当我们需要将spring boot应用部署至外部tomcat(或其他web容器)时，相关配置需作如下调整。

1.通过spring-boot-maven-plugin插件指定启动类，该类详见第3步。

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <configuration>
        <mainClass>com.sam.demo.bootstrap.AppStart</mainClass>
    </configuration>
</plugin>

2.为了正常编译，需在pom.xml文件中引入spring-boot-starter-tomcat和spring-boot-starter-tomcat,绑定到provided，同时在spring-boot-starter-web中排除tomcat依赖。最终生成的war包不包含任何与tomcat有关的jar包。可以将生成的war包部署至任何支持java web的容器中。

<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-tomcat</artifactId>
    <scope>provided</scope>
</dependency>
<dependency>
    <groupId>org.apache.tomcat</groupId>
    <artifactId>tomcat-servlet-api</artifactId>
    <version>7.0.68</version>
    <scope>provided</scope>
</dependency>

3.修改spring-boot-maven-plugin插件指定的启动类。不再以main方法启动项目，而是通过SpringApplicationBuilder来指定启动类。

import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.boot.web.support.SpringBootServletInitializer;

@SpringBootApplication
public class AppStart extends SpringBootServletInitializer {
    
    @Override  
    protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {  
        return application.sources(AppStart.class);  
    }
}

附：因为项目中不再有web.xml文件，所以在eclipse中会有错误提示。解决办法是在pom.xml中加入maven-war-plugin插件。如下：

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-war-plugin</artifactId>
    <configuration>
        <failOnMissingWebXml>false</failOnMissingWebXml>
    </configuration>
</plugin>