配置servlet出现这样的问题如何解决，谢谢了

javax.servlet.ServletException: Error instantiating servlet class com.servlet.test.HelloServlet
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:174)
    org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:879)
    org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
    org.apache.tomcat.(PoolTcpEndpoint.java:528)
    org.apache.tomcat.(LeaderFollowerWorkerThread.java:81)
    org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689)
    java.lang.Thread.run(Unknown Source)