SpringBoot如何默认引入Tomcat，并自动装配

SpringBoot引入Tomcat

其实就是直接在pom.xml文件默认引入Tomcat的。。。。

    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-json</artifactId>
      <version>2.3.7.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-tomcat</artifactId>
      <version>2.3.7.RELEASE</version>
      <scope>compile</scope>
    </dependency>

自动装配

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

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

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