springboot默认加载的是tomcat容器，它还有另外两个可选择的容器：jetty和undertow，还有一个Configurable是可以自定义的一个接口；本文不讨论tomcat，jetty，undertow的优劣。感兴趣的同学可以自己查询。

通过org.springframework.boot.web.servlet.server.ServletWebServerFactory的实现就可以知道；那怎么样将默认的tomcat换成jetty或者undertow呢？

其实很简单，以jetty为例，只需要引入org.springframework.boot:spring-boot-starter-jetty包并且排除spring-boot-starter-tomcat包即可;若是要使用undertow，则引用包org.springframework.boot:spring-boot-starter-undertow

gradle配置如下：

configurations {
    compile.exclude module: "spring-boot-starter-logging"
    compile.exclude module: "spring-boot-starter-tomcat"
}
dependencies {

    compile("com.alibaba:fastjson:1.2.75")
    compile("mysql:mysql-connector-java")

    //spring
    compile("org.springframework.boot:spring-boot-starter-jetty")
    compile("org.springframework.boot:spring-boot-starter")
    compile("org.mybatis.spring.boot:mybatis-spring-boot-starter:2.1.4")
    compile("org.springframework.boot:spring-boot-starter-web")
    compile("org.springframework.boot:spring-boot-starter-actuator")
    //compile("org.springframework.boot:spring-boot-starter-aop")
    //compile("org.springframework.boot:spring-boot-starter-log4j2")

    //lombok
    compileOnly 'org.projectlombok:lombok'
    annotationProcessor 'org.projectlombok:lombok'

    //log
    compile "org.apache.logging.log4j:log4j-api:2.14.0"
    compile "org.apache.logging.log4j:log4j-core:2.14.0"

    //test
    testCompile('org.springframework.boot:spring-boot-starter-test')
    testImplementation 'io.projectreactor:reactor-test'
}

那么我们从源码阶段分析一下，为什么这样配置即可换容器；在包spring-boot-2.4.3.jar中，路径：org.springframework.boot.web.embedded下，有4个包：jetty,netty,tomcat,undertow；统一接口为ServletWebServerFactory，它有三个常用的实现JettyServletWebServerFactory，UndertowServletWebServerFactory，TomcatServletWebServerFactory； 默认会使TomcatServletWebServerFactory该实现； servlet包的context包下ServletWebServerApplicationContext该类createWebServer方法会创建webServer，选择factory的源代码如下：

    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 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);
    }

从上面代码就可以看出，排除tomcat包并增加需要使用的容器的包即可

总结：springboot换容器，只需要引入相关包，并且排除默认的tomcat包即可