来细细研究。

首先入口从run方法开始。

public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) { return run(new Class<?>[] { primarySource }, args); } public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) { //这个构造方法中可以确定webApplicationType类型 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); //获取ApplicationContext实现类，根据不同的实现类，spring启动的流程略有不同，比如springboot就是利用ApplicationContext预留的方法来加载Web容器的。 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; }

createApplicationContext()

protected ConfigurableApplicationContext createApplicationContext() { Class<?> contextClass = this.applicationContextClass; if (contextClass == null) { try { //根据webApplicationType决定生成哪种类型 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); }

3个case分别对应了不同WebServer容器，主要是webmvc和响应式fluxmvc，看代码注释都能理解。

/** * 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;

所以创建哪一种的关键就在switch这个条件了。

switch (this.webApplicationType)

这个属性是在前面run方法中的构造方法里赋值的。

return new SpringApplication(primarySources).run(args); public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) { this.resourceLoader = resourceLoader; Assert.notNull(primarySources, "PrimarySources must not be null"); this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources)); //给webApplicationType赋值 this.webApplicationType = WebApplicationType.deduceFromClasspath(); setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class)); setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class)); this.mainApplicationClass = deduceMainApplicationClass(); }

从下面这段代码可以看出，主要就是根据你的jar中有哪一种class来判断，也就是你引入了哪种mvc方式就使用哪种，spring-boot-starter-web中使用的是springmvc，如果要使用fluxmvc应该引入spring-boot-starter-webflux。

springmvc对应的自然是org.springframework.web.servlet.DispatcherServlet

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"; 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; }

所以现在可以确定switch的条件是SERVLET，对应构建的对象就是AnnotationConfigServletWebServerApplicationContext

case SERVLET: contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS); public static final String DEFAULT_SERVLET_WEB_CONTEXT_CLASS = "org.springframework.boot." + "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";

ApplicationContext实现类确定后，回到run方法，此时就可以根据具体的实现类来决定spring在启动时需要做哪些事情了。

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); //获取ApplicationContext实现类，根据不同的实现类，spring启动的流程略有不同，比如springboot就是利用ApplicationContext预留的方法来加载Web容器的。 context = createApplicationContext(); exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[] { ConfigurableApplicationContext.class }, context); prepareContext(context, environment, listeners, applicationArguments, printedBanner); //spring加载核心方法 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; }

主要分析refreshContext方法，这个是spring启动初始化的核心方法。

protected void refresh(ConfigurableApplicationContext applicationContext) { applicationContext.refresh(); } @Override public final void refresh() throws BeansException, IllegalStateException { try { //调用父类的AbstractApplicationContext的refresh方法 super.refresh(); } catch (RuntimeException ex) { WebServer webServer = this.webServer; if (webServer != null) { webServer.stop(); } throw ex; } }

spring容器加载的核心方法，定义一系列方法，一种模板方法模式，其中嵌入web容器的入口就在onRefresh中。

@Override public void refresh() throws BeansException, IllegalStateException { synchronized (this.startupShutdownMonitor) { // Prepare this context for refreshing. prepareRefresh(); // Tell the subclass to refresh the internal bean factory. ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); // Prepare the bean factory for use in this context. prepareBeanFactory(beanFactory); try { // Allows post-processing of the bean factory in context subclasses. postProcessBeanFactory(beanFactory); // Invoke factory processors registered as beans in the context. invokeBeanFactoryPostProcessors(beanFactory); // Register bean processors that intercept bean creation. registerBeanPostProcessors(beanFactory); // Initialize message source for this context. initMessageSource(); // Initialize event multicaster for this context. initApplicationEventMulticaster(); // Initialize other special beans in specific context subclasses. //生成并加载webServer容器的入口 onRefresh(); // Check for listener beans and register them. registerListeners(); // Instantiate all remaining (non-lazy-init) singletons. finishBeanFactoryInitialization(beanFactory); // Last step: publish corresponding event. finishRefresh(); } catch (BeansException ex) { if (logger.isWarnEnabled()) { logger.warn("Exception encountered during context initialization - " + "cancelling refresh attempt: " + ex); } // Destroy already created singletons to avoid dangling resources. destroyBeans(); // Reset 'active' flag. cancelRefresh(ex); // Propagate exception to caller. throw ex; } finally { // Reset common introspection caches in Spring's core, since we // might not ever need metadata for singleton beans anymore... resetCommonCaches(); } } }

调用ServletWebServerApplicationContext的onRefresh方法，ServletWebServerApplicationContext是 AnnotationConfigServletWebServerApplicationContext的父类

@Override protected void onRefresh() { super.onRefresh(); try { createWebServer(); } catch (Throwable ex) { throw new ApplicationContextException("Unable to start web server", ex); } } private void createWebServer() { WebServer webServer = this.webServer; ServletContext servletContext = getServletContext(); if (webServer == null && servletContext == null) { //获取容器工厂对象 ServletWebServerFactory factory = getWebServerFactory(); //创建webServer 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(); } protected ServletWebServerFactory getWebServerFactory() { // Use bean names so that we don't consider the hierarchy //从spring容器中获取ServletWebServerFactory类型的beanNames String[] beanNames = getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class); //校验有且只有一个ServletWebServerFactory类型的beanNames 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)); } //从spring容器中获取具体对象 return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class); }

springboot会通过spi机制加载许多类，这些类都在spring.factories文件中，其中包含了与web容器相关的。

org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\

有Tomcat、Jetty、Undertow、Netty几种，当前class路径中只有Tomcat.class，所以会向容器中注册TomcatWebServerFactoryCustomizer，这个类是TomcatServletWebServerFactory的子类，而TomcatServletWebServerFactory又是ServletWebServerFactory的实现类。

@Configuration(proxyBeanMethods = false) @ConditionalOnWebApplication @EnableConfigurationProperties(ServerProperties.class) public class EmbeddedWebServerFactoryCustomizerAutoConfiguration { /** * Nested configuration if Tomcat is being used. */ @Configuration(proxyBeanMethods = false) @ConditionalOnClass({ Tomcat.class, UpgradeProtocol.class }) public static class TomcatWebServerFactoryCustomizerConfiguration { @Bean public TomcatWebServerFactoryCustomizer tomcatWebServerFactoryCustomizer(Environment environment, ServerProperties serverProperties) { return new TomcatWebServerFactoryCustomizer(environment, serverProperties); } } /** * Nested configuration if Jetty is being used. */ @Configuration(proxyBeanMethods = false) @ConditionalOnClass({ Server.class, Loader.class, WebAppContext.class }) public static class JettyWebServerFactoryCustomizerConfiguration { @Bean public JettyWebServerFactoryCustomizer jettyWebServerFactoryCustomizer(Environment environment, ServerProperties serverProperties) { return new JettyWebServerFactoryCustomizer(environment, serverProperties); } } /** * Nested configuration if Undertow is being used. */ @Configuration(proxyBeanMethods = false) @ConditionalOnClass({ Undertow.class, SslClientAuthMode.class }) public static class UndertowWebServerFactoryCustomizerConfiguration { @Bean public UndertowWebServerFactoryCustomizer undertowWebServerFactoryCustomizer(Environment environment, ServerProperties serverProperties) { return new UndertowWebServerFactoryCustomizer(environment, serverProperties); } } /** * Nested configuration if Netty is being used. */ @Configuration(proxyBeanMethods = false) @ConditionalOnClass(HttpServer.class) public static class NettyWebServerFactoryCustomizerConfiguration { @Bean public NettyWebServerFactoryCustomizer nettyWebServerFactoryCustomizer(Environment environment, ServerProperties serverProperties) { return new NettyWebServerFactoryCustomizer(environment, serverProperties); } } }

所以最终下面这个方法得到的就是TomcatServletWebServerFactory

ServletWebServerFactory factory = getWebServerFactory();

最后看一下getWebServer方法，构建一个TomcatWebServer对象，并启动。

@Override public WebServer getWebServer(ServletContextInitializer... initializers) { if (this.disableMBeanRegistry) { Registry.disableRegistry(); } Tomcat tomcat = new Tomcat(); File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat"); tomcat.setBaseDir(baseDir.getAbsolutePath()); //构建Connector Connector connector = new Connector(this.protocol); connector.setThrowOnFailure(true); tomcat.getService().addConnector(connector); customizeConnector(connector); tomcat.setConnector(connector); tomcat.getHost().setAutoDeploy(false); //Engine configureEngine(tomcat.getEngine()); for (Connector additionalConnector : this.additionalTomcatConnectors) { tomcat.getService().addConnector(additionalConnector); } //构建TomcatEmbeddedContext prepareContext(tomcat.getHost(), initializers); // return getTomcatWebServer(tomcat); } protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) { return new TomcatWebServer(tomcat, getPort() >= 0, getShutdown()); } public TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) { Assert.notNull(tomcat, "Tomcat Server must not be null"); this.tomcat = tomcat; this.autoStart = autoStart; this.gracefulShutdown = (shutdown == Shutdown.GRACEFUL) ? new GracefulShutdown(tomcat) : null; 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 //启动tomcat 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); } } }