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

pm.makePersistent(obj);

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);

A DataNucleus extension is to pass in a String form of the identity to the above method. It accepts identity strings of the form

• {fully-qualified-class-name}:{key}
• {discriminator-name}:{key}

obj = pm.getObjectById("mydomain.MyClass:3");

When you call the method getObjectById if an object with that identity is found in the cache then a call is, by default, made to validate it still exists. You can avoid this call to the datastore by setting the persistence property datanucleus.findObject.validateWhenCached to false .

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);

Don't forget that you can also use deletion by query to delete objects. Alternatively use bulk deletion.

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.

Don't forget that you can also use bulk update to update a group of objects of a type.

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 datanucleus.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

pm.refresh(obj);

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.