hibernate problem

i have a hibernate problem , i am using hibernate to connect to the mysql database and i am using tomcat , when i restart my tomcat cat it works fine and database is connected , but after a while the connection is lost with the database and the following error is shown in the log file .

please tell me what to do ? .

net.sf.hibernate.exception.GenericJDBCException: Could not execute query

at net.sf.hibernate.exception.ErrorCodeConverter.handledNonSpecificException(ErrorCodeConverter.java:90)

at net.sf.hibernate.exception.ErrorCodeConverter.convert(ErrorCodeConverter.java:79)

at net.sf.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:30)

at net.sf.hibernate.impl.SessionImpl.convert(SessionImpl.java:4110)

at net.sf.hibernate.impl.SessionImpl.iterate(SessionImpl.java:1621)

at net.sf.hibernate.impl.QueryImpl.iterate(QueryImpl.java:27)

at us.hpsoft.etools.utils.UserDatabaseHandler.getAllUserData(UserDatabaseHandler.java:1135)

at us.hpsoft.etools.action.RegistrationAction.login(RegistrationAction.java:62)

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

at java.lang.reflect.Method.invoke(Method.java:585)

at org.apache.struts.actions.DispatchAction.dispatchMethod(DispatchAction.java:280)

at org.apache.struts.actions.DispatchAction.execute(DispatchAction.java:216)

at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:484)

at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)

at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)

at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:525)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:214)

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)

at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)

at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:306)

at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:385)

at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:745)

at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:675)

at org.apache.jk.common.SocketConnection.runIt(ChannelSocket.java:868)

at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)

at java.lang.Thread.run(Thread.java:595)

Caused by: com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception:

** BEGIN NESTED EXCEPTION **

java.net.SocketException

MESSAGE: Broken pipe

STACKTRACE:

java.net.SocketException: Broken pipe

at java.net.SocketOutputStream.socketWrite0(Native Method)

at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:92)

at java.net.SocketOutputStream.write(SocketOutputStream.java:136)

at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:65)

at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:123)

at com.mysql.jdbc.MysqlIO.send(MysqlIO.java:2689)

at com.mysql.jdbc.MysqlIO.send(MysqlIO.java:2618)

at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1551)

at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1665)

at com.mysql.jdbc.Connection.execSQL(Connection.java:2978)

at com.mysql.jdbc.Connection.execSQL(Connection.java:2902)

at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:930)

at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:1024)

at net.sf.hibernate.impl.BatcherImpl.getResultSet(BatcherImpl.java:88)

at net.sf.hibernate.loader.Loader.getResultSet(Loader.java:875)

at net.sf.hibernate.hql.QueryTranslator.iterate(QueryTranslator.java:868)

at net.sf.hibernate.impl.SessionImpl.iterate(SessionImpl.java:1618)