servlet入门示例出错，提示"Error instantiating servlet class"
大家好！我初学java的web开发，有些很基础的问题弄不通，还请大家指点。
我正在学servlet，在做一个入门示例时，就是访问不了正常的页面。不知道应该怎么解决这个问题。具体如下：
我是按这个页面：http://blog.csdn.net/wangdingqiaoit/article/details/7674367 来搭建环境了。我用的是myeclipse10, tomcat 7, jdk1.7。访问http://localhost:8080/FirstServlet/index.jsp文件没有问题，但访问http://localhost:8080/FirstServlet/servlet/Test这个页面时，提示：HTTP Status 500 - Error instantiating servlet class test.Test

具体错误为：
type Exception report
message Error instantiating servlet class test.Test
description The server encountered an internal error that prevented it from fulfilling this request.
exception 
javax.servlet.ServletException: Error instantiating servlet class test.Test
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408) org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1041) org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:603) org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:243
org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:241 java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) java.lang.Thread.run(Thread.java:744)

root cause 
java.lang.InstantiationException sun.reflect.InstantiationExceptionConstructorAccessorImpl.newInstance(InstantiationExceptionConstructorAccessorImpl.java:48)
java.lang.reflect.Constructor.newInstance(Constructor.java:526)
java.lang.Class.newInstance(Class.java:374) org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502) org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408) org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1041)
org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:603) org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2430
org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2419
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
java.lang.Thread.run(Thread.java:744)


note The full stack trace of the root cause is available in the Apache Tomcat/7.0.47 logs.