1.首先 修改pom.xml下的打包方式

<packaging>war</packaging>

2.添加servlet-api依赖

 <!--添加servlet-api的依赖-->
<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>3.1.0</version>
    <scope>provided</scope>
</dependency>

3.去除springboot 内嵌的tomcat模块

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <!-- 移除嵌入式tomcat插件 -->
    <exclusions>
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </exclusion>
    </exclusions>
</dependency>

4.修改编译设置

方法1:
<build>
  <plugins>
    <plugin>
      <artifactId>maven-war-plugin</artifactId>
      <version>2.6</version>
      <configuration>
        <!--如果想在没有web.xml文件的情况下构建WAR，请设置为false。-->      
        <failOnMissingWebXml>false</failOnMissingWebXml>                        
      </configuration>
    </plugin>
  </plugins>
</build>

方法2:
添加以下配置到pom.xml文件下

版本3.0.0的插件 web.xml不存在问题，所以可以通过升级插件来解决问题

<plugin>
  <artifactId>maven-war-plugin</artifactId>
  <version>3.0.0</version>
</plugin>

5.修改启动类

@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(this.getClass());
    }
}

6.打包编译