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 <code>Cloneable</code> 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.
* <p>
* Invoking Object's clone method on an instance that does not implement the
* <code>Cloneable</code> interface results in the exception
* <code>CloneNotSupportedException</code> being thrown.
* <p>
* By convention, classes that implement this interface should override
* <tt>Object.clone</tt> (which is protected) with a public method.
* See {@link java.lang.Object#clone()} for details on overriding this
* method.
* <p>
* Note that this interface does <i>not</i> contain the <tt>clone</tt> 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 <code>Object</code> is the root of the class hierarchy.
* Every class has <code>Object</code> 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 <tt>x</tt>, the expression:
* <blockquote>
* <pre>
* x.clone() != x</pre></blockquote>
* will be true, and that the expression:
* <blockquote>
* <pre>
* x.clone().getClass() == x.getClass()</pre></blockquote>
* will be <tt>true</tt>, but these are not absolute requirements.
* While it is typically the case that:
* <blockquote>
* <pre>
* x.clone().equals(x)</pre></blockquote>
* will be <tt>true</tt>, this is not an absolute requirement.
* <p>
* By convention, the returned object should be obtained by calling
* <tt>super.clone</tt>. If a class and all of its superclasses (except
* <tt>Object</tt>) obey this convention, it will be the case that
* <tt>x.clone().getClass() == x.getClass()</tt>.
* <p>
* 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 <tt>super.clone</tt> 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 <tt>super.clone</tt>
* need to be modified.
* <p>
* The method <tt>clone</tt> for class <tt>Object</tt> performs a
* specific cloning operation. First, if the class of this object does
* not implement the interface <tt>Cloneable</tt>, then a
* <tt>CloneNotSupportedException</tt> is thrown. Note that all arrays
* are considered to implement the interface <tt>Cloneable</tt>.
* 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.
* <p>
* The class <tt>Object</tt> does not itself implement the interface
* <tt>Cloneable</tt>, so calling the <tt>clone</tt> method on an object
* whose class is <tt>Object</tt> 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 <code>Cloneable</code> interface. Subclasses
* that override the <code>clone</code> 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> 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，