错误信息：
1 HTTP Status 500 – Internal Server Error 2 Type Exception Report 3 4 Message Error instantiating servlet class [Servlet.BeerSelect] 5 6 Description The server encountered an unexpected condition that prevented it from fulfilling the request. 7 8 Exception 9 10 javax.servlet.ServletException: Error instantiating servlet class [Servlet.BeerSelect] 11 org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490) 12 org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) 13 org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:668) 14 org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) 15 org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408) 16 org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) 17 org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:791) 18 org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1417) 19 org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) 20 java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) 21 java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) 22 org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) 23 java.lang.Thread.run(Unknown Source) 24 Root Cause 25 26 java.lang.UnsupportedClassVersionError: Servlet/BeerSelect has been compiled by a more recent version of the Java Runtime (class file version 54.0), this version of the Java Runtime only recognizes class file versions up to 52.0 (unable to load class [Servlet.BeerSelect]) 27 org.apache.catalina.loader.WebappClassLoaderBase.findClassInternal(WebappClassLoaderBase.java:2376) 28 org.apache.catalina.loader.WebappClassLoaderBase.findClass(WebappClassLoaderBase.java:834) 29 org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1297) 30 org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1157) 31 org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490) 32 org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) 33 org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:668) 34 org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) 35 org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408) 36 org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) 37 org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:791) 38 org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1417) 39 org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) 40 java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) 41 java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) 42 org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) 43 java.lang.Thread.run(Unknown Source) 44 Note The full stack trace of the root cause is available in the server logs.
一开始没有细看，以为是tomcat目录结构错误，网上搜了各种方法都没能解决
后来仔细一看Root Cause，是Java Runtime版本引起的。想了想，我安装的
是jdk10，默认的编译版本也是10，应该是编译版本过高。
解决方案：
将编译等级调低即可
