In JPA, objects detach automatically when they are serialized or when a persistence context ends. The specification does not define any way to explicitly detach objects. The extended KodoEntityManager, however, allows you to explicitly detach objects at any time.

public Object detach (Object pc):
public Object[] detachAll (Object... pcs):
public Collection detachAll (Collection pcs):

Each detach method returns detached copies of the given instances. The copy mechanism is similar to serialization, except that only certain fields are traversed. We will see how to control which fields are detached in a later section.

When detaching an instance that has been modified in the current transaction (and thus made dirty), the current transaction is flushed. This means that when subsequently re-attaching the detached instances, Kodo assumes that the transaction from which they were originally detached was committed; if it has been rolled back, then the re-attachment process will throw an optimistic concurrency exception.

You can stop Kodo from assuming the transaction will commit by invoking setRollbackOnly prior to detaching your objects. Setting the RollbackOnly flag prevents Kodo from flushing when detaching dirty objects; instead Kodo just runs its pre-flush actions (see the KodoEntityManager.preFlush or KodoPersistenceManager.preFlush Javadoc for details).

This allows you to use the same instances in multiple attach/modify/detach/rollback cycles. Alternatively, you might also prevent a flush by making your modifications outside of a transaction (with NontransactionalWrite enabled) before detaching.

When attaching, Kodo uses several strategies to determine the optimal way to merge changes made to the detached instance. As you will see, these strategies can even be used to attach changes made to a transient instance which was never detached in the first place.

These strategies will be assigned on a per-instance basis, such that during the attachment of an object graph more than one of the above strategies may be used.

If you attempt to attach a versioned instance whose representation has changed in the datastore since detachment, Kodo will throw an optimistic concurrency exception upon commit or flush, just as if a normal optimistic conflict was detected. When attaching an instance whose database record has been deleted since detaching, or when attaching a detached instance into a manager that has a stale version of the object, Kodo will throw an optimistic concurrency exception from the attach method. In these cases, Kodo sets the RollbackOnly flag on the transaction.

When detached objects lose their association with the Kodo runtime, they also lose the ability to load additional state from the datastore. It is important, therefore, to populate objects with all the persistent state you will need before detaching them. While you are free to do this manually, Kodo includes facilities for automatically populating objects when they detach. The kodo.DetachState configuration property determines which fields and relations are detached by default. All settings are recursive. They are:

Any field that is not included in the set determined by the detach mode is set to its Java default value in the detached instance.

The kodo.DetachState option is actually a plugin string (see Section 2.5, “Plugin Configuration”) that allows you to also configure the following options related to detached state:

<property name="kodo.DetachState" value="fgs(DetachedStateField=true)"/>

kodo.DetachState: fgs(DetachStateFielded=true)

You can also alter the set of fields that will be included in the detached graph at runtime. KodoEntityManagers expose the following APIs for controlling detached state:

public static final int DETACH_LOADED;
public static final int DETACH_FGS;
public static final int DETACH_ALL;
public int getDetachState ();
public void setDetachState (int mode);

The JDO FetchPlan expose the following APIs for controlling detached state:

public static final int DETACH_LOAD_FIELDS;
public static final int DETACH_UNLOAD_FIELDS;
public int getDetachmentOptions ();
public void setDetachmentOptions (int options);

The JDO Overview covers these APIs in Chapter 12, FetchPlan. In addition, the The KodoFetchPlan extension adds another option flag for detaching all fields:

public static final int DETACH_ALL_FIELDS;

import kodo.persistence.*;

@Entity
public class Magazine
    implements Serializable
{
    private String name;
    @DetachedState private Object state;
    ...
}

public class Magazine
    implements Serializable
{
    private String name;
    private Object detachedState;
    ...
}


<?xml version="1.0"?>
<jdo>
  <package name="org.mag">
    <class name="Magazine" detachable="true">
      <extension vendor-name="kodo" key="detached-state-field"
        value="detachedState"/>
    </class>
  </package>
</jdo>

The JPA Overview describes JPA's automatic detach behavior in Section 7.3, “Persistence Context”. We describe Kodo's options for automatic detach in JDO below.

Detachable JDO objects automatically detach when they are serialized. Kodo expands this automatic detach behavior with optional automatic detachment on various events: close, commit, and non-transactional read. On each configured event, all managed objects in the PersistenceManager cache become detached. Non-detachable objects become transient.

By using the proper set of these options, you can avoid unnecessary detach calls in your code. Some common use cases for this functionality are:

No automatic detachment is performed in JDO by default. You can turn on automatic detachment at runtime through the KodoPersistenceManager.setDetachAllOn* methods. You can also set detach detachment triggers with the kodo.AutoDetach configuration property:

kodo.AutoDetach: close, commit, nontx-read