/**
 * Redistribution and use of this software and associated documentation ("Software"), with or
 * without modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain copyright statements and notices. Redistributions
 * must also contain a copy of this document.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice, this list of
 * conditions and the following disclaimer in the documentation and/or other materials provided with
 * the distribution.
 *
 * 3. The name "Exolab" must not be used to endorse or promote products derived from this Software
 * without prior written permission of Intalio, Inc. For written permission, please contact
 * info@exolab.org.
 *
 * 4. Products derived from this Software may not be called "Exolab" nor may "Exolab" appear in
 * their names without prior written permission of Intalio, Inc. Exolab is a registered trademark of
 * Intalio, Inc.
 *
 * 5. Due credit should be given to the Exolab Project (http://www.exolab.org/).
 *
 * THIS SOFTWARE IS PROVIDED BY INTALIO, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESSED OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
 * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL INTALIO, INC. OR ITS
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
 * WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * Copyright 1999 (C) Intalio, Inc. All Rights Reserved.
 *
 * $Id$
 */

package org.exolab.castor.xml.schema;

/**
 * Implements a reference to an object that will be resolved at a later time using some resolver
 * mechanism. A resolvable reference can be created in both resolved and unresolved states.
 * Resolvable references are immutable by definition.
 * <p>
 * A resolverable reference has two states: resolved and unresolved. When in the resolved state, the
 * reference will always return the same resolved object. When in the unresolved state, the first
 * time the object is requested, it will be resolved and returned. At that point the reference
 * becomes resolved and the same object is returned in subsequent requests.
 * <p>
 * The following example creates a resolved and unresolved objects and then resolved the two:
 * 
 * <pre>
 * ResolvableReference resolved, unresolved;
 *
 * resolved = new ResolvableReference(myObject);
 * unresolved = new ResolvableReference("id", resolver);
 * if (resolved.get() == myObject)
 *   ; // This will always be true
 * if (unresolved.get() == resolver.resolve("id"))
 *   ; // This will always be true
 * </pre>
 * <p>
 *
 * @author <a href="mailto:arkin@intalio.com">Assaf Arkin</a>
 * @author <a href="mailto:kvisco@intalio.com">Keith Visco</a>
 * @see Resolver
 * @version $Revision$ $Date: 2003-03-03 00:05:44 -0700 (Mon, 03 Mar 2003) $
 **/
public final class ResolvableReference {

  /**
   * Determines whether or not the reference can be resolved at the time this method is called. A
   * call to this method does not guarantee a null value will not be returned by a call to the get()
   * method.
   **/
  public boolean resolvable() {
    // -- if resolver is null, the referent must be known, or
    // -- never will be known, so return true
    if (_resolver == null)
      return true;
    // -- otherwise check resolver to see if it can resolve the id
    return (_resolver.resolve(_id) != null);
  } // -- resolvable

  /**
   * Called to resolve the object and return it. The returned object must be cast to the proper
   * class. If a referent was specified in the constructor, that referent will be returned. If an
   * identifier was specified, the resolved will be called to resolve the object. The resolver will
   * be called only the first time this method is called. Subsequent calls will return the same
   * object.
   * <p>
   * Null is returned if the object was resolved to null.
   *
   * @return The resolved object
   */
  public Referable get() {
    // If resolver is null, the referent is known.
    if (_resolver == null)
      return _referent;

    // Must synchronize, resolving should only occur once.
    // If two get methods get to this point at once, the
    // first one will resolve, the second one will not.
    synchronized (this) {
      if (_resolver != null) {
        _referent = _resolver.resolve(_id);
        _resolver = null;
      }
      return _referent;
    }
  }

  /**
   * Constructs a resolvable reference for the named object. This reference will be resolved on
   * demand by calling the resolver with the specified identifier. The object need not be resolvable
   * until the {@link #get} method is called.
   *
   * @param id The object's identifier
   * @param resolver The resolve to use
   */
  public ResolvableReference(String id, Resolver resolver) {
    _id = id;
    _resolver = resolver;
  }


  /**
   * Constructs a resolvable reference for the given object. This reference will always resolve to
   * the specified referent.
   *
   * @param referent The object to resolve to
   */
  public ResolvableReference(Referable referent) {
    _referent = referent;
  }


  /**
   * The resolver used to resolve the object. This variable will only be held while the reference is
   * unresolved. Once the reference has been resolved, this variable will be set to null. Unused
   * resolvers will be garbage collected.
   */
  private Resolver _resolver;


  /**
   * References the resolved object if passed from the constructor or has been resolved by a call to
   * {@link #get}. Once an object has been resolved, the same object will always be returned.
   */
  private Referable _referent;


  /**
   * The identifier to use for resolving the reference.
   */
  private String _id;

}

