SpringBoot默认的容器为Tomcat，

依赖包在spring-boot-starter-web下

Xml代码

<dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>  

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>  

</dependencies>

SpringBoot把容器修改为Jetty

方法很简单，就是在pom.xml文件中，在引用的spring-boot-starter-web排除Tomcat的依赖包，然后再引入Jetty容器的依赖包，如下：

Xml代码

<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>  

        <!-- Jetty适合长连接应用，就是聊天类的长连接 -->
        <!-- 使用Jetty，需要在spring-boot-starter-web排除spring-boot-starter-tomcat，因为SpringBoot默认使用tomcat -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-jetty</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
</dependencies>

SpringBoot把容器修改为undertow

Xml代码

<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>  

        <!-- undertow不支持jsp -->
        <!-- 使用undertow，需要在spring-boot-starter-web排除spring-boot-starter-tomcat，因为SpringBoot默认使用tomcat -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-undertow</artifactId>
        </dependency>  

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
</dependencies>

为什么可以这样切换呢？

因为SpringBoot在org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration类中已经配置好，根据依赖的Jar包自动切换，代码如下：

Java代码

/**
     * Nested configuration if Tomcat is being used.
     */
    @Configuration
    @ConditionalOnClass({ Servlet.class, Tomcat.class })
    @ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedTomcat {  

    @Bean
    public TomcatEmbeddedServletContainerFactory tomcatEmbeddedServletContainerFactory() {
        return new TomcatEmbeddedServletContainerFactory();
    }  

}  

/**
     * Nested configuration if Jetty is being used.
     */
@Configuration
@ConditionalOnClass({ Servlet.class, Server.class, Loader.class,
                     WebAppContext.class })
@ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedJetty {  

    @Bean
    public JettyEmbeddedServletContainerFactory jettyEmbeddedServletContainerFactory() {
        return new JettyEmbeddedServletContainerFactory();
    }  

}  

/**
     * Nested configuration if Undertow is being used.
     */
@Configuration
@ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
@ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedUndertow {  

    @Bean
    public UndertowEmbeddedServletContainerFactory undertowEmbeddedServletContainerFactory() {
        return new UndertowEmbeddedServletContainerFactory();
    }  

}

方法上注解根据依赖的容器Jar包判断使用哪个容器：

如：

1、tomcat容器

Java代码

@ConditionalOnClass({ Servlet.class, Tomcat.class })

表示有使用类Tomcat.class则是tomcat容器

2、Jetty容器

Java代码

@ConditionalOnClass({ Servlet.class, Server.class, Loader.class,
            WebAppContext.class })

3、undertow容器

Java代码

@ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })