其实就是直接在pom.xml文件默认引入Tomcat的。。。。
1 <dependency> 2 <groupId>org.springframework.boot</groupId> 3 <artifactId>spring-boot-starter-json</artifactId> 4 <version>2.3.7.RELEASE</version> 5 <scope>compile</scope> 6 </dependency> 7 <dependency> 8 <groupId>org.springframework.boot</groupId> 9 <artifactId>spring-boot-starter-tomcat</artifactId> 10 <version>2.3.7.RELEASE</version> 11 <scope>compile</scope> 12 </dependency>
1. 通过spring-boot-autoconfigure的spring.factories中key为org.springframework.boot.autoconfigure.EnableAutoConfiguration，值为ServletWebServerFactoryAutoConfiguration引入配置文件
2. 通过@import注解引入EmbeddedTomcat的bean
3. 可以看到这里有@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })注解，由于spring-boot-web默认引入Tomcat.jar，所以这里会进行bean的注入，
看ServletWebServerFactoryAutoConfiguration类上的注解，其实也引入了Jetty、undertow，只是没有相应的jar包，所以没有注入相应的bean
4. 注入bean之后，怎么启动Tomcat呢？答案就在org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext里面createWebServer方法，这里类也是继承AbstractApplicationContext类的
5. 该方法里面会调用通过bean的byType去找servlet工厂bean，至此就找到第三步注入的bean--TomcatServletWebServerFactory
1 protected ServletWebServerFactory getWebServerFactory() { 2 // Use bean names so that we don‘t consider the hierarchy 3 String[] beanNames = getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class); 4 if (beanNames.length == 0) { 5 throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to missing " 6 + "ServletWebServerFactory bean."); 7 } 8 if (beanNames.length > 1) { 9 throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple " 10 + "ServletWebServerFactory beans : " + StringUtils.arrayToCommaDelimitedString(beanNames)); 11 } 12 return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class); 13 }
6. 剩下就是调用Tomcat工厂实现socket的监听了，具体可以参考 https://blog.csdn.net/qq_31086797/article/details/107418371
原文：https://www.cnblogs.com/volun/p/15201613.html