初学tomcat，jsp，servlet的时候老碰到这样的错误，求助。

初学tomcat，jsp，servlet的时候老碰到下面的错误，求大神帮助。
`Illegal access: this web application instance has been stopped already.
Could not load com.mysql.jdbc.Util. The eventual following stack trace is caused by an error
thrown for debugging purposes as well as to attempt to terminate the thread which caused
the illegal access, and has no functional impact.
java.lang.IllegalStateException`