Caused by: org.springframework.boot.web.server.WebServerException: Unable to start embedded Tomcat
 at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.initialize(TomcatWebServer.java:126) ~[spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
 at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.<init>(TomcatWebServer.java:86) ~[spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
 at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getTomcatWebServer(TomcatServletWebServerFactory.java:413) ~[spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
 at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getWebServer(TomcatServletWebServerFactory.java:174) ~[spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
 at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.createWebServer(ServletWebServerApplicationContext.java:179) ~[spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
 at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(ServletWebServerApplicationContext.java:152) ~[spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
 ... 8 common frames omitted
Caused by: org.apache.catalina.LifecycleException: Failed to start component [StandardServer[-1]]
 at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:167) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 at org.apache.catalina.startup.Tomcat.start(Tomcat.java:367) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.initialize(TomcatWebServer.java:107) ~[spring-boot-2.0.4.RELEASE.jar:2.0.4.RELEASE]
 ... 13 common frames omitted
Caused by: org.apache.catalina.LifecycleException: Failed to start component [StandardService[Tomcat]]
 at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:167) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 at org.apache.catalina.core.StandardServer.startInternal(StandardServer.java:793) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 ... 15 common frames omitted
Caused by: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Tomcat]]
 at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:167) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 at org.apache.catalina.core.StandardService.startInternal(StandardService.java:422) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 ... 17 common frames omitted
Caused by: org.apache.catalina.LifecycleException: A child container failed during start
 at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:949) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 at org.apache.catalina.core.StandardEngine.startInternal(StandardEngine.java:262) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) ~[tomcat-embed-core-8.5.32.jar:8.5.32]
 ... 19 common frames omitted

排查了半天发现是jar包冲突
servlet-api 和springboot 内部tomcat 中servlet-api jar包冲突，
解决方案1：剔除手动引入的servlet-api jar
解决方案2：升级servlet-api jar 为4.0+（我用的servlet-api 2.5）