clone() 方法

在应用开发过程中，我们可能会需要拷贝（copy）一个现有的对象，即得到一个新对象并希望其与现有对象封装完全相同的信息（属性值），主要是为了此后两者互不相干，修改其中的一个对象不会影响到另外一个。简单地引用变量间的赋值是不能解决问题的，因为并没有创建新对象；而自己编写代码先创建一个新对象，再将原始对象的属性一一复制过来也比较繁琐，且存在“浅度拷贝”问题；这种情况下，利用clone() 方法来实现对象拷贝不失为一种明智的选择。

 

Object类中的clone()方法专门提供拷贝当前对象的功能，其原型如下：

   /**
     * 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 {@code x}, 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 {@code true}, but these are not absolute requirements.
     * While it is typically the case that:
     * <blockquote>
     * <pre>
     * x.clone().equals(x)</pre></blockquote>
     * will be {@code true}, this is not an absolute requirement.
     * <p>
     * By convention, the returned object should be obtained by calling
     * {@code super.clone}.  If a class and all of its superclasses (except
     * {@code Object}) obey this convention, it will be the case that
     * {@code x.clone().getClass() == x.getClass()}.
     * <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 {@code 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 {@code super.clone}
     * need to be modified.
     * <p>
     * The method {@code clone} for class {@code Object} performs a
     * specific cloning operation. First, if the class of this object does
     * not implement the interface {@code Cloneable}, then a
     * {@code CloneNotSupportedException} is thrown. Note that all arrays
     * are considered to implement the interface {@code Cloneable} and that
     * the return type of the {@code clone} method of an array type {@code T[]}
     * is {@code T[]} where T is any reference or primitive type.
     * 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 {@code Object} does not itself implement the interface
     * {@code Cloneable}, so calling the {@code clone} method on an object
     * whose class is {@code 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 {@code Cloneable} interface. Subclasses
     *               that override the {@code 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;

 

其中的修饰符native标明此方法是一个本地方法，即调用了其运行时在平台/操作系统的底层功能。该方法能够创建并返回当前对象的一个副本，可以理解为将当前对象的所有信息（一连串的内存空间中存储的数据）直接复制一份并单独保存，因此其返回的是已经包含了原有对象信息的一个新对象，而不是原有对象的引用。

 

clone()方法在Object类中也被定义为protected的，因此只有在其子类中进行重写才能真正发挥作用，Java语言规定，所有要进行“克隆”的对象所属的类必须实现java.lang.Cloneable接口，这是一个安全性保护。

 

下面看一个具体的例子：

实现简单的克隆操作

Person.java

package org.fool.clone;

import org.apache.commons.lang3.builder.ToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;

public class Person implements Cloneable
{
	private String name;
	private int age;

	public Person(String name, int age)
	{
		this.name = name;
		this.age = age;
	}

	public String getName()
	{
		return name;
	}

	public void setName(String name)
	{
		this.name = name;
	}

	public int getAge()
	{
		return age;
	}

	public void setAge(int age)
	{
		this.age = age;
	}

	@Override
	public Object clone()
	{
		Person p = null;

		try
		{
			p = (Person) super.clone();
		}
		catch (CloneNotSupportedException e)
		{
			e.printStackTrace();
		}

		return p;
	}

	@Override
	public String toString()
	{

		return ToStringBuilder.reflectionToString(this,
				ToStringStyle.SHORT_PREFIX_STYLE);
	}
}

 

TestClone.java

package org.fool.clone;

public class TestClone
{
	public static void main(String[] args)
	{
		Person p1 = new Person("Spring", 18);
		Person p2 = (Person) p1.clone();
		
		System.out.println(p1 == p2);
		
		p2.setAge(30);
		
		System.out.println(p1);
		System.out.println(p2);
	}
}

 

 程序运行结果：

false
Person[name=Spring,age=18]
Person[name=Spring,age=30]

 

 

这里我们可以查看java.long.Cloneable接口的源代码，可以发现该接口中没有任何内容，其源代码如下：

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
 * @see     java.lang.CloneNotSupportedException
 * @see     java.lang.Object#clone()
 * @since   JDK1.0
 */
public interface Cloneable {
}

这样的接口被称为空接口，实际上只是起到标记的作用——必须是该接口的实现类的实例才能进行克隆操作，因此这样的接口也称为“标记性接口”。

 

 

需要小心的是，使用上述的clone()方法进行对象拷贝可能出现浅度拷贝（Low Copy）的问题，我们来看一个直观的例子。

Book.java

package org.fool.clone;

import org.apache.commons.lang3.builder.ToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;

public class Book implements Cloneable
{
	private String bookName;
	private double price;
	private Person author;

	public Book(String bookName, double price, Person author)
	{
		this.bookName = bookName;
		this.price = price;
		this.author = author;
	}

	public String getBookName()
	{
		return bookName;
	}

	public void setBookName(String bookName)
	{
		this.bookName = bookName;
	}

	public double getPrice()
	{
		return price;
	}

	public void setPrice(double price)
	{
		this.price = price;
	}

	public Person getAuthor()
	{
		return author;
	}

	public void setAuthor(Person author)
	{
		this.author = author;
	}

	@Override
	public Object clone()
	{
		Book b = null;

		try
		{
			b = (Book) super.clone();
		}
		catch (CloneNotSupportedException e)
		{
			e.printStackTrace();
		}

		// b.author = (Person) b.getAuthor().clone();

		return b;
	}

	@Override
	public String toString()
	{
		return ToStringBuilder.reflectionToString(this,
				ToStringStyle.SHORT_PREFIX_STYLE);
	}

}

 

 

TestLowCopy.java

package org.fool.clone;

public class TestLowCopy
{
	public static void main(String[] args)
	{
		Book b1 = new Book("Spring", 99.5, new Person("Google", 18));
		Book b2 = (Book) b1.clone();

		b2.setBookName("Hibernate");
		b2.setPrice(89.5);
		b2.getAuthor().setName("Facebook");
		b2.getAuthor().setAge(30);

		System.out.println(b1);
		System.out.println(b2);
	}
}

 

 

程序运行结果：

Book[bookName=Spring,price=99.5,author=Person[name=Facebook,age=30]]
Book[bookName=Hibernate,price=89.5,author=Person[name=Facebook,age=30]]

 

 

可以看出克隆后的b2对象对其属性author所引用的Person对象的改动影响到了b1，这是由clone()方法的实现机制决定的——clone()方法先在内存中开辟一块目标对象所需的存储空间（只要是同属于一个类型，则对象占有的存储空间大小也一定相同），然后直接将原始对象存储空间中的内容（包括各属性的值）原样拷贝过来。对基本类型的属性，其属性值就是真正要用到的信息，这样的操作当然没有问题，但对于引用类型的属性，其值只是所引用的其他对象的句柄，这就导致clone后“副本”对象与原始对象引用类型属性指向同样的对象，为了更加直观，读者可以自行画个内存分析图分析一下。

 

这种不够彻底的拷贝也称为浅度拷贝，浅度拷贝可能造成“原件”和“副本”对象之间的“藕断丝连”，往往导致我们所不希望的结果。与之对应的彻底拷贝操作被称为深度拷贝（Deep Copy），实现起来也不算困难，只需要在拷贝目标对象时对与其有关联的对象，比如拷贝的Book对象通过其属性author所引用的Person对象，也同时进行显式拷贝处理。

 

这里我们只需要将之前Book.java中代码的注释去掉，就可以实现深度拷贝。

b.author = (Person) b.getAuthor().clone();

@Override
public Object clone()
{
	Book b = null;

	try
	{
		b = (Book) super.clone();
	}
	catch (CloneNotSupportedException e)
	{
		e.printStackTrace();
	}

	b.author = (Person) b.getAuthor().clone();

	return b;
}

 

 

TestDeepCopy.java

package org.fool.clone;

public class TestDeepCopy
{
	public static void main(String[] args)
	{
		Book b1 = new Book("Spring", 99.5, new Person("Google", 18));
		Book b2 = (Book) b1.clone();

		b2.setBookName("Hibernate");
		b2.setPrice(89.5);
		b2.getAuthor().setName("Facebook");
		b2.getAuthor().setAge(30);

		System.out.println(b1);
		System.out.println(b2);
	}
}

 

程序运行结果：

Book[bookName=Spring,price=99.5,author=Person[name=Google,age=18]]
Book[bookName=Hibernate,price=89.5,author=Person[name=Facebook,age=30]]