As you read in the guide for PersistenceManagerFactory, to control the persistence of your objects
you will require at least one PersistenceManagerFactory. Once you have obtained this object you then use
this to obtain a PersistenceManager. A PersistenceManager provides access to the operations for persistence
of your objects. This short guide will demonstrate some of the more common operations.
You obtain a PersistenceManager
as follows
PersistenceManager pm = pmf.getPersistenceManager();
In general you will be performing all operations on a PersistenceManager within a transaction, whether your
transactions are controlled by your J2EE container, by a framework such as Spring, or by locally defined transactions.
In the examples below we will omit the transaction demarcation for clarity.
The main thing that you will want to do with the data layer of a JDO-enabled application is persist your objects
into the datastore. As we mentioned earlier, a PersistenceManagerFactory represents the datastore where the
objects will be persisted. So you create a normal Java object in your application, and you then persist this as follows
This will result in the object being persisted into the datastore, though clearly it will not be persistent until
you commit the transaction. The LifecycleState of the object changes from Transient to PersistentClean
(after makePersistent), to Hollow (at commit).
Once you have persisted an object, it has an "identity". This is a unique way of identifying it. You can
obtain the identity by calling
Object id = pm.getObjectId(obj);
So what ? Well the identity can be used to retrieve the object again at some other part in your application.
So you pass the identity into your application, and the user clicks on some button on a web page and that button
corresponds to a particular object identity. You can then go back to your data layer and retrieve the object
as follows
Object obj = pm.getObjectById(id);
When you need to delete an object that you had previous persisted, deleting it is simple.
Firstly you need to get the object itself, and then delete it as follows
Object obj = pm.getObjectById(id); // Retrieves the object to delete
pm.deletePersistent(obj);
To modify a previously persisted object you only need to enlist the (updated) object in
a transaction and its changes will be propagated to the datastore at commit of the transaction.
You often have a previously persisted object and you want to use it away from the data-access layer
of your application. In this case you want to detach the object (and its related objects) so that
they can be passed across to the part of the application that requires it. To do this you do
Object detachedObj = pm.detachCopy(obj); // Returns a copy of the persisted object, in detached state
The detached object is like the original object except that it has no StateManager connected, and it
stores its JDO identity and version. It retains a list of all fields that are modified while it is detached.
This means that when you want to "attach" it to the data-access layer it knows what to update.
As an alternative, to make the detachment process transparent, you can set the PMF property
javax.jdo.option.DetachAllOnCommit to true and when you commit your transaction all objects enlisted in
the transaction will be detached.
You've detached an object (shown above), and have modified it in your application, and you now want to
attach it back to the persistence layer. You do this as follows
Object attachedObj = pm.makePersistent(obj); // Returns a copy of the detached object, in attached state
In the situation where you have an object and you think that its values may have changed in
the datastore you can update its values to the latest using the following
What this will do is as follows
- Refresh the values of all FetchPlan fields in the object
- Unload all non-FetchPlan fields in the object
If the object had any changes they will be thrown away by this step, and replaced by
the latest datastore values.
