Oracle Database Discussions

I am using Tomcat 5.5 and Oracle 10g, my application using this driver

DriverManager.registerDriver (new oracle.jdbc.driver.OracleDriver());
DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:XE", "PTRS", "ptrs");

My Application sometimes give me this error during execution.

type Exception report

message

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

exception

java.lang.RuntimeException: An error occurred while trying to create a java.sql.Connection: Listener refused the connection with the following error:
ORA-12519, TNS:no appropriate service handler found
The Connection descriptor used by the client was:
localhost:1521:XE

com.ptrs.dataaccesstier.ConnectionFactory.getConnection(ConnectionFactory.java:53)
com.ptrs.dataaccesstier.ExamDAO.findByPrimaryKey(ExamDAO.java:60)
com.ptrs.dataaccesstier.DataAccessFacade.findByPrimaryKey(DataAccessFacade.java:357)
com.ptrs.dataaccesstier.InspectionDAO.find(InspectionDAO.java:405)
com.ptrs.dataaccesstier.DataAccessFacade.find(DataAccessFacade.java:424)
com.ptrs.businesstier.BusinessFacade.find(BusinessFacade.java:308)
com.ptrs.presentationtier.InspectorOADetailReportCommand.execute(InspectorOADetailReportCommand.java:41)
com.ptrs.presentationtier.ControllerServlet.service(ControllerServlet.java:42)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:362)


Any Suggestions?