SpringBoot为Web应用提供了内置Web服务器，我们不用再额外下载Tomcat、Jetty、Undertow等服务器。
spring-boot-autoconfigure中提供了自动配置内置Web服务器的功能，只要添加了相关依赖，就会配置对应的Web服务器。
对于spring-boot-starter-web：
spring-boot-starter-tomcat（默认）：内置Tomcat服务器。spring-boot-starter-jetty：内置Jetty服务器。spring-boot-starter-undertow：内置Undertow服务器。
对于spring-boot-starter-webflux：
spring-boot-starter-reactor-netty（默认）：使用Netty监听网络请求。spring-boot-starter-tomcat：内置Tomcat服务器。spring-boot-starter-jetty：内置Jetty服务器。spring-boot-starter-undertow：内置Undertow服务器。
如果我们不想使用默认内置Web服务器，需要先移除默认值，然后导入需要的：
<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-jetty</artifactId>
<version>2.7.8</version>
</dependency>
内置Web服务器的自动配置基于SpringBoot自动配置SPI机制和BeanPostProcessor机制。简单来说包括以下步骤：
- 在
org.springframework.boot.autoconfigure.AutoConfiguration.imports中定义自动配置类：- EmbeddedWebServerFactoryCustomizerAutoConfiguration
- ServletWebServerFactoryAutoConfiguration
- ReactiveWebServerFactoryAutoConfiguration
- 在
EmbeddedWebServerFactoryCustomizerAutoConfiguration中注册WebServerFactoryCustomizer实现类。 - 在
ServletWebServerFactoryAutoConfiguration或ReactiveWebServerFactoryAutoConfiguration中注册WebServerFactoryCustomizerBeanPostProcessor和XxxWebServerFactory。 - 在
XxxWebServerApplicationContext的onfresh()阶段，使用XxxWebServerFactory创建WebServer，并监听指定端口。 - 在
WebServerFactoryCustomizerBeanPostProcessor中，使用WebServerFactory对WebServerFactory的bean对象进行自定义配置。
1 内置Web服务器自动配置原理（以Tomcat为例）
上图展示了内置Tomcat自动配置的相关类图，包括五个核心模块：
- EmbeddedWebServerFactoryCustomizerAutoConfiguration：注册TomcatWebServerFactoryCustomizer。
- ServletWebServerFactoryAutoConfiguration/ReactiveWebServerFactoryAutoConfiguration：注册WebServerFactoryCustomizerBeanPostProcessor和TomcatServletWebServerFactory
- WebServerFactoryCustomizerBeanPostProcessor：在TomcatServletWebServerFactory初始化前，使用TomcatWebServerFactoryCustomizer对其进行配置。
- TomcatServletWebServerFactory：创建TomcatWebServer。
- ServletWebServerApplicationContext：使用TomcatServletWebServerFactory创建TomcatWebServer。
1.1 EmbeddedWebServerFactoryCustomizerAutoConfiguration
EmbeddedWebServerFactoryCustomizerAutoConfiguration会根据是否添加依赖，来注册对应的WebServerFactoryCustomizer实现类：
- TomcatWebServerFactoryCustomizer
- JettyWebServerFactoryCustomizer
- UndertowWebServerFactoryCustomizer
- NettyWebServerFactoryCustomizer
EmbeddedWebServerFactoryCustomizerAutoConfiguration源码如下：
@AutoConfiguration
@ConditionalOnWebApplication
@EnableConfigurationProperties(ServerProperties.class) // 注册ServerProperties，读取配置信息
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);
}
}
}
1.2 ServletWebServerFactoryAutoConfiguration
ServletWebServerFactoryAutoConfiguration会注册以下bean：
- ServletWebServerFactoryCustomizer、TomcatServletWebServerFactoryCustomizer（Tomcat内置服务器时）
- WebServerFactoryCustomizerBeanPostProcessor
- TomcatServletWebServerFactory/JettyServletWebServerFactory/UndertowServletWebServerFactory
ServletWebServerFactoryAutoConfiguration部分源码如下：
@AutoConfiguration
@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 {
@Bean
public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties,
ObjectProvider<WebListenerRegistrar> webListenerRegistrars,
ObjectProvider<CookieSameSiteSupplier> cookieSameSiteSuppliers) {
return new ServletWebServerFactoryCustomizer(serverProperties,
webListenerRegistrars.orderedStream().collect(Collectors.toList()),
cookieSameSiteSuppliers.orderedStream().collect(Collectors.toList()));
}
@Bean
@ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")
public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(
ServerProperties serverProperties) {
return new TomcatServletWebServerFactoryCustomizer(serverProperties);
}
}
通过@Import(ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class)会注册WebServerFactoryCustomizerBeanPostProcessor：
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,
BeanDefinitionRegistry registry) {
if (this.beanFactory == null) {
return;
}
registerSyntheticBeanIfMissing(registry, "webServerFactoryCustomizerBeanPostProcessor",
WebServerFactoryCustomizerBeanPostProcessor.class,
WebServerFactoryCustomizerBeanPostProcessor::new);
registerSyntheticBeanIfMissing(registry, "errorPageRegistrarBeanPostProcessor",
ErrorPageRegistrarBeanPostProcessor.class, ErrorPageRegistrarBeanPostProcessor::new);
}
通过@Import(ServletWebServerFactoryConfiguration.EmbeddedXxx.class)会注册对应的XxxServletWebServerFactory，例如：
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
static class EmbeddedTomcat {
@Bean
TomcatServletWebServerFactory tomcatServletWebServerFactory(
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;
}
}
1.3 XxxWebServerApplicationContext
在XxxWebServerApplicationContext#onfresh()中，会创建webServer。需要注意的是，此时单例bean还没有初始化。
例如，ServletWebServerApplicationContext#onRefresh()：
protected void onRefresh() {
super.onRefresh();
try {
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}
在ServletWebServerApplicationContext#createWebServer()方法中，会创建ServletWebServerFactory的bean对象，此时会执行WebServerFactoryCustomizerBeanPostProcessor回调。然后再创建webServer，监听指定端口：
private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
// 从容器中获取ServletWebServerFactory
ServletWebServerFactory factory = getWebServerFactory();
createWebServer.tag("factory", factory.getClass().toString());
// 创建webServer
this.webServer = factory.getWebServer(getSelfInitializer());
createWebServer.end();
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);
}
}
// 初始化servletContext环境配置
initPropertySources();
}
ServletWebServerApplicationContext#getWebServerFactory()方法会从容器中获取ServletWebServerFactory。由于此时还没有初始化单例对象，因此会执行创建bean的流程：
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 MissingWebServerFactoryBeanException(getClass(), ServletWebServerFactory.class,
WebApplicationType.SERVLET);
}
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);
}
根据之前注册的不同XxxWebServerFactory，此时会调用对应实现类创建webServer。例如，TomcatServletWebServerFactory#getWebServer()：
public WebServer getWebServer(ServletContextInitializer... initializers) {
if (this.disableMBeanRegistry) {
Registry.disableRegistry();
}
// 创建Tomcat容器
Tomcat tomcat = new Tomcat();
File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
tomcat.setBaseDir(baseDir.getAbsolutePath());
for (LifecycleListener listener : this.serverLifecycleListeners) {
tomcat.getServer().addLifecycleListener(listener);
}
// 创建连接器
Connector connector = new Connector(this.protocol);
connector.setThrowOnFailure(true);
tomcat.getService().addConnector(connector);
customizeConnector(connector);
tomcat.setConnector(connector);
tomcat.getHost().setAutoDeploy(false);
// 配置引擎
configureEngine(tomcat.getEngine());
for (Connector additionalConnector : this.additionalTomcatConnectors) {
tomcat.getService().addConnector(additionalConnector);
}
prepareContext(tomcat.getHost(), initializers);
// 创建Tomcat服务器，并启动
return getTomcatWebServer(tomcat);
}
1.4 WebServerFactoryCustomizerBeanPostProcessor
在ServletWebServerFactory的创建过程中，会触发WebServerFactoryCustomizerBeanPostProcessor#postProcessBeforeInitialization()方法：
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
if (bean instanceof WebServerFactory) {
postProcessBeforeInitialization((WebServerFactory) bean);
}
return bean;
}
它会调用之前注册的所有WebServerFactoryCustomizer实现类，对WebServerFactory进行自定义配置：
private void postProcessBeforeInitialization(WebServerFactory webServerFactory) {
LambdaSafe.callbacks(WebServerFactoryCustomizer.class, getCustomizers(), webServerFactory)
.withLogger(WebServerFactoryCustomizerBeanPostProcessor.class)
.invoke((customizer) -> customizer.customize(webServerFactory));
}