刚开始尝试写Servlet代码，第一天就碰到这个错误，在网上找了很多资料才找到解决办法，在此记录一下。
org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext[/FirstServlet]]
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:1247)
at org.apache.catalina.startup.HostConfig$DeployDirectory.run(HostConfig.java:1897)
at java.util.concurrent.Executors$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: The servlets named [LoginServlet] and [com.liu.control.LoginServlet] are both mapped to the url-pattern [/LoginServlet] which is not permitted
at org.apache.catalina.deploy.WebXml.addServletMapping(WebXml.java:293)
at org.apache.catalina.startup.ContextConfig.processAnnotationWebServlet(ContextConfig.java:2410)
at org.apache.catalina.startup.ContextConfig.processAnnotationsStream(ContextConfig.java:2085)
at org.apache.catalina.startup.ContextConfig.processAnnotationsFile(ContextConfig.java:2046)
at org.apache.catalina.startup.ContextConfig.processAnnotationsFile(ContextConfig.java:2039)
at org.apache.catalina.startup.ContextConfig.processAnnotationsFile(ContextConfig.java:2039)
at org.apache.catalina.startup.ContextConfig.processAnnotationsFile(ContextConfig.java:2039)
at org.apache.catalina.startup.ContextConfig.webConfig(ContextConfig.java:1304)
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
五月 10, 2017 6:59:01 下午 org.apache.catalina.startup.HostConfig deployDirectory
严重: Error deploying web application directory E:MyEclipse2016文件.metadata.me_tcat7webappsFirstServlet
java.lang.IllegalStateException: ContainerBase.addChild: start: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext[/FirstServlet]]
at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:904)
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:1247)
at org.apache.catalina.startup.HostConfig$DeployDirectory.run(HostConfig.java:1897)
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)
在网上找到的答案大部分是说web.xml配置错误，最后在一个博客中找到了最终解决办法。原来是因为/LoginServlet一个请求映射了
[LoginServlet] and [com.liu.control.LoginServlet] 两个Servlet。
就是这个servlet已经在下面定义过了，
@WebServlet("/LoginServlet")//在这里自动定义了
public class LoginServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
所以在web.xml文件中就不需要再重复定义了，将servlet-mapping属性删除即可。