在学习Servlet时，运行程序时报错：Server Tomcat v7.0 Server at localhost failed to start.如图
网上查的原因主要是改web.xml配置文件，但修改后并未成功。后来才发现原因不在配置文件。
严重: A child container failed during start java.util.concurrent.ExecutionException: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext[/Health]] at java.util.concurrent.FutureTask.report(FutureTask.java:122) at java.util.concurrent.FutureTask.get(FutureTask.java:192) at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1119) at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:819) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1571) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1561) 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: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext[/Health]] at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:162) ... 6 more Caused by: java.lang.NoClassDefFoundError: Ltrain/CreateImage; at java.lang.Class.getDeclaredFields0(Native Method) at java.lang.Class.privateGetDeclaredFields(Class.java:2583) at java.lang.Class.getDeclaredFields(Class.java:1916) at org.apache.catalina.util.Introspection.getDeclaredFields(Introspection.java:106) at org.apache.catalina.startup.WebAnnotationSet.loadFieldsAnnotation(WebAnnotationSet.java:270) at org.apache.catalina.startup.WebAnnotationSet.loadApplicationServletAnnotations(WebAnnotationSet.java:139) at org.apache.catalina.startup.WebAnnotationSet.loadApplicationAnnotations(WebAnnotationSet.java:65) at org.apache.catalina.startup.ContextConfig.applicationAnnotationsConfig(ContextConfig.java:417) at org.apache.catalina.startup.ContextConfig.configureStart(ContextConfig.java:891) at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:388) 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:5522) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145) ... 6 more Caused by: java.lang.ClassNotFoundException: train.CreateImage at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1892) at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1735) ... 20 more 十二月 12, 2017 9:54:17 上午 org.apache.catalina.core.ContainerBase startInternal 严重: A child container failed during start java.util.concurrent.ExecutionException: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost]] at java.util.concurrent.FutureTask.report(FutureTask.java:122) at java.util.concurrent.FutureTask.get(FutureTask.java:192) at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1119) at org.apache.catalina.core.StandardEngine.startInternal(StandardEngine.java:300) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145) at org.apache.catalina.core.StandardService.startInternal(StandardService.java:444) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145) at org.apache.catalina.core.StandardServer.startInternal(StandardServer.java:758) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145) at org.apache.catalina.startup.Catalina.start(Catalina.java:694) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:294) at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:428) Caused by: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost]] at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:162) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1571) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1561) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at java.lang.Thread.run(Thread.java:745) Caused by: org.apache.catalina.LifecycleException: A child container failed during start at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1127) at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:819) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145) ... 6 more 十二月 12, 2017 9:54:17 上午 org.apache.catalina.startup.Catalina start 严重: The required Server component failed to start so Tomcat is unable to start. org.apache.catalina.LifecycleException: Failed to start component [StandardServer[8005]] at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:162) at org.apache.catalina.startup.Catalina.start(Catalina.java:694) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:294) at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:428) Caused by: org.apache.catalina.LifecycleException: Failed to start component [StandardService[Catalina]] at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:162) at org.apache.catalina.core.StandardServer.startInternal(StandardServer.java:758) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145) ... 7 more Caused by: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina]] at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:162) at org.apache.catalina.core.StandardService.startInternal(StandardService.java:444) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145) ... 9 more Caused by: org.apache.catalina.LifecycleException: A child container failed during start at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1127) at org.apache.catalina.core.StandardEngine.startInternal(StandardEngine.java:300) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145) ... 11 more
发现自己的错误在于：ClassNotFoundException，但是检查了好几遍类引用无误。最后解决的方法也很智障。改了包名train和类名CreateImage，改成trainClass和CreateRandomImages，程序运行成功。再将包名和类名改回train和CreateImage，程序仍运行成功。原因不知道为什么..
思考：在报错时，要先通过Caused by 找到自己程序错误原因所在，然后针对定位的问题改。而不是报错就直接搜广义的报错原因。