1 /* 2 * Copyright (c) 1996, 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 26 package java.io; 27 28 /** 29 * Serializability of a class is enabled by the class implementing the 30 * java.io.Serializable interface. Classes that do not implement this 31 * interface will not have any of their state serialized or 32 * deserialized. All subtypes of a serializable class are themselves 33 * serializable. The serialization interface has no methods or fields 34 * and serves only to identify the semantics of being serializable. <p> 35 * 36 * To allow subtypes of non-serializable classes to be serialized, the 37 * subtype may assume responsibility for saving and restoring the 38 * state of the supertype's public, protected, and (if accessible) 39 * package fields. The subtype may assume this responsibility only if 40 * the class it extends has an accessible no-arg constructor to 41 * initialize the class's state. It is an error to declare a class 42 * Serializable if this is not the case. The error will be detected at 43 * runtime. <p> 44 * 45 * During deserialization, the fields of non-serializable classes will 46 * be initialized using the public or protected no-arg constructor of 47 * the class. A no-arg constructor must be accessible to the subclass 48 * that is serializable. The fields of serializable subclasses will 49 * be restored from the stream. <p> 50 * 51 * When traversing a graph, an object may be encountered that does not 52 * support the Serializable interface. In this case the 53 * NotSerializableException will be thrown and will identify the class 54 * of the non-serializable object. <p> 55 * 56 * Classes that require special handling during the serialization and 57 * deserialization process must implement special methods with these exact 58 * signatures: 59 * 60 * <PRE> 61 * private void writeObject(java.io.ObjectOutputStream out) 62 * throws IOException 63 * private void readObject(java.io.ObjectInputStream in) 64 * throws IOException, ClassNotFoundException; 65 * private void readObjectNoData() 66 * throws ObjectStreamException; 67 * </PRE> 68 * 69 * <p>The writeObject method is responsible for writing the state of the 70 * object for its particular class so that the corresponding 71 * readObject method can restore it. The default mechanism for saving 72 * the Object's fields can be invoked by calling 73 * out.defaultWriteObject. The method does not need to concern 74 * itself with the state belonging to its superclasses or subclasses. 75 * State is saved by writing the individual fields to the 76 * ObjectOutputStream using the writeObject method or by using the 77 * methods for primitive data types supported by DataOutput. 78 * 79 * <p>The readObject method is responsible for reading from the stream and 80 * restoring the classes fields. It may call in.defaultReadObject to invoke 81 * the default mechanism for restoring the object's non-static and 82 * non-transient fields. The defaultReadObject method uses information in 83 * the stream to assign the fields of the object saved in the stream with the 84 * correspondingly named fields in the current object. This handles the case 85 * when the class has evolved to add new fields. The method does not need to 86 * concern itself with the state belonging to its superclasses or subclasses. 87 * State is saved by writing the individual fields to the 88 * ObjectOutputStream using the writeObject method or by using the 89 * methods for primitive data types supported by DataOutput. 90 * 91 * <p>The readObjectNoData method is responsible for initializing the state of 92 * the object for its particular class in the event that the serialization 93 * stream does not list the given class as a superclass of the object being 94 * deserialized. This may occur in cases where the receiving party uses a 95 * different version of the deserialized instance's class than the sending 96 * party, and the receiver's version extends classes that are not extended by 97 * the sender's version. This may also occur if the serialization stream has 98 * been tampered; hence, readObjectNoData is useful for initializing 99 * deserialized objects properly despite a "hostile" or incomplete source 100 * stream. 101 * 102 * <p>Serializable classes that need to designate an alternative object to be 103 * used when writing an object to the stream should implement this 104 * special method with the exact signature: 105 * 106 * <PRE> 107 * ANY-ACCESS-MODIFIER Object writeReplace() throws ObjectStreamException; 108 * </PRE><p> 109 * 110 * This writeReplace method is invoked by serialization if the method 111 * exists and it would be accessible from a method defined within the 112 * class of the object being serialized. Thus, the method can have private, 113 * protected and package-private access. Subclass access to this method 114 * follows java accessibility rules. <p> 115 * 116 * Classes that need to designate a replacement when an instance of it 117 * is read from the stream should implement this special method with the 118 * exact signature. 119 * 120 * <PRE> 121 * ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException; 122 * </PRE><p> 123 * 124 * This readResolve method follows the same invocation rules and 125 * accessibility rules as writeReplace.<p> 126 * 127 * The serialization runtime associates with each serializable class a version 128 * number, called a serialVersionUID, which is used during deserialization to 129 * verify that the sender and receiver of a serialized object have loaded 130 * classes for that object that are compatible with respect to serialization. 131 * If the receiver has loaded a class for the object that has a different 132 * serialVersionUID than that of the corresponding sender's class, then 133 * deserialization will result in an {@link InvalidClassException}. A 134 * serializable class can declare its own serialVersionUID explicitly by 135 * declaring a field named <code>"serialVersionUID"</code> that must be static, 136 * final, and of type <code>long</code>: 137 * 138 * <PRE> 139 * ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L; 140 * </PRE> 141 * 142 * If a serializable class does not explicitly declare a serialVersionUID, then 143 * the serialization runtime will calculate a default serialVersionUID value 144 * for that class based on various aspects of the class, as described in the 145 * Java(TM) Object Serialization Specification. However, it is <em>strongly 146 * recommended</em> that all serializable classes explicitly declare 147 * serialVersionUID values, since the default serialVersionUID computation is 148 * highly sensitive to class details that may vary depending on compiler 149 * implementations, and can thus result in unexpected 150 * <code>InvalidClassException</code>s during deserialization. Therefore, to 151 * guarantee a consistent serialVersionUID value across different java compiler 152 * implementations, a serializable class must declare an explicit 153 * serialVersionUID value. It is also strongly advised that explicit 154 * serialVersionUID declarations use the <code>private</code> modifier where 155 * possible, since such declarations apply only to the immediately declaring 156 * class--serialVersionUID fields are not useful as inherited members. Array 157 * classes cannot declare an explicit serialVersionUID, so they always have 158 * the default computed value, but the requirement for matching 159 * serialVersionUID values is waived for array classes. 160 * 161 * Android implementation of serialVersionUID computation will change slightly 162 * for some classes if you're targeting android N. In order to preserve compatibility, 163 * this change is only enabled is the application target SDK version is set to 164 * 24 or higher. It is highly recommended to use an explicit serialVersionUID 165 * field to avoid compatibility issues. 166 * 167 * <h3>Implement Serializable Judiciously</h3> 168 * Refer to <i>Effective Java</i>'s chapter on serialization for thorough 169 * coverage of the serialization API. The book explains how to use this 170 * interface without harming your application's maintainability. 171 * 172 * <h3>Recommended Alternatives</h3> 173 * <strong>JSON</strong> is concise, human-readable and efficient. Android 174 * includes both a {@link android.util.JsonReader streaming API} and a {@link 175 * org.json.JSONObject tree API} to read and write JSON. Use a binding library 176 * like <a href="http://code.google.com/p/google-gson/">GSON</a> to read and 177 * write Java objects directly. 178 * 179 * @author unascribed 180 * @see java.io.ObjectOutputStream 181 * @see java.io.ObjectInputStream 182 * @see java.io.ObjectOutput 183 * @see java.io.ObjectInput 184 * @see java.io.Externalizable 185 * @since JDK1.1 186 */ 187 public interface Serializable { 188 } 189