SpringBoot嵌入式容器(自动配置原理、自定义嵌入式容器)

1. 自动配置原理

SpringBoot默认嵌入Tomcat作为Servlet容器
自动配置类是ServletWebServerFactoryAutoConfiguration、EmbeddedWebServerFactoryCustomizerAutoConfiguration
从自动配置类开始分析功能。如xxxxAutoConfiguration

先分析ServletWebServerFactoryAutoConfiguration自动配置类：

......省略部分......
@AutoConfiguration(after = {SslAutoConfiguration.class}
)
@AutoConfigureOrder(Integer.MIN_VALUE)
@ConditionalOnClass({ServletRequest.class})  
@ConditionalOnWebApplication(type = Type.SERVLET
)
@EnableConfigurationProperties({ServerProperties.class})
@Import({BeanPostProcessorsRegistrar.class, ServletWebServerFactoryConfiguration.EmbeddedTomcat.class, ServletWebServerFactoryConfiguration.EmbeddedJetty.class, ServletWebServerFactoryConfiguration.EmbeddedUndertow.class})
public class ServletWebServerFactoryAutoConfiguration {
......省略部分......
}

ServletWebServerFactoryAutoConfiguration自动配置了嵌入式容器场景
绑定了ServerProperties配置类，所有和服务器有关的配置都是server开头，和tomcat有关的配置都是server.tomcat开头
ServletWebServerFactoryAutoConfiguration导入了嵌入式的三大服务器 Tomcat、Jetty、Undertow
1.导入Tomcat、Jetty、Undertow都有条件注解。系统中有这个类才行，也就是导了这个包
2.默认 Tomcat配置生效。给容器中放了TomcatServletWebServerFactory，这是一个 web服务器工厂，造web服务器的

......省略部分......@Configuration(proxyBeanMethods = false)@ConditionalOnClass({Servlet.class, Tomcat.class, UpgradeProtocol.class})@ConditionalOnMissingBean(value = {ServletWebServerFactory.class},search = SearchStrategy.CURRENT)static class EmbeddedTomcat {EmbeddedTomcat() {}@BeanTomcatServletWebServerFactory tomcatServletWebServerFactory(ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers, ObjectProvider<TomcatContextCustomizer> contextCustomizers, ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {......省略部分......}......省略部分......}
......省略部分......

3.web服务器工厂都有一个功能，getWebServer获取web服务器
4.TomcatServletWebServerFactory创建了tomcat

......省略部分......
public class TomcatServletWebServerFactory extends AbstractServletWebServerFactory implements ConfigurableTomcatWebServerFactory, ResourceLoaderAware {......省略部分......public WebServer getWebServer(ServletContextInitializer... initializers) {......省略部分......Tomcat tomcat = new Tomcat();......省略部分......}
......省略部分......
}

ServletWebServerApplicationContextioc容器，启动的时候createWebServer方法会调用TomcatServletWebServerFactory.getWebServer创建web服务器
Spring容器刷新(启动)的时候，执行经典的十二大步，会预留一个时机，调用onRefresh()刷新子容器

......省略部分......
public class ServletWebServerApplicationContext extends GenericWebApplicationContextimplements ConfigurableWebServerApplicationContext {......省略部分......@Overrideprotected void onRefresh() {super.onRefresh();try {createWebServer();}catch (Throwable ex) {throw new ApplicationContextException("Unable to start web server", ex);}}......省略部分......
}

经典的十二大步调用

......省略部分......
public abstract class AbstractApplicationContext extends DefaultResourceLoaderimplements ConfigurableApplicationContext {......省略部分......@Overridepublic void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {......省略部分......try {// Allows post-processing of the bean factory in context subclasses.postProcessBeanFactory(beanFactory);StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");// Invoke factory processors registered as beans in the context.invokeBeanFactoryPostProcessors(beanFactory);// Register bean processors that intercept bean creation.registerBeanPostProcessors(beanFactory);beanPostProcess.end();// Initialize message source for this context.initMessageSource();// Initialize event multicaster for this context.initApplicationEventMulticaster();// Initialize other special beans in specific context subclasses.onRefresh();// Check for listener beans and register them.registerListeners();// Instantiate all remaining (non-lazy-init) singletons.finishBeanFactoryInitialization(beanFactory);// Last step: publish corresponding event.finishRefresh();}......省略部分......}......省略部分......
}

总结：Web场景的Spring容器启动，在onRefresh的时候，会调用创建web服务器的方法。Web服务器的创建是通过WebServerFactory搞定的。容器中又会根据导人员的包进行条件注解，启动相关的服务器配置，默认EmbeddedTomcat会给容器中放一个TomcatServletWebServerFactory，进行项目启动，自动创建出Tomcat

2. 自定义嵌入式容器

切换嵌入式服务器。如下所示：

    <properties><!-- springboot3使用jakarta-servlet 6，但是jetty使用jakarta-servlet 5- --><jakarta-servlet.version>5.0.0</jakarta-servlet.version></properties><dependencies><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></dependency></dependencies>