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.

Name Description
Transient Any object created by the developer that do are not persisted. These don’t have a JDO identity.
Persistent New Any object that is newly persisted in the current transaction. A JDO identity has been assigned to these objects.
Persistent Dirty Any persistent object that has been changed in the current transaction.
Hollow Any persistent object that represents data in the datastore, but whose values are not in the instance.
Persistent Clean Any persistent object that represents data in the datastore, and whose values have not been changed in the current transaction.
Persistent Deleted Any persistent object that represents data in the datastore, and that has been deleted in the current transaction.
Persistent New Deleted Any object that have been newly made persistent and then deleted in the same current transaction.
Persistent Non transactional Any persistent object that represents data in the datastore, whose values are loaded but not transactionally consistent.
Persistent Non transactional Dirty Any persistent object that represents data in the datastore, whose values are loaded but not transactionally consistent, and that has been modified.
Transient Clean Any transient object that represents a transactional instance whose values have not been changed in the current transaction.
Transient Dirty Any transient object that represents a transactional instance whose values have been changed in the current transaction.
Detached Clean Any detached object that represents a persistent instance whose values have not been changed since detaching.
Detached Dirty Any 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.

  1. 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

  1. Transaction tx=pm.currentTransaction();
  2. try
  3. {
  4.     tx.begin();
  5.     Product product = new Product("Plate", 9.99);
  6.     pm.makePersistent(product);
  7.     tx.commit();
  8. }
  9. finally
  10. {
  11.     if (tx.isActive())
  12.     {
  13.         tx.rollback();
  14.     }
  15. }

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

Object States - 图1

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

  1. Transaction tx=pm.currentTransaction();
  2. try
  3. {
  4.     tx.begin();
  5.     String product_name = product.getName();
  6.     ...
  7.     product.setPrice(7.50);
  8.     tx.commit();
  9. }
  10. finally
  11. {
  12.     if (tx.isActive())
  13.     {
  14.         tx.rollback();
  15.     }
  16. }

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

Object States - 图2

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

  1. Transaction tx=pm.currentTransaction();
  2. try
  3. {
  4.     tx.begin();
  5.     String product_name = product.getName();
  6.     ...
  7.     pm.deletePersistent(product);
  8.     tx.commit();
  9. }
  10. finally
  11. {
  12.     if (tx.isActive())
  13.     {
  14.         tx.rollback();
  15.     }
  16. }

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

Object States - 图3

Possible state transitions

The following diagram shows the state transitions possible with JDO.

Object States - 图4