1 /**
2 * Redistribution and use of this software and associated documentation
3 * ("Software"), with or without modification, are permitted provided
4 * that the following conditions are met:
5 *
6 * 1. Redistributions of source code must retain copyright
7 * statements and notices. Redistributions must also contain a
8 * copy of this document.
9 *
10 * 2. Redistributions in binary form must reproduce the
11 * above copyright notice, this list of conditions and the
12 * following disclaimer in the documentation and/or other
13 * materials provided with the distribution.
14 *
15 * 3. The name "Exolab" must not be used to endorse or promote
16 * products derived from this Software without prior written
17 * permission of Intalio, Inc. For written permission,
18 * please contact info@exolab.org.
19 *
20 * 4. Products derived from this Software may not be called "Exolab"
21 * nor may "Exolab" appear in their names without prior written
22 * permission of Intalio, Inc. Exolab is a registered
23 * trademark of Intalio, Inc.
24 *
25 * 5. Due credit should be given to the Exolab Project
26 * (http://www.exolab.org/).
27 *
28 * THIS SOFTWARE IS PROVIDED BY INTALIO, INC. AND CONTRIBUTORS
29 * ``AS IS'' AND ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT
30 * NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
31 * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
32 * INTALIO, INC. OR ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
33 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
34 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
35 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
36 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
37 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
38 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
39 * OF THE POSSIBILITY OF SUCH DAMAGE.
40 *
41 * Copyright 1999 (C) Intalio, Inc. All Rights Reserved.
42 *
43 * $Id$
44 */
45
46 package org.exolab.castor.xml.schema;
47
48 /**
49 * Implements a reference to an object that will be resolved at a later
50 * time using some resolver mechanism. A resolvable reference can be
51 * created in both resolved and unresolved states. Resolvable
52 * references are immutable by definition.
53 * <p>
54 * A resolverable reference has two states: resolved and unresolved.
55 * When in the resolved state, the reference will always return the
56 * same resolved object. When in the unresolved state, the first time
57 * the object is requested, it will be resolved and returned. At that
58 * point the reference becomes resolved and the same object is returned
59 * in subsequent requests.
60 * <p>
61 * The following example creates a resolved and unresolved objects
62 * and then resolved the two:
63 * <pre>
64 * ResolvableReference resolved, unresolved;
65 *
66 * resolved = new ResolvableReference( myObject );
67 * unresolved = new ResolvableReference( "id", resolver );
68 * if ( resolved.get() == myObject )
69 * ; // This will always be true
70 * if ( unresolved.get() == resolver.resolve( "id" ) )
71 * ; // This will always be true
72 * </pre>
73 * <p>
74 *
75 * @author <a href="mailto:arkin@intalio.com">Assaf Arkin</a>
76 * @author <a href="mailto:kvisco@intalio.com">Keith Visco</a>
77 * @see Resolver
78 * @version $Revision$ $Date: 2003-03-03 00:05:44 -0700 (Mon, 03 Mar 2003) $
79 **/
80 public final class ResolvableReference
81 {
82
83 /**
84 * Determines whether or not the reference can be resolved at the
85 * time this method is called. A call to this method does not guarantee
86 * a null value will not be returned by a call to the get() method.
87 **/
88 public boolean resolvable() {
89 //-- if resolver is null, the referent must be known, or
90 //-- never will be known, so return true
91 if ( _resolver == null) return true;
92 //-- otherwise check resolver to see if it can resolve the id
93 return (_resolver.resolve(_id) != null);
94 } //-- resolvable
95
96 /**
97 * Called to resolve the object and return it. The returned object
98 * must be cast to the proper class. If a referent was specified
99 * in the constructor, that referent will be returned. If an
100 * identifier was specified, the resolved will be called to resolve
101 * the object. The resolver will be called only the first time
102 * this method is called. Subsequent calls will return the same
103 * object.
104 * <p>
105 * Null is returned if the object was resolved to null.
106 *
107 * @return The resolved object
108 */
109 public Referable get()
110 {
111 // If resolver is null, the referent is known.
112 if ( _resolver == null )
113 return _referent;
114
115 // Must synchronize, resolving should only occur once.
116 // If two get methods get to this point at once, the
117 // first one will resolve, the second one will not.
118 synchronized ( this ) {
119 if ( _resolver != null ) {
120 _referent = _resolver.resolve( _id );
121 _resolver = null;
122 }
123 return _referent;
124 }
125 }
126
127 /**
128 * Constructs a resolvable reference for the named object.
129 * This reference will be resolved on demand by calling the
130 * resolver with the specified identifier. The object need
131 * not be resolvable until the {@link #get} method is called.
132 *
133 * @param id The object's identifier
134 * @param resolver The resolve to use
135 */
136 public ResolvableReference( String id, Resolver resolver )
137 {
138 _id = id;
139 _resolver = resolver;
140 }
141
142
143 /**
144 * Constructs a resolvable reference for the given object.
145 * This reference will always resolve to the specified referent.
146 *
147 * @param referent The object to resolve to
148 */
149 public ResolvableReference(Referable referent)
150 {
151 _referent = referent;
152 }
153
154
155 /**
156 * The resolver used to resolve the object. This variable will
157 * only be held while the reference is unresolved. Once the
158 * reference has been resolved, this variable will be set to
159 * null. Unused resolvers will be garbage collected.
160 */
161 private Resolver _resolver;
162
163
164 /**
165 * References the resolved object if passed from the constructor
166 * or has been resolved by a call to {@link #get}. Once an
167 * object has been resolved, the same object will always be
168 * returned.
169 */
170 private Referable _referent;
171
172
173 /**
174 * The identifier to use for resolving the reference.
175 */
176 private String _id;
177
178 }
179