JDO State Transition

JDO manages the lifecycle of an object, from creation (Transient) through to persistence in the datastore (Hollow, Persistent Clean) and all of the various states between these. The transition between these states are achieved by using methods on the Persistence Manager such as makePersistent(), makeTransient(), deletePersistent(), and by commiting the changes made by these operations, or by rolling them back.

The various lifecycle states supported by JDO are shown below.

NameDescription
TransientAny object created by the developer that do are not persisted. These don't have a JDO identity.
Persistent NewAny object that is newly persisted in the current transaction. A JDO identity has been assigned to these objects.
Persistent DirtyAny persistent object that has been changed in the current transaction.
HollowAny persistent object that represents data in the datastore, but whose values are not in the instance.
Persistent CleanAny persistent object that represents data in the datastore, and whose values have not been changed in the current transaction.
Persistent DeletedAny persistent object that represents data in the datastore, and that has been deleted in the current transaction.
Persistent New DeletedAny object that have been newly made persistent and then deleted in the same current transaction.
Persistent Non transactionalAny persistent object that represents data in the datastore, whose values are loaded but not transactionally consistent.
Persistent Non transactional DirtyAny persistent object that represents data in the datastore, whose values are loaded but not transactionally consistent, and that has been modified.
Transient CleanAny transient object that represents a transactional instance whose values have not been changed in the current transaction.
Transient DirtyAny transient object that represents a transactional instance whose values have been changed in the current transaction.
Detached CleanAny detached object that represents a persistent instance whose values have not been changed since detaching.
Detached DirtyAny detached object that represents a persistent instance whose values have been changed since detaching.

Detecting Object State

JDO provides a class JDOHelper that allows you to interrogate the object state via its attributes (isPersistent(), isDeleted(), etc). In JDO 2.1 for JDKs 1.5+ JDOHelper is extended to also provide a method that gives the full object state. 

ObjectState state = JDOHelper.getObjectState(obj);


Persisting an object

The most basic thing you can do with JDO is persist an object. The following code is an example of how you can do this 

Transaction tx=pm.currentTransaction();
try
{
    tx.begin();
    Product product = new Product("Plate", 9.99);
    pm.makePersistent(product);
    tx.commit();
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }
}

The Product object progresses from Transient (initial, unpersisted state), through to Persistent New, and then finally to Hollow when it reaches the data store (after the "commit"). If the persist failed, it would "rollback" and hence end up in the same state as when it started. The following diagram shows this graphically





Updating an object

When you have persisted objects you need to update them. The following code is an example of how you can do this 

Transaction tx=pm.currentTransaction();
try
{
    tx.begin();
    String product_name = product.getName();
    ...
    product.setPrice(7.50);
    tx.commit();
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }
}

The Product object starts off in Hollow state and progresses to Persistent Clean when the user requires to read from it. It then migrates to Persistent Dirty when the price is updated. Finally it returns to Hollow when the user commits/rolls back the transaction. The following diagram shows this graphically





Deleting an object

When you no longer need an object persisted, you can delete it. The following code is an example of how you can do this 

Transaction tx=pm.currentTransaction();
try
{
    tx.begin();
    String product_name = product.getName();
    ...
    pm.deletePersistent(product);
    tx.commit();
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }
}

The Product object starts off in Hollow state and progresses to Persistent Clean when the user requires to read from it. It then migrates to Persistent Deleted when the deletePersistent() called. Finally it either progresses to Transient when commit is called, or returns to Hollow if it is rolled back. The following diagram shows this graphically





Possible state transitions

The following diagram shows the state transitions possible with JDO.



JDO State Transition