Cannot initialize context because there is already a root application context present - check whether you have multiple ContextLoader* definitions in your web.xml!

报错信息

23-Aug-2019 13:40:42.269 严重 [localhost-startStop-1] org.apache.catalina.core.StandardContext.listenerStart Exception sending context initialized event to listener instance of class [org.springframework.web.context.ContextLoaderListener]
 java.lang.IllegalStateException: Cannot initialize context because there is already a root application context present - check whether you have multiple ContextLoader* definitions in your web.xml!
	at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:262)
	at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:103)
	at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4792)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5256)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
	at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:754)
	at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:730)
	at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:734)
	at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:1140)
	at org.apache.catalina.startup.HostConfig$DeployDirectory.run(HostConfig.java:1875)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

pom.xml 关键配置

<parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.1.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
<dependencies>
        <!-- 打包时移除嵌入式Tomcat插件 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
            <scope>provided</scope>
        </dependency>
    </dependencies>

ConsumerApplication启动配置

@SpringBootApplication(exclude = {DataSourceAutoConfiguration.class,DruidDataSourceAutoConfigure.class})
public class ConsumerApplication  extends SpringBootServletInitializer {

    public static void main(String[] args) {
        new SpringApplicationBuilder(SaferycomConsumerApplication.class).web(WebApplicationType.SERVLET).run(args);
    }

    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        return builder.sources(SaferycomConsumerApplication.class);
    }
}

以上关键点在与 main方法中的写法，我项目部署时使用的分包，也就是消费者和服务者。
服务者：里面使用 ### SpringApplication.run(SaferycomConsumerApplication.class, args); 可以在外部tomcat下正常启动。
消费者：里面使用 ### new SpringApplicationBuilder(SaferycomConsumerApplication.class).web(WebApplicationType.SERVLET).run(args);
可以在外部tomcat下正常启动。