EclipseLink/DesignDocs/214661

Functional Specification: VM managed Entity detachment

ER 214661

Project overview

Allowing a Persistence Context to reference managed objects in a weak manner provides an automatic memory management mechanism for rich clients and like users as well providing a general performance improvement for EclipseLink

Currently JPA clients must manage memory foot print manually by controlling the lifetime of the PersistenceContext as well as the contents of the Persistence Context.

Within a Rich Client environment it is much easier for application developers to simply use the managed objects as the UI backing objects and leverage transparent persistence instead of creating a DOA layer. Currently the only limitation with this approach is memory management. Persistence Contexts tend to grow and detaching objects is problematic.

With this feature memory management can be delegated to the VMs garbage collection functionality through the use of weak references. Having non changed objects removed from the Persistence Context also reduces the number of objects that must be processed during database flush, improving performance.

Functionality

Adding a Reference Mode Enum and API’s to allow users to control how EclipseLink references managed objects.
• When “HARD” mode is chosen all objects are referenced by hard Java references and are not available for GC.
• When “WEAK“ mode is chosen EclipseLink will reference all changed tracked objects though weak references. This means any object no longer referenced directly or indirectly will be available for garbage collection. If the object is gc’d before the EM/UnitOfWork flushes to the database then this object and any others like it will not be checked for changes. When a change is made to a change tracked object that object is moved to a hard reference and will not be available for GC until flushed. New and removed objects are also held by hard references. Non change tracked objects will always be held by “hard” references and are not available for GC. This is the default mode for EclipsLink once this feature is checked-in.
• When “FORCE-WEAK” mode is chosen all objects including non change tracked objects are held by weak references. When a change is made to a change tracked object that object is moved to a hard reference and will not be available for GC until flushed. New and removed objects are also held by hard references until flush.

package org.eclipse.persistence.sessions.factories;
 
public enum ReferenceMode {
    /**
     * References to Objects will be through hard references.  These objects will not be available for
     * garbage collection until the referencing artifact (usually a Persistence Context or UnitOfWork)
     * released or closed/
     */
    HARD,
 
    /**
     * References to Objects that support active change tracking will be held by weak references.  Objects
     * with defered change tracking will have hard references to prevent the loss of changes.
     * See: {@link java.lang.ref.WeakReference}
     */
    WEAK,
 
    /**
     * Same as weak reference except Objects that can not be changed tracked (Deferred Change Detection) will
     * not be prevented from being garbage collected.
     */
    FORCE_WEAK
}

A particular EM/UnitOfWork mode is configured through the acquireUnitOfWork API or through the Entity Manager property "eclipselink.persistence.context.reference-mode" during the createEntityManager(Map) call. The PersistenceUnit/Session wide default can be set through the Session.setDefaultReferenceMode() or "eclipselink.persistence.context.reference-mode.default".

Design

This functionality is achieved by altering two sections of the UnitOfWork. The first is altering the cloneMapping to use the new WeakIdentityHashMap that supports keys or values being garbage collected. The second part is extending the current UnitOfWorkIdentity map to support weak references in the same manner as the WeakIdentityMap. If the reference mode is WEAK or FORCE_WEAK the cloneMapping will use the WeakIdentityHashMap and the IdentityMap for each class will be set based on the ChangeTrackingPolicy of the descriptor in combination with the ReferenceMode.

The only interesting design item is the use of a ReferenceQueue to improve the performance of cleaning up the dead references. This ensures only those references that are actually GC'd will need to be processed instead of the entire list as we currently do. This process may improve our current performance with Weak Identity Maps.

Open Issues

This section lists the open issues that are still pending that must be decided prior to fully implementing this project's requirements.

Date	Author	Version Description & Notes
April 23, 2008	Gordon Yorke	Initial Proposal

Decisions

This section lists decisions made. These are intended to document the resolution of open issues or constraints added to the project that are important.

Issue #	Owner	Description / Notes

Future Considerations

Issue #	Description / Notes	Decision