Hi All, I'm getting a ConcurrentModificationException when I'm trying to remove an item from a Set in a persisted class.

The scenario:

I load a persisted object from the database, close the session and put the detached object into httpSession. The object is then modified later, a "passenger" is removed from the "passengers" Set of a Carpool object, and the error occurs.

The database tables are Carpool and User with an assocation table called passenger. My goal is to just remove the link info in the passenger table. I've tried using all-delete-orphan, but that deletes the the entry in the User table--gosh, but I just don't want that to happen, no sir.

Tell me I'm missing some _extremely_ elementry item.

Thanks, info below.

John

Hibernate version:2

Mapping documents:
<?xml version="1.0"?>

<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
<class
name="com.wamu.carpool.Carpool"
table="carpool"
dynamic-update="false"
dynamic-insert="false"
>

<id
name="id"
column="id"
type="long"
unsaved-value="-1"
>
<generator class="net.sf.hibernate.id.TableHiLoGenerator">
<param name="table">Sequencer</param>
<param name="column">seed</param>
<param name="max_lo">10</param>
</generator>
</id>

<property
name="comments"
type="java.lang.String"
update="true"
insert="true"
access="property"
column="comments"
length="255"
/>

<property
name="capacity"
type="int"
update="true"
insert="true"
access="property"
column="capacity"
/>

<property
name="departureTime"
type="java.util.Date"
update="true"
insert="true"
access="property"
column="departureTime"
/>

<many-to-one
name="driver"
class="com.wamu.carpool.security.User"
cascade="none"
outer-join="auto"
update="true"
insert="true"
access="property"
column="driverId"
/>

<many-to-one
name="exit"
class="com.wamu.carpool.Exit"
cascade="none"
outer-join="auto"
update="true"
insert="true"
access="property"
column="exitId"
/>

<set
name="passengers"
table="passenger"
lazy="false"
inverse="false"
cascade="all-delete-orphan"
sort="unsorted"
>

<key
column="carpoolId"
>
</key>

<many-to-many
class="com.wamu.carpool.security.User"
column="userId"
outer-join="auto"
/>

</set>

<!--
To add non XDoclet property mappings, create a file named
hibernate-properties-Carpool.xml
containing the additional properties and place it in your merge dir.
-->

</class>

</hibernate-mapping>


Code between sessionFactory.openSession() and session.close():
try {
Session session = HibernateSessionFactory.currentSession();

Transaction tx = session.beginTransaction();

//reassociate the carpool with the hibernate session.
Carpool refreshedCarpool = (Carpool)session.load(Carpool.class, new Long(carpool.getId()));

logger.debug("removing passenger " + passenger.getUserName());
logger.debug("passenger size:" + String.valueOf(refreshedCarpool.getPassengers().size()));
Iterator i = refreshedCarpool.getPassengers().iterator();

while(i.hasNext()){
User currentPassenger = (User)i.next();
if(currentPassenger.getUserName().equals(passenger.getUserName())){
refreshedCarpool.getPassengers().remove(currentPassenger);
}
}
logger.debug("passenger size after remove():" + String.valueOf(refreshedCarpool.getPassengers().size()));

session.update(refreshedCarpool);
tx.commit();
} catch (HibernateException e) {
logger.error(e);
throw new RuntimeException(e);
} finally {
try {
HibernateSessionFactory.closeSession();
} catch (HibernateException e1) {
logger.error(e1);
}
}

Full stack trace of any exception that occurs:
Caused by: java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:782)
at java.util.HashMap$KeyIterator.next(HashMap.java:818)
at net.sf.hibernate.collection.PersistentCollection$IteratorProxy.next(PersistentCollection.java:441)
at com.wamu.carpool.CarpoolManagerHibernateImpl.removePassenger(CarpoolManagerHibernateImpl.java:142)
at com.wamu.carpool.CarpoolHandler.removePassenger(CarpoolHandler.java:255)
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:324)
at com.sun.faces.el.MethodBindingImpl.invoke(MethodBindingImpl.java:126)
... 34 more

Name and version of the database you are using:
HSQL v1.7.2

The problem is solved by overriding the equals() and hashcode() methods of Object. Hibernate uses these methods when doing it's internal methods. Once I did this, things went smoothly. Here's the change to my code:





The moral of the story is to override the equals() and hashcode() methods when working with objects in a persisted Collection.

The concurrent modification exception was a Java error - nothing to do with Hibernate, or the equals method, etc.

You can't remove an object from a Collection the way you were, when you're iterating through it. If you want to remove an object while iterating, use the remove method available on the iterator.