src/main/asciidoc/attach_detach.adoc

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== JDO Attach/Detachanchor:JDO_AttachDetach[]

JDO provides an interface to the persistence of objects. JDO 1.0 doesn't
provide a way of taking an object that was just persisted and just work
on it and update the persisted object later. The user has to copy the
fields manually and copy them back to the persisted object later. JDO
2.0 introduces a new way of handling this situation, by *detaching* an
object from the persistence graph, allowing it to be worked on in the
users application. It can then be *attached* to the persistence graph
later. Please refer to link:state_transition.html[Object Lifecycle] for
where this fits in. The first thing to do to use a class with this
facility is to tag it as "detachable". This is done by adding the
attribute

....
<class name="MyClass" detachable="true">
....

This acts as an instruction to the link:enhancement.html[enhancement
process] to add methods necessary to utilise the attach/detach process.

The following code fragment highlights how to use the attach/detach
mechanism

....
Product working_product=null;
Transaction tx=pm.currentTransaction();
try
{
    tx.begin();

    Product prod=new Product(name,description,price);
    pm.makePersistent(prod);

    // Detach the product for use
    working_product = (Product)pm.detachCopy(prod);

    tx.commit();
}
catch (Exception e)
{
    // Handle the exception
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }
}

// Work on the detached object in our application
working_product.setPrice(new_price);

...

// Reattach the updated object
tx = pm.currentTransaction();
try
{
    tx.begin();

    Product attached_product = pm.makePersistent(working_product);

    tx.commit();
}
catch (Exception e)
{
    // Handle the exception
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }
}
....

So we now don't need to do any manual copying of object fields just
using a simple call to detach the object, and then attach it again
later. Here are a few things to note with _attach/detach_ :

* Calling _detachCopy_ on an object that is not detachable will return a
*transient* instance that is a COPY of the original, so use the COPY
thereafter.
* Calling _detachCopy_ on an object that is detachable will return a
*detached* instance that is a COPY of the original, so use this COPY
thereafter
* A _detached_ object retain the id of its datastore entity. Detached
objects should be used where you want to update the objects and attach
them later (updating the associated object in the datastore. If you want
to create copies of the objects in the datastore with their own
identities you should use _makeTransient_ instead of _detachCopy_.
* Calling _detachCopy_ will detach all fields of that object that are in
the current link:fetchgroups.html[Fetch Group] for that class for that
_PersistenceManager_.
* By default the fields of the object that will be detached are those in
the _Default Fetch Group_.
* You should choose your link:fetchgroups.html[Fetch Group] carefully,
bearing in mind which object(s) you want to access whilst detached.
Detaching a relation field will detach the related object as well.
* If you don't detach a field of an object, you [.underline]#cannot#
access the value for that field while the object is detached.
* If you don't detach a field of an object, you [.underline]#can# update
the value for that field while detached, and thereafter you can access
the value for that field.
* Calling _makePersistent_ will return an (attached) copy of the
detached object. It will attach all fields that were originally
detached, and will also attach any other fields that were modified
whilst detached.

anchor:detach_all_on_commit[]

=== Detach All On Commitanchor:Detach_All_On_Commit[]

JDO2 also provides a mechanism whereby all objects that were enlisted in
a transaction are automatically detached when the transaction is
committed. You can enable this in one of 3 ways. If you want to use this
mode globally for all __PersistenceManager__s (PMs) from a
_PersistenceManagerFactory_ (PMF) you could either set the PMF property
"datanucleus.DetachAllOnCommit", or you could create your PMF and call
the PMF method *setDetachAllOnCommit(true)*. If instead you wanted to
use this mode only for a particular PM, or only for a particular
transaction for a particular PM, then you can call the PM method
*setDetachAllOnCommit(true)* before the commit of the transaction, and
it will apply for all transaction commits thereafter, until turned off
(*setDetachAllOnCommit(false)*. Here's an example

....
// Create a PMF
...

// Create an object
MyObject my = new MyObject();

PersistenceManager pm = pmf.getPersistenceManager();
Transaction tx = pm.currentTransaction();
try
{
    tx.begin();

    // We want our object to be detached when it's been persisted
    pm.setDetachAllOnCommit(true);

    // Persist the object that we created earlier
    pm.makePersistent(my);

    tx.commit();
    // The object "my" is now in detached state and can be used further
}
finally
{
    if (tx.isActive)
    {
        tx.rollback();
    }
}
....

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anchor:copy_on_attach[]

=== Copy On Attachanchor:Copy_On_Attach[]

By default when you are attaching a detached object it will return an
attached copy of the detached object. JDO2.1 provides a new feature that
allows this attachment to just migrate the existing detached object into
attached state.

You enable this by setting the _PersistenceManagerFactory_ (PMF)
property *datanucleus.CopyOnAttach* to false. Alternatively you can use
the methods _PersistenceManagerFactory.setCopyOnAttach(boolean flag)_ or
_PersistenceManager.setCopyOnAttach(boolean flag)_. If we return to the
example at the start of this page, this now becomes

....
// Reattach the updated object
pm.setCopyOnAttach(false);
tx = pm.currentTransaction();
try
{
    tx.begin();

    // working product is currently in detached state

    pm.makePersistent(working_product);
    // working_product is now in persistent (attached) state

    tx.commit();
}
catch (Exception e)
{
    // Handle the exception
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }
}
....

Please note that if you try to attach two detached objects representing
the same underlying persistent object within the same transaction (i.e a
persistent object with the same identity already exists in the level 1
cache), then a JDOUserException will be thrown.

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=== Serialization of Detachable classesanchor:Serialization_of_Detachable_classes[]

During enhancement of Detachable classes, a field called
_jdoDetachedState_ is added to the class definition. This field allows
reading and changing tracking of detached objects while they are not
managed by a PersistenceManager.

When serialization occurs on a Detachable object, the _jdoDetachedState_
field is written to the serialized object stream. On deserialize, this
field is written back to the new deserialized instance. This process
occurs transparently to the application. However, if deserialization
occurs with an un-enhanced version of the class, the detached state is
lost.

Serialization and deserialization of Detachable classes and un-enhanced
versions of the same class is only possible if the field
_serialVersionUID_ is added. It's recommended during development of the
class, to define the _serialVersionUID_ and make the class to implement
the _java.io.Serializable_ interface, as the following example:

....
class MyClass implements java.io.Serializable
{
    private static final long serialVersionUID = 2765740961462495537L; // any random value here
    
    //.... other fields
}
....

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