我是新手，谁可以告诉我，为什么会产生这个错误吗？

HTTP Status 500 -

type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

java.lang.NullPointerException
	acec.dbConn.executeQuery(dbConn.java:77)
	acec.LoginCheck.doGet(LoginCheck.java:34)
	acec.LoginCheck.doPost(LoginCheck.java:54)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

note The full stack trace of the root cause is available in the Apache Tomcat/5.0.28 logs.

----------------解决方案--------------------------------------------------------
空指针异常。。
----------------解决方案--------------------------------------------------------
JAVA中有指针？？？
----------------解决方案--------------------------------------------------------