Your persistent classes can implement the javax.jdo.InstanceCallbacks interface to receive callbacks when certain JDO lifecycle events take place. This interface consists of four methods:
The jdoPostLoad method is called by the JDO implementation after the default fetch group fields of your class have been loaded from the data store. Default fetch groups are explained in the section on JDO metadata; for now think of the default fetch group as all of the primitive fields of the object. No other persistent fields can be accessed in this method.
jdoPostLoad is often used to initialize non-persistent fields whose values depend on the values of persistent fields. An example of this is presented below.
jdoPreStore is called just before the persistent values in your object are flushed to the data store. You can access all persistent fields in this method.
jdoPreStore is the complement to jdoPostLoad. While jdoPostLoad is most often used to initialize non-persistent values from persistent data, jdoPreStore is usually used to set persistent fields with information cached in non-persistent ones. See the example below.
The jdoPreClear method is called before the persistent fields of your object are cleared. JDO implementations clear the persistent state of objects for several reasons, most of which will be covered later in this document. jdoPreClear can be used to clear non-persistent cached data and null relations to other objects. You should not access the values of persistent fields in this method.
jdoPreDelete is called before an object is deleted from the data store. Access to persistent fields is valid within this method. You might implement privately-owned relations by using this method to delete other related objects.
Unlike the PersistenceCapable interface, you must implement the InstanceCallbacks interface explicitly if you want to receive lifecycle callbacks.
Example 4.3. Using the InstanceCallbacks Interface
/** * Example demonstrating the use of the InstanceCallbacks interface to * persist a java.net.InetAddress and implement a privately-owned relation. */ public class Host implements InstanceCallbacks { // the InetAddress field cannot be persisted directly by JDO, so we // use the jdoPostLoad and jdoPreStore methods below to persist it // indirectly through its host name string private transient InetAddress address; // non-persistent private String hostName; // persistent // set of devices attached to this host private Set devices = new HashSet (); // setters, getters, and business logic omitted public void jdoPostLoad () { // form the InetAddress using the persistent host name try { address = InetAddress.getByName (hostName); } catch (IOException ioe) { throw new JDOException ("Invalid host name: " + hostName, ioe); } } public void jdoPreStore () { // store the host name information based on the InetAddress values hostName = address.getHostName (); } public void jdoPreDelete () { // delete all related devices when this host is deleted JDOHelper.getPersistenceManager (this).deletePersistentAll (devices); } public void jdoPreClear () { } }