原文链接：Java对象克隆（Clone）及Cloneable接口、Serializable接口的深入探讨
Part I 没啥好说的，直接开始Part II吧。
Part II 谈到了对象的克隆，就不得不说为什么要对对象进行克隆。java中所有的对象都是保存在堆中，而堆是供全局共享的。也就是说，如果同一个Java程序的不同方法，只要能拿到某个对象的引用，引用者就可以随意的修改对象的内部数据（前提是这个对象的内部数据通过get/set方法曝露出来）。有的时候，我们编写的代码想让调用者只获得该对象的一个拷贝（也就是一个内容完全相同的对象，但是在内存中存在两个这样的对象），有什么办法可以做到呢？当然是克隆咯。
Part III 首先，我们是程序员，当然是用我们程序员的语言来交流。
import java.util.Date; public class User implements Cloneable { private String username; private String password; private Date birthdate; public User(String username, String password, Date birthdate) { this.username = username; this.password = password; this.birthdate = birthdate; } @Override protected Object clone() throws CloneNotSupportedException { return super.clone(); } @Override public int hashCode() { // 省略equals的实现(可用eclipse自动生成) } @Override public boolean equals(Object obj) { // 省略equals的实现(可用eclipse自动生成) } // 省略一大堆get/set方法 }上述代码构建了一个User类，并且实现了java.lang.Cloneable接口。顾名思义，Cloneable的意思就是说明这个类可以被克隆的意思。 而我们先去看看java.lang.Cloneable这个接口有些什么。
/* * @(#)Cloneable.java 1.17 05/11/17 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.lang; /** * A class implements the Cloneable interface to * indicate to the {@link java.lang.Object#clone()} method that it * is legal for that method to make a * field-for-field copy of instances of that class. * * Invoking Object's clone method on an instance that does not implement the * Cloneable interface results in the exception * CloneNotSupportedException being thrown. * * By convention, classes that implement this interface should override * Object.clone (which is protected) with a public method. * See {@link java.lang.Object#clone()} for details on overriding this * method. * * Note that this interface does not contain the clone method. * Therefore, it is not possible to clone an object merely by virtue of the * fact that it implements this interface. Even if the clone method is invoked * reflectively, there is no guarantee that it will succeed. * * @author unascribed * @version 1.17, 11/17/05 * @see java.lang.CloneNotSupportedException * @see java.lang.Object#clone() * @since JDK1.0 */ public interface Cloneable { }不要惊讶，没错，除了一大堆的鸡肠以外，这个接口没有定义任何的方法签名。也就是说，我们要克隆一个对象，但是他又不给我提供一个方法。那该怎么办呢？不怕，我们还有全能的Object类，别忘记他可是所有类的始祖啊（神一般的存在着），所以，有事没事都该去问候一下他老人家。
/* * @(#)Object.java 1.73 06/03/30 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.lang; /** * Class Object is the root of the class hierarchy. * Every class has Object as a superclass. All objects, * including arrays, implement the methods of this class. * * @author unascribed * @version 1.73, 03/30/06 * @see java.lang.Class * @since JDK1.0 */ public class Object { // 省略N多的代码 /** * Creates and returns a copy of this object. The precise meaning * of "copy" may depend on the class of the object. The general * intent is that, for any object x, the expression: * * x.clone() != x * will be true, and that the expression: * * x.clone().getClass() == x.getClass() * will be true, but these are not absolute requirements. * While it is typically the case that: * * x.clone().equals(x) * will be true, this is not an absolute requirement. * * By convention, the returned object should be obtained by calling * super.clone. If a class and all of its superclasses (except * Object) obey this convention, it will be the case that * x.clone().getClass() == x.getClass(). * * By convention, the object returned by this method should be independent * of this object (which is being cloned). To achieve this independence, * it may be necessary to modify one or more fields of the object returned * by super.clone before returning it. Typically, this means * copying any mutable objects that comprise the internal "deep structure" * of the object being cloned and replacing the references to these * objects with references to the copies. If a class contains only * primitive fields or references to immutable objects, then it is usually * the case that no fields in the object returned by super.clone * need to be modified. * * The method clone for class Object performs a * specific cloning operation. First, if the class of this object does * not implement the interface Cloneable, then a * CloneNotSupportedException is thrown. Note that all arrays * are considered to implement the interface Cloneable. * Otherwise, this method creates a new instance of the class of this * object and initializes all its fields with exactly the contents of * the corresponding fields of this object, as if by assignment; the * contents of the fields are not themselves cloned. Thus, this method * performs a "shallow copy" of this object, not a "deep copy" operation. * * The class Object does not itself implement the interface * Cloneable, so calling the clone method on an object * whose class is Object will result in throwing an * exception at run time. * * @return a clone of this instance. * @exception CloneNotSupportedException if the object's class does not * support the Cloneable interface. Subclasses * that override the clone method can also * throw this exception to indicate that an instance cannot * be cloned. * @see java.lang.Cloneable */ protected native Object clone() throws CloneNotSupportedException; }呵呵，又是一大串的鸡肠，别以为我是来凑字数的，这些都是Sun公司Java开发人员写的技术文章，多看看少说话吧。 没错，又是个native方法，果然是个高深的东西，不过我们还是要占一下他的便宜。而且他这个方法是protected的，分明就是叫我们去占便宜的。 再继续看看下面测试代码。
import java.util.Date; import org.junit.Test; public class TestCase { @Test public void testUserClone() throws CloneNotSupportedException { User u1 = new User("Kent", "123456", new Date()); User u2 = u1; User u3 = (User) u1.clone(); System.out.println(u1 == u2); // true System.out.println(u1.equals(u2)); // true System.out.println(u1 == u3); // false System.out.println(u1.equals(u3)); // true } }这个clone()方法果然牛，一下子就把我们的对象克隆了一份，执行结果也符合我们的预期，u1和u3的地址不同但是内容相同。
Part IV 通过上述的例子，我们可以看出，要让一个对象进行克隆，其实就是两个步骤： \1. 让该类实现java.lang.Cloneable接口； \2. 重写（override）Object类的clone()方法。 但是，事实上真的是如此简单吗？再看下面的代码。
public class Administrator implements Cloneable { private User user; private Boolean editable; public Administrator(User user, Boolean editable) { this.user = user; this.editable = editable; } @Override protected Object clone() throws CloneNotSupportedException { return super.clone(); } @Override public int hashCode() { // 老规矩 } @Override public boolean equals(Object obj) { // 老规矩 } // 老规矩 }上面定义了一个Administrator类，这个类持有一个User类的对象。接下来我们看看对Administrator对象进行克隆会有什么效果。
import java.util.Date; import org.junit.Test; public class TestCase { @Test public void testAdministratorClone() throws CloneNotSupportedException { Administrator a1 = new Administrator(new User("Kent", "123456", new Date()), true); Administrator a2 = a1; Administrator a3 = (Administrator) a1.clone(); System.out.println(a1 == a2); // true System.out.println(a1.equals(a2)); // true System.out.println(a1 == a3); // false System.out.println(a1.equals(a3)); // true System.out.println(a1.getUser() == a3.getUser()); //true ! It's not our expected!!!!! System.out.println(a1.getUser().equals(a3.getUser())); //true } }呵呵呵！出问题了吧。Java哪是那么容易就能驾驭的说！ 这里我们就可以引入两个专业的术语：浅克隆（shallow clone）和深克隆（deep clone）。 所谓的浅克隆，顾名思义就是很表面的很表层的克隆，如果我们要克隆Administrator对象，只克隆他自身以及他包含的所有对象的引用地址。 而深克隆，就是非浅克隆。克隆除自身以外所有的对象，包括自身所包含的所有对象实例。至于深克隆的层次，由具体的需求决定，也有“N层克隆”一说。 但是，所有的基本（primitive）类型数据，无论是浅克隆还是深克隆，都会进行原值克隆。毕竟他们都不是对象，不是存储在堆中。注意：基本数据类型并不包括他们对应的包装类。 如果我们想让对象进行深度克隆，我们可以这样修改Administrator类。
@Override protected Object clone() throws CloneNotSupportedException { Administrator admin = (Administrator) super.clone(); admin.user = (User) admin.user.clone(); return admin; }由于Boolean会对值进行缓存处理，所以我们没必要对Boolean的对象进行克隆。并且Boolean类也没有实现java.lang.Cloneable接口。
Part V \1. 让该类实现java.lang.Cloneable接口； \2. 确认持有的对象是否实现java.lang.Cloneable接口并提供clone()方法； \3. 重写（override）Object类的clone()方法，并且在方法内部调用持有对象的clone()方法； \4. …… \5. 多麻烦啊，调来调去的，如果有N多个持有的对象，那就要写N多的方法，突然改变了类的结构，还要重新修改clone()方法。 难道就没有更好的办法吗？
Part VI 接下来要重点介绍一下使用java.lang.Serializable来实现对象的深度克隆。 首先，我们编写一个工具类并提供cloneTo()方法。
import java.io.ByteArrayInputStream; import java.io.ByteArrayOutputStream; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; public abstract class BeanUtil { @SuppressWarnings("unchecked") public static T cloneTo(T src) throws RuntimeException { ByteArrayOutputStream memoryBuffer = new ByteArrayOutputStream(); ObjectOutputStream out = null; ObjectInputStream in = null; T dist = null; try { out = new ObjectOutputStream(memoryBuffer); out.writeObject(src); out.flush(); in = new ObjectInputStream(new ByteArrayInputStream(memoryBuffer.toByteArray())); dist = (T) in.readObject(); } catch (Exception e) { throw new RuntimeException(e); } finally { if (out != null) try { out.close(); out = null; } catch (IOException e) { throw new RuntimeException(e); } if (in != null) try { in.close(); in = null; } catch (IOException e) { throw new RuntimeException(e); } } return dist; } }看不懂，没关系，直接拿去用就可以了。嘻嘻。 接下来我们测试一下是否能通过这个工具来实现深度克隆。 又是这个可爱的TestCase，可怜的每次都要动他……
import java.util.Date; import org.junit.Test; public class TestCase { @Test public void testCloneTo() { Administrator src = new Administrator(new User("Kent", "123456", new Date()), true); Administrator dist = BeanUtil.cloneTo(src); System.out.println(src == dist); // false System.out.println(src.equals(dist)); // true System.out.println(src.getUser() == dist.getUser()); //false ! Well done! System.out.println(src.getUser().equals(dist.getUser())); //true } }好了，无论你的对象有多么的复杂，只要这些对象都能够实现java.lang.Serializable接口，就可以进行克隆，而且这种克隆的机制是JVM完成的，不需要修改实体类的代码，方便多了。
为什么这么简单就可以实现对象的克隆呢？java.lang.Serializable接口又是干嘛用的呢？如果想知道这些问题的答案，请汇款到0123 4567 8901 2345678。 …… 开玩笑咔……这些问题的答案，我都留在了java.lang.Serializable的专题里面，请各位翻一下我的日志吧。PS.有可能这几天之内会出来，看哪天无聊写写吧……先睡够再说……
以上即为转载内容，作为一名初学java的，深知学习不易，学完也不易，打算把自己日常收集的资料整理学习一番，在这里打算列一个表单，记录一些知识点，由于实力不足以自己钻研，只好转载一部分自己看过的文章，整理一番。希望明年能够学习有所进步吧，自勉。 2017-08-06
