一、前言

Springboot源码解析是一件大工程，逐行逐句的去研究代码，会很枯燥，也不容易坚持下去。
我们不追求大而全，而是试着每次去研究一个小知识点，最终聚沙成塔，这就是我们的springboot源码管中窥豹系列。

二、web服务器

以前的的spring项目或者springmvc项目都需要一个web服务器，tomcat,或者其它的
使用springboot之后，我们不再需要配置web服务器，因为springboot帮我们集成了
今天我们来分析一下源码，看看在哪里实现的，知其然知其所以然

三、源码分析

还是从SpringApplication的run方法开始看
不熟悉的可以看之前的文章：springboot源码解析-管中窥豹系列之总体结构（一）

SpringApplication.javapublic ConfigurableApplicationContext run(String... args) {...try {...refreshContext(context);...}catch (Throwable ex) {...}...return context;}

接着进入到 refreshContext(context) 里面

AbstractApplicationContext.java@Overridepublic void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {...try {...// Initialize other special beans in specific context subclasses.onRefresh();...}catch (BeansException ex) {...}finally {...}}}

进入到 onRefresh() 方法

protected void onRefresh() throws BeansException {// For subclasses: do nothing by default.}

注意这个是一个protected方法，我们进入到子实现里面
具体用的哪个context，请看之前的文章：springboot源码解析-管中窥豹系列之项目类型（二）

public static final String DEFAULT_CONTEXT_CLASS = \"org.springframework.context.\"+ \"annotation.AnnotationConfigApplicationContext\";public static final String DEFAULT_SERVLET_WEB_CONTEXT_CLASS = \"org.springframework.boot.\"+ \"web.servlet.context.AnnotationConfigServletWebServerApplicationContext\";public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = \"org.springframework.\"+ \"boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext\";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);}

最常用的就是普通web项目，我们看这一个
我们到org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext里面找 onRefresh() 方法
没找到，在父类ServletWebServerApplicationContext里面找到了

ServletWebServerApplicationContext.java@Overrideprotected void onRefresh() {super.onRefresh();try {createWebServer();}catch (Throwable ex) {throw new ApplicationContextException(\"Unable to start web server\", ex);}}

我们到 createWebServer() 方法里面

private void createWebServer() {WebServer webServer = this.webServer;ServletContext servletContext = getServletContext();if (webServer == null && servletContext == null) {ServletWebServerFactory factory = getWebServerFactory();this.webServer = factory.getWebServer(getSelfInitializer());}else ifad8(servletContext != null) {try {getSelfInitializer().onStartup(servletContext);}catch (ServletException ex) {throw new ApplicationContextException(\"Cannot initialize servlet context\",ex);}}initPropertySources();}

用的工厂模式，先找到工厂getWebServerFactory()
再用工厂生成webServer, factory.getWebServer(getSelfInitializer())
先看看 getWebServerFactory() 这个方法

protected ServletWebServerFactory getWebServerFactory() {// Use bean names so that we don\'t consider the hierarchyString[] 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);}

获取唯一的工厂：ServletWebServerFactory，多了少了都不行
在哪加载进springboot的呢？

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter</artifactId><scope>compile</scope></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-autoconfigure</artifactId><scope>compile</scope></dependency>

spring-boot-starter-web里面是包含了spring-boot-starter依赖的
spring-boot-starter里面包含了spring-boot-autoconfigure依赖
spring-boot-autoconfigure里面有一个类：ServletWebServerFactoryConfiguration
这个类里面有一个静态类: EmbeddedTomcat

@Configuration(proxyBeanMethods = false)@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)static class EmbeddedTomcat {@BeanTomcatServletWebServerFactory tomcatServlet56cWebServerFactory(ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,ObjectProvider<TomcatContextCustomizer> contextCustomizers,ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();factory.getTomcatConnectorCustomizers().addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));factory.getTomcatContextCustomizers().addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));factory.getTomcatProtocolHandlerCustomizers().addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));return factory;}}

ConditionOnClass : 它依赖 Servlet.class, Tomcat.class, UpgradeProtocol.class
ConditionalOnMissingBean：防止重复加载
至此，factory怎么加载进spring就找到了
我们多看一点，这个类ServletWebServerFactoryConfiguration还有两个静态类

@Configuration(proxyBeanMethods = false)@ConditionalOnClass({ Servlet.class, Server.class, Loader.class, WebAppContext.class })@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRad0ENT)static class EmbeddedJetty {@BeanJettyServletWebServerFactory JettyServletWebServerFactory(ObjectProvider<JettyServerCustomizer> serverCustomizers) {JettyServletWebServerFactory factory = new JettyServletWebServerFactory();factory.getServerCustomizers().addAll(serverCustomizers.orderedStream().collect(Collectors.toList()));return factory;}}@Configuration(proxyBeanMethods = false)@ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)static class EmbeddedUndertow {@BeanUndertowServletWebServerFactory undertowServletWebServerFactory(ObjectProvider<UndertowDeploymentInfoCustomizer> deploymentInfoCustomizers,ObjectProvider<UndertowBuilderCustomizer> builderCustomizers) {UndertowServletWebServerFactory factory = new UndertowServletWebServerFactory();factory.getDeploymentInfoCustomizers().addAll(deploymentInfoCustomizers.orderedStream().collect(Collectors.toList()));factory.getBuilderCustomizers().addAll(builderCustomizers.orderedStream().collect(Collectors.toList()));return factory;}}

一个是生成jetty服务器工厂，一个是生成undertow服务器工厂
它们的加载，取决于依赖的class是否存在
tomcat: Servlet.class, Tomcat.class, UpgradeProtocol.class
jetty: Servlet.class, Server.class, Loader.class, WebAppContext.class
undertow: Servlet.class, Undertow.class, SslClientAuthMode.class

假如我们想换成undertow服务器，依赖改了就行了

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><exclusions><exclusion><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-tomcat</artifactId></exclusion></exclusions></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-undertow</artifactId></dependency>

假如我们想换成jetty服务器，同理

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><exclusions><exclusion><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-tomcat</artifactId>1602</exclusion></exclusions></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-jetty</artifactId></dependency>

思维拉回来，工厂有了，我们看看工厂怎么生成的webServer

TomcatServletWebServerFactory.java@Overridepublic 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());Connector connector = new Connector(this.protocol);connector.setThrowOnFailure(true);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);}