Some days ago i was working with hibernate to persist some POJOs, when i found a an "ERROR LazyInitializationException: - illegal access to loading collection".
I'm combining Spring with Hibernate in such work, so i tought it could be a problem in the way i did the management of Hibernate Sessions through Spring Transactions, but it was not the case.
The problem is bit more creeping, because it involves the way Hibernate manages the persistent class creation. So, let's start introducing some code to explate the point.

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public class Domain extends AbstractSecureObject {
 
 
    private Set mobileEquipments = Collections
            .synchronizedSet(new HashSet());
 
[...]
 
    public void setMobileEquipments(Set mobileEquipments) {
        this.mobileEquipments = mobileEquipments;
    }
 
    // used by Hibernate
    @SuppressWarnings("unused")
    private Set getMobileEquipments() {
        return mobileEquipments;
    }
 
    public MobileEquipment[] getActualMobileEquipments() {
        return mobileEquipments.toArray(new MobileEquipment[mobileEquipments
                .size()]);
    }
 
    public void addMobileEquimpent(MobileEquipment me) {
        mobileEquipments.add(me);
        me.defineParentDomain(this);
    }
 
    public void removeMobileEquimpent(MobileEquipment me) {
        mobileEquipments.remove(me);
        me.defineParentDomain(null);
    }
 
    public boolean containsMobileEquipment(MobileEquipment mobileEquipment) {
        return mobileEquipments.contains(mobileEquipment);
    }
 
}


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public class MobileEquipment extends AbstractSecureObject {
 
        private Domain parentDomain;
 
[..]
 
        public Domain getParentDomain() {
                return parentDomain;
        }
 
        public void setParentDomain(Domain parentDomain) {
                this.parentDomain = parentDomain;
                if (parentDomain != null &&
                 !parentDomain.containsMobileEquipment(this))
                        parentDomain.addMobileEquimpent(this);
        }
 
}

Both the Domain class and MobileEquipment class are a persistent, and as you can see, they are involved in a double-sided relation: a domain contains a set of MobileEquipments and a MobileEquipment contains a reference to its containing domain. As good programming practice, the MobileEquipment.setParentDomain() method sets either the domain member field of MobileEquipment and adds an entry for this MobileEquipment to the collection stored in the parent Domain.

This approach, that is correct from a programming perspective, led to the LazyInitializationException when Hibernate uses accessor methods to manage persistent objects. The problem is clear if you think of what happens when a class, like Domain, is loaded by Hibernate:

• Hibernate creates an instance for Domain
• When there is the need to load the MobileEquipments' Set, Hibernate tries to load it.
• To load the set, MobileEquipment objects are created
• To populate a MobileEquipment object, hibernate calls the setParentDomain method
• The method first sets the parentDomain field of MobileEquipment, then tries to access the MobileEquipments' Set in the Domain class, but this set is not already loaded (because Hibernate is now loading it) and so a LazyInitializationException is thrown

To solve the problem i decided to make accessor methods, lilke setParentDomain(), private, because they will be used just by Hibernate. This implies that the value that will be set will be correct (i.e. the parentDomain actually contains this MobileEquipment in its Set). To set the parentDomain, a programmer must use the defineParentDomain() method instead. This method, that is never accesed by Hibernate, will contain the logic to add the MobileEquipment to the parent Domain, in order to guarantee correctness (because this operation is performed by programmers that are humans and so can be wrong!). So, the precedent classes bacame:

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public class Domain extends AbstractSecureObject {
 
 
    private Set mobileEquipments = Collections
            .synchronizedSet(new HashSet());
 
[...]
 
    // used by Hibernate
    @SuppressWarnings("unused")
    private void setMobileEquipments(Set mobileEquipments) {
        this.mobileEquipments = mobileEquipments;
    }
 
    // used by Hibernate
    @SuppressWarnings("unused")
    private Set getMobileEquipments() {
        return mobileEquipments;
    }
 
    public MobileEquipment[] getActualMobileEquipments() {
        return mobileEquipments.toArray(new MobileEquipment[mobileEquipments
                .size()]);
    }
 
    public void addMobileEquimpent(MobileEquipment me) {
        mobileEquipments.add(me);
        me.defineParentDomain(this);
    }
 
    public void removeMobileEquimpent(MobileEquipment me) {
        mobileEquipments.remove(me);
        me.defineParentDomain(null);
    }
 
    public boolean containsMobileEquipment(MobileEquipment mobileEquipment) {
        return mobileEquipments.contains(mobileEquipment);
    }
 
}


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public class MobileEquipment extends AbstractSecureObject {
 
        private Domain parentDomain;
 
[..]
 
        public Domain getParentDomain() {
                return parentDomain;
        }
 
        @SuppressWarnings("unused")
        // used by Hibernate
        private void setParentDomain(Domain parentDomain) {
                this.parentDomain = parentDomain;
        }
 
        public void defineParentDomain(Domain parentDomain) {
                this.parentDomain = parentDomain;
                if (parentDomain != null && !parentDomain.containsMobileEquipment(this))
                        parentDomain.addMobileEquimpent(this);
        }
 
}