PersistenceDelegate.java

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package java.beans;

/**
 * The PersistenceDelegate class takes the responsibility
 * for expressing the state of an instance of a given class
 * in terms of the methods in the class's public API. Instead
 * of associating the responsibility of persistence with
 * the class itself as is done, for example, by the
 * {@code readObject} and {@code writeObject}
 * methods used by the {@code ObjectOutputStream}, streams like
 * the {@code XMLEncoder} which
 * use this delegation model can have their behavior controlled
 * independently of the classes themselves. Normally, the class
 * is the best place to put such information and conventions
 * can easily be expressed in this delegation scheme to do just that.
 * Sometimes however, it is the case that a minor problem
 * in a single class prevents an entire object graph from
 * being written and this can leave the application
 * developer with no recourse but to attempt to shadow
 * the problematic classes locally or use alternative
 * persistence techniques. In situations like these, the
 * delegation model gives a relatively clean mechanism for
 * the application developer to intervene in all parts of the
 * serialization process without requiring that modifications
 * be made to the implementation of classes which are not part
 * of the application itself.
 * <p>
 * In addition to using a delegation model, this persistence
 * scheme differs from traditional serialization schemes
 * in requiring an analog of the {@code writeObject}
 * method without a corresponding {@code readObject}
 * method. The {@code writeObject} analog encodes each
 * instance in terms of its public API and there is no need to
 * define a {@code readObject} analog
 * since the procedure for reading the serialized form
 * is defined by the semantics of method invocation as laid
 * out in the Java Language Specification.
 * Breaking the dependency between {@code writeObject}
 * and {@code readObject} implementations, which may
 * change from version to version, is the key factor
 * in making the archives produced by this technique immune
 * to changes in the private implementations of the classes
 * to which they refer.
 * <p>
 * A persistence delegate, may take control of all
 * aspects of the persistence of an object including:
 * <ul>
 * <li>
 * Deciding whether or not an instance can be mutated
 * into another instance of the same class.
 * <li>
 * Instantiating the object, either by calling a
 * public constructor or a public factory method.
 * <li>
 * Performing the initialization of the object.
 * </ul>
 * @see XMLEncoder
 *
 * @since 1.4
 *
 * @author Philip Milne
 */

public abstract class PersistenceDelegate {

    /**
     * Constructs a {@code PersistenceDelegate}.
     */
    protected PersistenceDelegate() {}

    /**
     * The {@code writeObject} is a single entry point to the persistence
     * and is used by an {@code Encoder} in the traditional
     * mode of delegation. Although this method is not final,
     * it should not need to be subclassed under normal circumstances.
     * <p>
     * This implementation first checks to see if the stream
     * has already encountered this object. Next the
     * {@code mutatesTo} method is called to see if
     * that candidate returned from the stream can
     * be mutated into an accurate copy of {@code oldInstance}.
     * If it can, the {@code initialize} method is called to
     * perform the initialization. If not, the candidate is removed
     * from the stream, and the {@code instantiate} method
     * is called to create a new candidate for this object.
     *
     * @param oldInstance The instance that will be created by this expression.
     * @param out The stream to which this expression will be written.
     *
     * @throws NullPointerException if {@code out} is {@code null}
     */
    public void writeObject(Object oldInstance, Encoder out) {
        Object newInstance = out.get(oldInstance);
        if (!mutatesTo(oldInstance, newInstance)) {
            out.remove(oldInstance);
            out.writeExpression(instantiate(oldInstance, out));
        }
        else {
            initialize(oldInstance.getClass(), oldInstance, newInstance, out);
        }
    }

    /**
     * Returns true if an <em>equivalent</em> copy of {@code oldInstance} may be
     * created by applying a series of statements to {@code newInstance}.
     * In the specification of this method, we mean by equivalent that the modified instance
     * is indistinguishable from {@code oldInstance} in the behavior
     * of the relevant methods in its public API. [Note: we use the
     * phrase <em>relevant</em> methods rather than <em>all</em> methods
     * here only because, to be strictly correct, methods like {@code hashCode}
     * and {@code toString} prevent most classes from producing truly
     * indistinguishable copies of their instances].
     * <p>
     * The default behavior returns {@code true}
     * if the classes of the two instances are the same.
     *
     * @param oldInstance The instance to be copied.
     * @param newInstance The instance that is to be modified.
     * @return True if an equivalent copy of {@code newInstance} may be
     *         created by applying a series of mutations to {@code oldInstance}.
     */
    protected boolean mutatesTo(Object oldInstance, Object newInstance) {
        return (newInstance != null && oldInstance != null &&
                oldInstance.getClass() == newInstance.getClass());
    }

    /**
     * Returns an expression whose value is {@code oldInstance}.
     * This method is used to characterize the constructor
     * or factory method that should be used to create the given object.
     * For example, the {@code instantiate} method of the persistence
     * delegate for the {@code Field} class could be defined as follows:
     * <pre>
     * Field f = (Field)oldInstance;
     * return new Expression(f, f.getDeclaringClass(), "getField", new Object[]{f.getName()});
     * </pre>
     * Note that we declare the value of the returned expression so that
     * the value of the expression (as returned by {@code getValue})
     * will be identical to {@code oldInstance}.
     *
     * @param oldInstance The instance that will be created by this expression.
     * @param out The stream to which this expression will be written.
     * @return An expression whose value is {@code oldInstance}.
     *
     * @throws NullPointerException if {@code out} is {@code null}
     *                              and this value is used in the method
     */
    protected abstract Expression instantiate(Object oldInstance, Encoder out);

    /**
     * Produce a series of statements with side effects on {@code newInstance}
     * so that the new instance becomes <em>equivalent</em> to {@code oldInstance}.
     * In the specification of this method, we mean by equivalent that, after the method
     * returns, the modified instance is indistinguishable from
     * {@code newInstance} in the behavior of all methods in its
     * public API.
     * <p>
     * The implementation typically achieves this goal by producing a series of
     * "what happened" statements involving the {@code oldInstance}
     * and its publicly available state. These statements are sent
     * to the output stream using its {@code writeExpression}
     * method which returns an expression involving elements in
     * a cloned environment simulating the state of an input stream during
     * reading. Each statement returned will have had all instances
     * the old environment replaced with objects which exist in the new
     * one. In particular, references to the target of these statements,
     * which start out as references to {@code oldInstance} are returned
     * as references to the {@code newInstance} instead.
     * Executing these statements effects an incremental
     * alignment of the state of the two objects as a series of
     * modifications to the objects in the new environment.
     * By the time the initialize method returns it should be impossible
     * to tell the two instances apart by using their public APIs.
     * Most importantly, the sequence of steps that were used to make
     * these objects appear equivalent will have been recorded
     * by the output stream and will form the actual output when
     * the stream is flushed.
     * <p>
     * The default implementation, calls the {@code initialize}
     * method of the type's superclass.
     *
     * @param type the type of the instances
     * @param oldInstance The instance to be copied.
     * @param newInstance The instance that is to be modified.
     * @param out The stream to which any initialization statements should be written.
     *
     * @throws NullPointerException if {@code out} is {@code null}
     */
    protected void initialize(Class<?> type,
                              Object oldInstance, Object newInstance,
                              Encoder out)
    {
        Class<?> superType = type.getSuperclass();
        PersistenceDelegate info = out.getPersistenceDelegate(superType);
        info.initialize(superType, oldInstance, newInstance, out);
    }
}