java jsp mysql数据库数据到达上限

这是网页上显示的
The server encountered an internal error () that prevented it from fulfilling this request.

exception

java.lang.NullPointerException
    dao.Extends.executeQuery(Extends.java:52)
    dao.ZhangDao.getZhangByid(ZhangDao.java:14)
    dao.RecordDao.getZhang(RecordDao.java:160)
    dao.RecordDao.getItem(RecordDao.java:74)
    dao.NewtonDao.Df(NewtonDao.java:49)
    dao.NewtonDao.Newton(NewtonDao.java:123)
    servlet.BeginServlet.doPost(BeginServlet.java:126)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:803)


note The full stack trace of the root cause is available in the Apache Tomcat/6.0.16 logs.
这是程序显示的：
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: The driver was unable to create a connection due to an inability to establish the client portion of a socket.

This is usually caused by a limit on the number of sockets imposed by the operating system. This limit is usually configurable.

For Unix-based platforms, see the manual page for the 'ulimit' command. Kernel or system reconfiguration may also be required.

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