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public interface Activatable
Activatable must be implemented by classes in order to support
Transparent Activation.
The Activatable interface may be added to persistent classes
by hand or by using the db4o enhancer. For further information
on the enhancer see the chapter "Enhancement" in the db4o
tutorial.
The basic idea for Transparent Activation is as follows:
Objects have an activation depth of 0, i.e. by default they are
not activated at all. Whenever a method is called on such an object,
the first thing to do before actually executing the method body is
to activate the object to level 1, i.e. populating its direct
members.
To illustrate this approach, we will use the following simple class.
public class Item {
The basic sequence of actions to get the above scheme to work is the
following:
private Item _next;
public Item(Item next) {
_next = next;
}
public Item next() {
return _next;
}
}
- Whenever an object is instantiated from db4o, the database registers
an activator for this object. To enable this, the object has to implement
the Activatable interface and provide the according bind(Activator)
method. The default implementation of the bind method will simply store
the given activator reference for later use.
public class Item implements Activatable {
- The first action in every method body of an activatable object should
be a call to the corresponding Activator's activate() method. (Note that
this is not enforced by any interface, it is rather a convention, and
other implementations are possible.)
transient Activator _activator;
public void bind(Activator activator) {
if (null != _activator) {
throw new IllegalStateException();
}
_activator = activator;
}
// ...
}
public class Item implements Activatable {
- The activate() method will check whether the object is already activated.
If this is not the case, it will request the container to activate the
object to level 1 and set the activated flag accordingly.
public void activate() {
if (_activator == null) return;
_activator.activate();
}
public Item next() {
activate();
return _next;
}
}
To instruct db4o to actually use these hooks (i.e. to register the
database when instantiating an object), TransparentActivationSupport
has to be registered with the db4o configuration.
Configuration config = ...
config.add(new TransparentActivationSupport());
Method Summary | |
---|---|
void |
activate(ActivationPurpose purpose)
should be called by every reading field access of an object. |
void |
bind(Activator activator)
called by db4o upon instantiation. |
Method Detail |
---|
void bind(Activator activator)
Activator
in a transient field of the object.
activator
- the Activatorvoid activate(ActivationPurpose purpose)
Activator.activate(ActivationPurpose)
on the Activator
that was
previously passed to bind(Activator)
.
purpose
- TODO
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