Java对象克隆（Clone）及Cloneable接口、Serializable接口的深入探讨

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，