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| 1 | /* | ||
| 2 | * Copyright (c) 2000, 2010, 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</code> and <code>writeObject</code> | ||
| 34 | * methods used by the <code>ObjectOutputStream</code>, streams like | ||
| 35 | * the <code>XMLEncoder</code> 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</code> | ||
| 55 | * method without a corresponding <code>readObject</code> | ||
| 56 | * method. The <code>writeObject</code> analog encodes each | ||
| 57 | * instance in terms of its public API and there is no need to | ||
| 58 | * define a <code>readObject</code> 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</code> | ||
| 63 | * and <code>readObject</code> 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</code> is a single entry point to the persistence | ||
| 92 | * and is used by a <code>Encoder</code> 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</code> method is called to see if | ||
| 99 | * that candidate returned from the stream can | ||
| 100 | * be mutated into an accurate copy of <code>oldInstance</code>. | ||
| 101 | * If it can, the <code>initialize</code> method is called to | ||
| 102 | * perform the initialization. If not, the candidate is removed | ||
| 103 | * from the stream, and the <code>instantiate</code> 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</code> may be | ||
| 124 | * created by applying a series of statements to <code>newInstance</code>. | ||
| 125 | * In the specification of this method, we mean by equivalent that the modified instance | ||
| 126 | * is indistinguishable from <code>oldInstance</code> 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</code> | ||
| 130 | * and <code>toString</code> prevent most classes from producing truly | ||
| 131 | * indistinguishable copies of their instances]. | ||
| 132 | * <p> | ||
| 133 | * The default behavior returns <code>true</code> | ||
| 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</code> may be | ||
| 139 | * created by applying a series of mutations to <code>oldInstance</code>. | ||
| 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</code>. | ||
| 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</code> method of the persistence | ||
| 151 | * delegate for the <code>Field</code> 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</code>) | ||
| 158 | * will be identical to <code>oldInstance</code>. | ||
| 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</code>. | ||
| 163 | * | ||
| 164 | * @throws NullPointerException if {@code out} is {@code null} | ||
| 165 | */ | ||
| 166 | protected abstract Expression instantiate(Object oldInstance, Encoder out); | ||
| 167 | |||
| 168 | /** | ||
| 169 | * Produce a series of statements with side effects on <code>newInstance</code> | ||
| 170 | * so that the new instance becomes <em>equivalent</em> to <code>oldInstance</code>. | ||
| 171 | * In the specification of this method, we mean by equivalent that, after the method | ||
| 172 | * returns, the modified instance is indistinguishable from | ||
| 173 | * <code>newInstance</code> in the behavior of all methods in its | ||
| 174 | * public API. | ||
| 175 | * <p> | ||
| 176 | * The implementation typically achieves this goal by producing a series of | ||
| 177 | * "what happened" statements involving the <code>oldInstance</code> | ||
| 178 | * and its publicly available state. These statements are sent | ||
| 179 | * to the output stream using its <code>writeExpression</code> | ||
| 180 | * method which returns an expression involving elements in | ||
| 181 | * a cloned environment simulating the state of an input stream during | ||
| 182 | * reading. Each statement returned will have had all instances | ||
| 183 | * the old environment replaced with objects which exist in the new | ||
| 184 | * one. In particular, references to the target of these statements, | ||
| 185 | * which start out as references to <code>oldInstance</code> are returned | ||
| 186 | * as references to the <code>newInstance</code> instead. | ||
| 187 | * Executing these statements effects an incremental | ||
| 188 | * alignment of the state of the two objects as a series of | ||
| 189 | * modifications to the objects in the new environment. | ||
| 190 | * By the time the initialize method returns it should be impossible | ||
| 191 | * to tell the two instances apart by using their public APIs. | ||
| 192 | * Most importantly, the sequence of steps that were used to make | ||
| 193 | * these objects appear equivalent will have been recorded | ||
| 194 | * by the output stream and will form the actual output when | ||
| 195 | * the stream is flushed. | ||
| 196 | * <p> | ||
| 197 | * The default implementation, calls the <code>initialize</code> | ||
| 198 | * method of the type's superclass. | ||
| 199 | * | ||
| 200 | * @param oldInstance The instance to be copied. | ||
| 201 | * @param newInstance The instance that is to be modified. | ||
| 202 | * @param out The stream to which any initialization statements should be written. | ||
| 203 | * | ||
| 204 | * @throws NullPointerException if {@code out} is {@code null} | ||
| 205 | */ | ||
| 206 | protected void initialize(Class<?> type, | ||
| 207 | Object oldInstance, Object newInstance, | ||
| 208 | Encoder out) | ||
| 209 | { | ||
| 210 | Class superType = type.getSuperclass(); | ||
| 211 | PersistenceDelegate info = out.getPersistenceDelegate(superType); | ||
| 212 | info.initialize(superType, oldInstance, newInstance, out); | ||
| 213 | } | ||
| 214 | } | ||