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 {
}
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]]
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