tomcat 500错误解决系列一：

由于web.xml中的servlet-class写错，导致了无法实例化servlet。

HTTP状态 500 - 内部服务器错误
类型 异常报告

消息 实例化Servlet类[package.SessionDemo_01_getSession]异常

描述 服务器遇到一个意外的情况，阻止它完成请求。

例外情况

javax.servlet.ServletException: 实例化Servlet类[package.SessionDemo_01_getSession]异常
	org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
	org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
	org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
	org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
	org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373)
	org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
	org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
	org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1590)
	org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
	java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	java.lang.Thread.run(Thread.java:748)
根本原因。

java.lang.ClassNotFoundException: package.SessionDemo_01_getSession
	org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1365)
	org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1188)
	org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
	org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
	org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
	org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
	org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373)
	org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
	org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
	org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1590)
	org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
	java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	java.lang.Thread.run(Thread.java:748)
):注意 主要问题的全部 stack 信息可以在 server logs 里查看