<packaging>war</packaging>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <!--移除tomcat依赖-->
    <exclusions>
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </exclusion>
    </exclusions>
</dependency>
或者
<dependency>
　　<groupId>org.springframework.boot</groupId>
　　<artifactId>spring-boot-starter-tomcat</artifactId>
　　<scope>provided</scope>
</dependency>

为什么继承该类，SpringBootServletInitializer源码注释：
Note that a WebApplicationInitializer is only needed if you are building a war file and deploying it. If you prefer to run an embedded web server then you won’t need this at all.
注意，如果您正在构建WAR文件并部署它，则需要WebApplicationInitializer。如果你喜欢运行一个嵌入式Web服务器，那么你根本不需要这个。
启动类代码：

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

@SpringBootApplication
public class Application extends SpringBootServletInitializer {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        return builder.sources(Application.class);
    }
}

public class ServletInitializer extends SpringBootServletInitializer {
    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
         //此处的Application.class为带有@SpringBootApplication注解的启动类
        return builder.sources(Application.class);
     }


}

使用外部Tomcat部署访问的时候，application.properties(或者application.yml)中配置的

server.port=
server.servlet.context-path=

将失效，请使用tomcat的端口，tomcat，webapps下项目名进行访问。为了防止应用上下文所导致的项目访问资源加载不到的问题，建议pom.xml文件中标签下添加标签：

<build>
    <!-- 应与application.properties(或application.yml)中context-path保持一致 -->
    <finalName>war包名称</finalName>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

在idea菜单栏中找到Build---->build Arufacts生成war，生成的war包会在target文件夹中找到