今天调试了很久，重装tomcat都没用，后来发现是xml servlet的url-pattern的配置少写一个"/",添加后启动即可。
org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext
at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:154)
at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:901)
at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:877)
at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:649)
at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:1245)
at org.apache.catalina.startup.HostConfig$DeployDirectory.run(HostConfig.java:1895)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalArgumentException: Invalid <url-pattern> servlet/HelloServlet in servlet mapping
at org.apache.catalina.core.StandardContext.addServletMapping(StandardContext.java:3300)
at org.apache.catalina.core.StandardContext.addServletMapping(StandardContext.java:3275)
at org.apache.catalina.deploy.WebXml.configureContext(WebXml.java:1438)
at org.apache.catalina.startup.ContextConfig.webConfig(ContextConfig.java:1357)
at org.apache.catalina.startup.ContextConfig.configureStart(ContextConfig.java:889)
at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:386)
at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:117)
at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:90)
at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5380)
at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
... 10 more