生成一个html页面，实现文件上传接口：
<html>
<head>
<title>File Uploading Form</title>
</head>
<body>
<h3>File Upload:</h3>
Select a file to upload: <br />
<form action="UploadServlet" method="post"
enctype="multipart/form-data">
<input type="file" name="file" size="50" />
<br />
<input type="submit" value="Upload File" />
</form>
</body>
</html>
编写一个Java servlet，实现文件上传的具体功能，编译好Java类文件，部署到tomcat服务器。
通过html页面上传文件时，碰到如下错误：
type Exception report
message Error instantiating servlet class UploadServlet
description The server encountered an internal error that prevented it from fulfilling this request.
exception
javax.servlet.ServletException: Error instantiating servlet class UploadServlet org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610) org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:526) org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078) org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:655) org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:277) org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2381) org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2370) java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) java.lang.Thread.run(Unknown Source)
root cause
java.lang.NoClassDefFoundError: org/apache/commons/fileupload/FileItemFactory java.lang.Class.getDeclaredConstructors0(Native Method) java.lang.Class.privateGetDeclaredConstructors(Unknown Source) java.lang.Class.getConstructor0(Unknown Source) java.lang.Class.newInstance(Unknown Source) org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610) org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:526) org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078) org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:655) org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:277) org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2381) org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2370) java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) java.lang.Thread.run(Unknown Source)
root cause
java.lang.ClassNotFoundException: org.apache.commons.fileupload.FileItemFactory org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1324) org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1177) java.lang.Class.getDeclaredConstructors0(Native Method) java.lang.Class.privateGetDeclaredConstructors(Unknown Source) java.lang.Class.getConstructor0(Unknown Source) java.lang.Class.newInstance(Unknown Source) org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610) org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:526) org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078) org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:655) org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:277) org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2381) org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2370) java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) java.lang.Thread.run(Unknown Source)解决办法：在$Tomcat\webapps\ROOT\WEB-INF下新建lib子路径，将commons-fileupload-1.3.1.jar和commons-io-2.4.jar拷贝到lib中，重启tomcat服务器即可。