/*
* ome.util.mem.Handle
*
* Copyright 2006 University of Dundee. All rights reserved.
* Use is subject to license terms supplied in LICENSE.txt
*/
package ome.util.mem;
/**
* Provides the basic machinery to share the same logical state across objects
* with different identities.
* <p>
* This class calls for a distinction between object identity and object state.
* Made this distinction, it becomes possible to share the same state across
* different objects upon copy operations. When an object is updated, a new
* state representation is bound to that object, while the other objects can
* still share the previous state. This way we can have shallow copy with the
* semantics of deep copy. This can result in dramatically reduced memory
* footprint when:
* </p>
* <ul>
* <li>A considerable amount of copies of a given master object are needed.</li>
* <li>You can't use references to the master object, because the copied
* objects need to have their own identity.</li>
* <li>The number of copied objects that are going to change their state after
* the copy operation is small compared to the total number of copies.</li>
* </ul>
* <p>
* For example, think of a class <code>R</code> that represents a rectangle in
* the plane with <code>4</code> integer fields <code>x, y, w, h</code>,
* and say you want to use instances of this class to describe an ROI (region of
* interest) selection in a given image 3D-stack composed of <code>100</code>
* planes — each plane would contain a rectangle selection and all those
* selections would make up your ROI. Let's assume that the initial selection is
* a discrete 3D-rectangle that spans all planes in the stack — you would
* have one rectangle per plane, every rectangle would have exactly the same
* state, say <code>s[x=0, y=0, w=3, h=4]</code>. Moreover, let's assume that
* you will have to modify slightly this initial ROI in order to get the final
* selection — for example by resizing/moving a couple of rectangles within
* the selection. Now, when you start off with the initial selection, you could
* decide to clone an initial master object whose state is <code>s</code>
* — this way, you can later modify one of the copies without affecting the
* others. However, because <i>Java</i> makes no distinction between object
* identity and state, you would have in memory <code>100</code> references
* and <code>100</code> copies of the same logical state <code>s</code>,
* while you actually only need one.
* <p>
* <p>
* The purpose of this class is to help you save memory in situations like that
* just described by approximating the semantics of the well known Handle/Body
* and Counted Body idioms often found in <i>C++</i> programs. A given class
* abstraction is implemented by two actual classes which replicate the same
* class interface. One class, the Handle, takes on the role of an object
* identifier and forwards all calls to the other class, the Body, which
* implements the actual functionality. Clients can only access instances of the
* Handle which can all share the same Body object whenever appropriate.
* </p>
* <p>
* The way this works in our case is pretty easy. An Handle class extends this
* base <code>Handle</code> class and provides a reference to an instance of
* the corresponding Body class. The concrete Handle class exposes the same
* interface as its corresponding Body (this is not an absolute requirement, but
* usually an implementation trade-off) and has <i>no state</i> — in fact,
* the state is hold by the associated Body object. The Handle just forwards to
* the Body any call that only reads the Body's state. However, it must call the
* {@link #breakSharing() breakSharing} protected method <i>before</i>
* forwarding any call that modifies the Body's state. It is crucial that
* concrete <code>Handle</code> classes stick to this rule. In fact, the
* {@link #copy() copy} method simply rebinds a new <code>Handle</code> to the
* existing Body, so subclasses must notify any incumbent change to the Body's
* state for the <code>Handle</code> to break state sharing. Lastly, it's also
* fundamental that the Body class implements the {@link Copiable} interface
* correctly for all this to work properly.
* </p>
*
* @author Jean-Marie Burel <a
* href="mailto:j.burel@dundee.ac.uk">j.burel@dundee.ac.uk</a>
* @author <br>
* Andrea Falconi <a
* href="mailto:a.falconi@dundee.ac.uk"> a.falconi@dundee.ac.uk</a>
* @version 2.2 <small> (<b>Internal version:</b> $Revision$ $Date:
* 2005/06/09 15:01:57 $) </small>
* @since OME2.2
*/
public abstract class Handle implements Copiable, Cloneable {
/** Reference to the Body object. */
private Copiable body;
/**
* Tells whether the {@link #body} is referenced by other
* <code>Handle</code> objects.
*/
private boolean shared;
/**
* Subclasses use this constructor to specify the Body instance this handle
* will be paired up with. Subclasses must pass in a <i>newly</i> created
* object.
*
* @param body
* Reference to the Body object. Mustn't be <code>null</code>.
*/
protected Handle(Copiable body) {
if (body == null) {
throw new NullPointerException("No body.");
}
this.body = body;
shared = false;
}
/**
* Returns a reference to the Body object that is <i>currently</i> paired
* up with this handle. The type of the returned object is the same as the
* one of the object that was passed to this class' protected constructor.
* However, the object returned by this method could be different from the
* one initially passed in at creation time if the
* {@link #breakSharing() breakSharing} method has been invoked. For this
* reason, subclasses mustn't cache a reference to the object returned by
* this method. Moreover, subclasses must never leak out a reference to the
* returned Body object.
*
* @return The Body object.
*/
protected Object getBody() {
return body;
}
/**
* Subclasses must call this method <i>before</i> forwarding any call that
* modifies the Body's state.
*/
protected final void breakSharing() {
if (shared) {
body = (Copiable) body.copy();
shared = false;
}
}
/**
* Returns a deep copy of this object. To be precise, this method returns an
* object that will behave like a deep copy, but has a negligible memory
* footprint until an attempt to change its state is made. Then the whole
* original state is restored in memory so that the state change operation
* can take place.
*
* @return A deep copy of this object. The class of the returned object is
* the same as the class of this object.
*/
public final Object copy() {
Handle h;
// Make a shallow copy of this object. This is fine b/c subclasses
// are not supposed to hold any state, never mind references to other
// objects :)
try {
h = (Handle) clone(); // Class of h is this instance's class.
} catch (CloneNotSupportedException cnse) {
// Shouldn't happen as this class implements Cloneable.
throw new InternalError(
"JVM Internal Error: couldn't clone object that "
+ "implements Cloneable.");
}
h.body = this.body; // Not actually needed, added for clarity.
// Set state sharing flag.
h.shared = true;
this.shared = true;
return h;
}
}