1 /*
2 * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25 package java.beans;
26
27 /**
28 * The PersistenceDelegate class takes the responsibility
29 * for expressing the state of an instance of a given class
30 * in terms of the methods in the class's public API. Instead
31 * of associating the responsibility of persistence with
32 * the class itself as is done, for example, by the
33 * {@code readObject} and {@code writeObject}
34 * methods used by the {@code ObjectOutputStream}, streams like
35 * the {@code XMLEncoder} which
36 * use this delegation model can have their behavior controlled
37 * independently of the classes themselves. Normally, the class
38 * is the best place to put such information and conventions
39 * can easily be expressed in this delegation scheme to do just that.
40 * Sometimes however, it is the case that a minor problem
41 * in a single class prevents an entire object graph from
42 * being written and this can leave the application
43 * developer with no recourse but to attempt to shadow
44 * the problematic classes locally or use alternative
45 * persistence techniques. In situations like these, the
46 * delegation model gives a relatively clean mechanism for
47 * the application developer to intervene in all parts of the
48 * serialization process without requiring that modifications
49 * be made to the implementation of classes which are not part
50 * of the application itself.
51 * <p>
52 * In addition to using a delegation model, this persistence
53 * scheme differs from traditional serialization schemes
54 * in requiring an analog of the {@code writeObject}
55 * method without a corresponding {@code readObject}
56 * method. The {@code writeObject} analog encodes each
57 * instance in terms of its public API and there is no need to
58 * define a {@code readObject} analog
59 * since the procedure for reading the serialized form
60 * is defined by the semantics of method invocation as laid
61 * out in the Java Language Specification.
62 * Breaking the dependency between {@code writeObject}
63 * and {@code readObject} implementations, which may
64 * change from version to version, is the key factor
65 * in making the archives produced by this technique immune
66 * to changes in the private implementations of the classes
67 * to which they refer.
68 * <p>
69 * A persistence delegate, may take control of all
70 * aspects of the persistence of an object including:
71 * <ul>
72 * <li>
73 * Deciding whether or not an instance can be mutated
74 * into another instance of the same class.
75 * <li>
76 * Instantiating the object, either by calling a
77 * public constructor or a public factory method.
78 * <li>
79 * Performing the initialization of the object.
80 * </ul>
81 * @see XMLEncoder
82 *
83 * @since 1.4
84 *
85 * @author Philip Milne
86 */
87
88 public abstract class PersistenceDelegate {
89
90 /**
91 * The {@code writeObject} is a single entry point to the persistence
92 * and is used by an {@code Encoder} in the traditional
93 * mode of delegation. Although this method is not final,
94 * it should not need to be subclassed under normal circumstances.
95 * <p>
96 * This implementation first checks to see if the stream
97 * has already encountered this object. Next the
98 * {@code mutatesTo} method is called to see if
99 * that candidate returned from the stream can
100 * be mutated into an accurate copy of {@code oldInstance}.
101 * If it can, the {@code initialize} method is called to
102 * perform the initialization. If not, the candidate is removed
103 * from the stream, and the {@code instantiate} method
104 * is called to create a new candidate for this object.
105 *
106 * @param oldInstance The instance that will be created by this expression.
107 * @param out The stream to which this expression will be written.
108 *
109 * @throws NullPointerException if {@code out} is {@code null}
110 */
111 public void writeObject(Object oldInstance, Encoder out) {
112 Object newInstance = out.get(oldInstance);
113 if (!mutatesTo(oldInstance, newInstance)) {
114 out.remove(oldInstance);
115 out.writeExpression(instantiate(oldInstance, out));
116 }
117 else {
118 initialize(oldInstance.getClass(), oldInstance, newInstance, out);
119 }
120 }
121
122 /**
123 * Returns true if an <em>equivalent</em> copy of {@code oldInstance} may be
124 * created by applying a series of statements to {@code newInstance}.
125 * In the specification of this method, we mean by equivalent that the modified instance
126 * is indistinguishable from {@code oldInstance} in the behavior
127 * of the relevant methods in its public API. [Note: we use the
128 * phrase <em>relevant</em> methods rather than <em>all</em> methods
129 * here only because, to be strictly correct, methods like {@code hashCode}
130 * and {@code toString} prevent most classes from producing truly
131 * indistinguishable copies of their instances].
132 * <p>
133 * The default behavior returns {@code true}
134 * if the classes of the two instances are the same.
135 *
136 * @param oldInstance The instance to be copied.
137 * @param newInstance The instance that is to be modified.
138 * @return True if an equivalent copy of {@code newInstance} may be
139 * created by applying a series of mutations to {@code oldInstance}.
140 */
141 protected boolean mutatesTo(Object oldInstance, Object newInstance) {
142 return (newInstance != null && oldInstance != null &&
143 oldInstance.getClass() == newInstance.getClass());
144 }
145
146 /**
147 * Returns an expression whose value is {@code oldInstance}.
148 * This method is used to characterize the constructor
149 * or factory method that should be used to create the given object.
150 * For example, the {@code instantiate} method of the persistence
151 * delegate for the {@code Field} class could be defined as follows:
152 * <pre>
153 * Field f = (Field)oldInstance;
154 * return new Expression(f, f.getDeclaringClass(), "getField", new Object[]{f.getName()});
155 * </pre>
156 * Note that we declare the value of the returned expression so that
157 * the value of the expression (as returned by {@code getValue})
158 * will be identical to {@code oldInstance}.
159 *
160 * @param oldInstance The instance that will be created by this expression.
161 * @param out The stream to which this expression will be written.
162 * @return An expression whose value is {@code oldInstance}.
163 *
164 * @throws NullPointerException if {@code out} is {@code null}
165 * and this value is used in the method
166 */
167 protected abstract Expression instantiate(Object oldInstance, Encoder out);
168
169 /**
170 * Produce a series of statements with side effects on {@code newInstance}
171 * so that the new instance becomes <em>equivalent</em> to {@code oldInstance}.
172 * In the specification of this method, we mean by equivalent that, after the method
173 * returns, the modified instance is indistinguishable from
174 * {@code newInstance} in the behavior of all methods in its
175 * public API.
176 * <p>
177 * The implementation typically achieves this goal by producing a series of
178 * "what happened" statements involving the {@code oldInstance}
179 * and its publicly available state. These statements are sent
180 * to the output stream using its {@code writeExpression}
181 * method which returns an expression involving elements in
182 * a cloned environment simulating the state of an input stream during
183 * reading. Each statement returned will have had all instances
184 * the old environment replaced with objects which exist in the new
185 * one. In particular, references to the target of these statements,
186 * which start out as references to {@code oldInstance} are returned
187 * as references to the {@code newInstance} instead.
188 * Executing these statements effects an incremental
189 * alignment of the state of the two objects as a series of
190 * modifications to the objects in the new environment.
191 * By the time the initialize method returns it should be impossible
192 * to tell the two instances apart by using their public APIs.
193 * Most importantly, the sequence of steps that were used to make
194 * these objects appear equivalent will have been recorded
195 * by the output stream and will form the actual output when
196 * the stream is flushed.
197 * <p>
198 * The default implementation, calls the {@code initialize}
199 * method of the type's superclass.
200 *
201 * @param type the type of the instances
202 * @param oldInstance The instance to be copied.
203 * @param newInstance The instance that is to be modified.
204 * @param out The stream to which any initialization statements should be written.
205 *
206 * @throws NullPointerException if {@code out} is {@code null}
207 */
208 protected void initialize(Class<?> type,
209 Object oldInstance, Object newInstance,
210 Encoder out)
211 {
212 Class<?> superType = type.getSuperclass();
213 PersistenceDelegate info = out.getPersistenceDelegate(superType);
214 info.initialize(superType, oldInstance, newInstance, out);
215 }
216 }