private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = this.getServletContext();
    if (webServer == null && servletContext == null) {
        StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
        // 獲取ServletWebFactory
        ServletWebServerFactory factory = this.getWebServerFactory();
        createWebServer.tag("factory", factory.getClass().toString());
        // 這里會(huì)去調(diào)用系統(tǒng)中獲取到的web容器工廠類(lèi)的getWebServer()方法
        this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()});
        createWebServer.end();
        this.getBeanFactory().registerSingleton("webServerGracefulShutdown", new WebServerGracefulShutdownLifecycle(this.webServer));
        this.getBeanFactory().registerSingleton("webServerStartStop", new WebServerStartStopLifecycle(this, this.webServer));
    } else if (servletContext != null) {
        try {
            this.getSelfInitializer().onStartup(servletContext);
        } catch (ServletException var5) {
            throw new ApplicationContextException("Cannot initialize servlet context", var5);
        }
    }
    this.initPropertySources();
}

protected ServletWebServerFactory getWebServerFactory() {
    String[] beanNames = this.getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);
    if (beanNames.length == 0) {
        throw new MissingWebServerFactoryBeanException(this.getClass(), ServletWebServerFactory.class, WebApplicationType.SERVLET);
    } else if (beanNames.length > 1) {
        throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple ServletWebServerFactory beans : " + StringUtils.arrayToCommaDelimitedString(beanNames));
    } else {
        return (ServletWebServerFactory)this.getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
    }
}

@AutoConfiguration
@AutoConfigureOrder(-2147483648)
@ConditionalOnClass({ServletRequest.class})
@ConditionalOnWebApplication(
    type = Type.SERVLET
)
@EnableConfigurationProperties({ServerProperties.class})
@Import({ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class, EmbeddedTomcat.class, EmbeddedJetty.class, EmbeddedUndertow.class})
public class ServletWebServerFactoryAutoConfiguration {
    public ServletWebServerFactoryAutoConfiguration() {
    }
    ...

public TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) {
    this.monitor = new Object();
    this.serviceConnectors = new HashMap();
    Assert.notNull(tomcat, "Tomcat Server must not be null");
    this.tomcat = tomcat;
    this.autoStart = autoStart;
    this.gracefulShutdown = shutdown == Shutdown.GRACEFUL ? new GracefulShutdown(tomcat) : null;
    // 初始化方法initialize---會(huì)調(diào)用this.tomcat.start();啟動(dòng)容器
    this.initialize();
}

<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-undertow</artifactId>
</dependency>

package com.decade.config;
import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory;
import org.springframework.boot.web.servlet.server.ConfigurableServletWebServerFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
public class MyConfig {
    @Bean
    public ConfigurableServletWebServerFactory defineWebServletFactory() {
        final TomcatServletWebServerFactory tomcatServletWebServerFactory = new TomcatServletWebServerFactory();
        tomcatServletWebServerFactory.setPort(8081);
        return tomcatServletWebServerFactory;
    }
}