I am new to hibernate and am running into issues deleting objects in a many-to-many bi-directional relationship while the inverse="true" flag is set.
When a person is deleted, I want that person to automatically be deregistered from all registered events.
When an event is deleted, I want that event to be removed from all registered persons.
I am not sure if this is a parent-child relationship...it seems more like a peer-to-peer relationship (if such a thing exists?...maybe this is bad practice?)
The problem I run into is that when I try to delete the object that has the inverse="true" flag in its mapping file, hibernate throws an exception upon flushing/committing.
When I remove the inverse="true" flag from both mapping files, everything works fine, but I worry that this might be inefficient, or that I am missing some simple concept.
1) Why did hibernate throw an exception when deleting the object that has the inverse="true" in its mapping file?
3) Is there a better way to delete objects in a (peer-to-peer?) bidirectional many-to-many relationship who have the inverse="true" flag in its mapping file?
web.xml and hibernate.cfg.xml files are identical to the hibernate tutorial. I'll happily provide them if needed.
package events;
// Imports
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletException;
import java.io.IOException;
import java.io.PrintWriter;
import java.text.SimpleDateFormat;
import java.text.ParseException;
import java.util.Date;
import util.HibernateUtil;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
public class EventManagerServlet extends HttpServlet {
protected void doGet( HttpServletRequest request, HttpServletResponse response )
throws ServletException, IOException {
SimpleDateFormat dateFormatter = new SimpleDateFormat( "dd.MM.yyyy" );
try {
// Begin unit of work
HibernateUtil.getSessionFactory().getCurrentSession().beginTransaction();
// Process request and render page...
// Write HTML header
PrintWriter out = response.getWriter();
out.println( "<html><head><title>Event Manager</title></head><body>" );
//Handle actions
String action = request.getParameter( "action" );
if ( action != null ) {
if ( action.equals( "addPerson" ) ) {
String firstname = request.getParameter( "firstname" );
String lastname = request.getParameter( "lastname" );
String age = request.getParameter( "age" );
createAndStorePerson( firstname, lastname, new Integer( age ).intValue() );
out.println( "<b><i>Added person.</i></b>" );
} else if ( action.equals( "removePerson" ) ) {
String id = request.getParameter( "personId" );
removePerson( new Long( id ) );
out.println( "<b><i>Removed Person.</i></b>" );
} else if ( action.equals( "addEvent" ) ) {
String eventTitle = request.getParameter( "eventTitle" );
String eventDate = request.getParameter( "eventDate" );
try {
createAndStoreEvent( eventTitle, dateFormatter.parse( eventDate ) );
out.println( "<b><i>Added event.</i></b>" );
} catch ( ParseException ex ) {
out.println( "<b><i>Parse Exception!</i></b>" );
}
} else if ( action.equals( "removeEvent" ) ) {
String id = request.getParameter( "eventId" );
try {
removeEvent( new Long( id ) );
} catch ( Exception ex ) {
out.println( "Error during remove event!: " + ex );
return;
}
out.println( "<b><i>Removed Event.</i></b>" );
} else if ( action.equals( "addPersonToEvent" ) ) {
String personId = request.getParameter( "personId" );
String eventId = request.getParameter( "eventId" );
try {
addPersonToEvent( new Long( personId ), new Long( eventId ) );
} catch ( Exception ex ) {
out.println( "error adding person to event: " + ex );
ex.printStackTrace();
}
out.println( "Added person to event" );
} else if ( action.equals( "removePersonFromEvent" ) ) {
String personId = request.getParameter( "personId" );
String eventId = request.getParameter( "eventId" );
removePersonFromEvent( new Long( personId ), new Long( eventId ) );
out.println( "Removed person from event" );
}
try {
HibernateUtil.getSessionFactory().getCurrentSession().flush();
} catch ( Exception ex ) {
out.println( "Error during flush!: " + ex );
ex.printStackTrace();
return;
}
}
out.println( "<br/>" );
try {
//Print list
listPeople( out );
try {
//Print events
listEvents( out, dateFormatter );
} catch ( Exception ex ) {
out.println( "Error during printing events! " + ex );
ex.printStackTrace();
return;
}
//Print the action form
printForm( out );
} catch ( Exception ex ) {
out.println( "Error during printing! " + ex );
ex.printStackTrace();
return;
}
try {
// End unit of work
HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().commit();
} catch ( Exception ex ) {
out.println( "Error during commit!: " + ex );
ex.printStackTrace();
}
// Write HTML footer
out.println( "</body></html>" );
out.flush();
out.close();
} catch ( Exception ex ) {
HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().rollback();
throw new ServletException( ex.getMessage() );
}
}
private void printForm( PrintWriter out ) {
out.println( "<table border='1'><tr><td>" );
out.println( "<h2>Add new event:</h2>" );
out.println( "<form>" );
out.println( "Title: <input name='eventTitle' length='50'/><br/>" );
out.println( "Date (e.g. 24.12.2009): <input name='eventDate' length='10'/><br/>" );
out.println( "<input type='submit' name='action' value='addEvent' />" );
out.println( "</form>" );
out.println( "</td><td>" );
out.println( "<h2>Remove event:</h2>" );
out.println( "<form>" );
out.println( "ID: <input name='eventId' length='50'/><br/>" );
out.println( "<input type='submit' name='action' value='removeEvent' />" );
out.println( "</form>" );
out.println( "</td></tr></table>" );
out.println( "<br/>" );
out.println( "<table border='1'><tr><td>" );
out.println( "<h2>Add new person:</h2>" );
out.println( "<form>" );
out.println( "First Name: <input name='firstname' length='50'/><br/>" );
out.println( "Last Name: <input name='lastname' length='50' /><br/>" );
out.println( "Age: <input name='age' length='4' /><br/>" );
out.println( "<input type='submit' name='action' value='addPerson' />" );
out.println( "</form>" );
out.println( "</td><td>" );
out.println( "<h2>Remove Person:</h2>" );
out.println( "<form>" );
out.println( "ID: <input name='personId' length='50'/><br/>" );
out.println( "<input type='submit' name='action' value='removePerson' />" );
out.println( "</form>" );
out.println( "</td></tr></table>" );
out.println( "<br/>" );
out.println( "<table border='1'><tr><td>" );
out.println( "<h2>Register person for event:</h2>" );
out.println( "<form>" );
out.println( "Person ID: <input name='personId' length='50'/><br/>" );
out.println( "Event ID: <input name='eventId' length='50'/><br/>" );
out.println( "<input type='submit' name='action' value='addPersonToEvent' />" );
out.println( "</form>" );
out.println( "</td><td>" );
out.println( "<h2>Remove person from event:</h2>" );
out.println( "<form>" );
out.println( "Person ID: <input name='personId' length='50'/><br/>" );
out.println( "Event ID: <input name='eventId' length='50'/><br/>" );
out.println( "<input type='submit' name='action' value='removePersonFromEvent' />" );
out.println( "</form>" );
out.println( "</td></tr></table>" );
}
private void listPeople( PrintWriter out ) {
List result = HibernateUtil.getSessionFactory().getCurrentSession().createCriteria(
Person.class ).list();
if ( result.size() > 0 ) {
out.println( "<h2>People in database:</h2>" );
out.println( "<table border='1'>" );
out.println( "<tr>" );
out.println( "<th>ID</th>" );
out.println( "<th>Name</th>" );
out.println( "<th>Age</th>" );
out.println( "<th>Registered Events</th>" );
out.println( "</tr>" );
for ( Iterator it = result.iterator(); it.hasNext(); ) {
Person person = (Person)it.next();
out.println( "<tr>" );
out.println( "<td>" + person.getId() + "</td>" );
out
.println( "<td>" + person.getLastname() + ", " + person.getFirstname()
+ "</td>" );
out.println( "<td>" + person.getAge() + "</td>" );
Set events = person.getEvents();
if ( events == null || events.size() < 1 ) {
out.println( "<td>Not registered for any events.</td>" );
} else {
out.println( "<td>Registered for " + events.size() + " events.</td>" );
}
out.println( "</tr>" );
}
out.println( "</table>" );
} else {
out.println( "<h2>No people found.</h2>" );
}
}
private void listEvents( PrintWriter out, SimpleDateFormat dateFormatter ) {
List result = HibernateUtil.getSessionFactory().getCurrentSession().createCriteria(
Event.class ).list();
if ( result.size() > 0 ) {
out.println( "<h2>Events in database:</h2>" );
out.println( "<table border='1'>" );
out.println( "<tr>" );
out.println( "<th>Id</th>" );
out.println( "<th>Event title</th>" );
out.println( "<th>Event date</th>" );
out.println( "<th>Participants</th>" );
out.println( "</tr>" );
for ( Iterator it = result.iterator(); it.hasNext(); ) {
Event event = (Event)it.next();
out.println( "<tr>" );
out.println( "<td>" + event.getId() + "</td>" );
out.println( "<td>" + event.getTitle() + "</td>" );
out.println( "<td>" + dateFormatter.format( event.getDate() ) + "</td>" );
Set participants = event.getParticipants();
if ( participants == null || participants.size() < 1 ) {
out.println( "<td>No participants.</td>" );
} else {
out.println( "<td>" + participants.size() + " participants found.</td>" );
}
out.println( "</tr>" );
}
out.println( "</table>" );
} else {
out.println( "<h2>No events found.</h2>" );
}
}
protected void createAndStorePerson( String firstname, String lastname, int age ) {
Person thePerson = new Person();
thePerson.setFirstname( firstname );
thePerson.setLastname( lastname );
thePerson.setAge( age );
HibernateUtil.getSessionFactory().getCurrentSession().save( thePerson );
}
protected void createAndStoreEvent( String title, Date theDate ) {
Event theEvent = new Event();
theEvent.setTitle( title );
theEvent.setDate( theDate );
HibernateUtil.getSessionFactory().getCurrentSession().save( theEvent );
}
protected void removePerson( Long id ) {
Person person = (Person)HibernateUtil.getSessionFactory().getCurrentSession().load(
Person.class, id );
HibernateUtil.getSessionFactory().getCurrentSession().delete( person );
}
protected void removeEvent( Long id ) {
Event event = (Event)HibernateUtil.getSessionFactory().getCurrentSession().load(
Event.class, id );
HibernateUtil.getSessionFactory().getCurrentSession().delete( event );
}
private void addPersonToEvent( Long personId, Long eventId ) {
Person aPerson = (Person)HibernateUtil.getSessionFactory().getCurrentSession().load(
Person.class, personId );
Event anEvent = (Event)HibernateUtil.getSessionFactory().getCurrentSession().load(
Event.class, eventId );
aPerson.getEvents().add( anEvent );
//anEvent.getParticipants().add(aPerson);
}
private void removePersonFromEvent( Long personId, Long eventId ) {
Person aPerson = (Person)HibernateUtil.getSessionFactory().getCurrentSession().load(
Person.class, personId );
Event anEvent = (Event)HibernateUtil.getSessionFactory().getCurrentSession().load(
Event.class, eventId );
aPerson.getEvents().remove( anEvent );
//anEvent.getParticipants().remove(aPerson);
}
}
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