java深浅克隆

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java基础

1.首先理解下浅克隆和深克隆的含义吧：

浅克隆：简单说就是对当前要克隆的对象的复制，但是不会复制当前对象对其他对象的引用，即引用还是指向之前被引用对象的地址。

深克隆：就是把要克隆对象以及他所有引用对象全部复制一份。

2.浅克隆的代码演示：通过这段代码你就会明白浅克隆到底是什么意思了。

class Teacher{
	public String name;
	public int age;
	public Teacher(String name,int age){
		this.name = name;
		this.age = age;
	}
}
class Student implements Cloneable{
	public String name;
	public int age;
	public Teacher teacher;
	public Student(String name,int age,Teacher t){
		this.name = name;
		this.age = age;
		this.teacher = t;
	}
	
	@Override
	public Student clone() throws CloneNotSupportedException {
		Student s= (Student)super.clone();
		return s;
	}
}
public class TestClone {
	public static void main(String[] args) throws Exception {
		Teacher teacher = new Teacher("Job",50);
		Student stu1 = new Student("Kobe",24,teacher);
		Student stu2 =  stu1.clone();
		System.out.println("stu1 == stu2?-->" + (stu1 == stu2));
		System.out.println("stu1[name = "+ stu1.name + ",age = " + stu1.age 
				+ "],stu2[name = " + stu2.name + ",age = " + stu2.age + "]");
		System.out.println("stu1.teacher == stu2.teacher?-->" 
				+ (stu1.teacher == stu2.teacher));
		/**
		 * 此处改变stu2引用对象的值，会改变stu1引用对象的值
		 * 
		 */
		stu2.teacher.name = "Mike";
		stu2.teacher.age = 60;
		System.out.println("stu1[name = " + stu1.teacher.name 
				+ ",age = " + stu1.teacher.age + "]");
	}
}

由上段代码的结果可以看出：stu1和stu2的内存地址不同，只是形成了值得拷贝，Student对象中的Teacher对象的引用没有改变。一旦改变stu2引用Teacher的值，stu1所引用的Teacher的值也会改变，其实他俩指向的是同一内存地址，这就是所谓的浅克隆。

顺便说一下Object基类中clone()方法的含义：Object中的clone()方法可以识别出我们要复制的是哪一个对象，然后会为此对象分配内存空间，并进行对象的复制，将原始对象的内容一一复制到新对象的存储空间中。

引用下官网API的一段话：

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.

3.深克隆代码的演示：

先来一段代码：

class Teacher implements Cloneable{
	public String name;
	public int age;
	public Teacher(String name,int age){
		this.name = name;
		this.age = age;
	}
	@Override
	public Object clone() throws CloneNotSupportedException {
		return super.clone();
	}
}
class Student implements Cloneable{
	public String name;
	public int age;
	public Teacher teacher;
	public Student(String name,int age,Teacher t){
		this.name = name;
		this.age = age;
		this.teacher = t;
	}
	
	@Override
	public Student clone() throws CloneNotSupportedException {
		Student s= (Student)super.clone();
		s.teacher = (Teacher)teacher.clone();
		return s;
	}
}
public class TestClone {
	public static void main(String[] args) throws Exception {
		Teacher teacher = new Teacher("Job",50);
		Student stu1 = new Student("Kobe",24,teacher);
		Student stu2 =  stu1.clone();
		System.out.println("stu1 == stu2?-->" + (stu1 == stu2));
		System.out.println("stu1[name = "+ stu1.name + ",age = " + stu1.age 
				+ "],stu2[name = " + stu2.name + ",age = " + stu2.age + "]");
		System.out.println("stu1.teacher == stu2.teacher?-->" 
				+ (stu1.teacher == stu2.teacher));
		/**
		 * 此处改变stu2引用对象的值，不会改变stu1引用对象的值，因为在Student的clone()方法中已经对引用Teacher进行了复制。
		 * 
		 */
		stu2.teacher.name = "Mike";
		stu2.teacher.age = 60;
		System.out.println("stu1[name = " + stu1.teacher.name 
				+ ",age = " + stu1.teacher.age + "]");
	}
}

从结果可以看出改变stu2引用对象属性的值，stu1中的引用对象属性值并没有改变，这在要拷贝对象中引用对象内部没有其他引用的时候是成立的，如果Teacher中有其他对象的引用，则这样又会产生浅克隆的现象，这样并不能叫做深克隆，除非一层层的拷贝下去，所以网上很多举例解释是错误的。

还有一种是通过串行化来进行深拷贝，也就是流的过程。把对象写入流--->串行化，从流中读出对象--->并行化，对象本身还是存在于JVM中，流也是对对象的一种拷贝。代码如下：

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

class Teacher implements Serializable{
	private static final long serialVersionUID = 1L;
	public String name;
	public int age;
	public Teacher(String name,int age){
		this.name = name;
		this.age = age;
	}
}
class Student implements Serializable{
	private static final long serialVersionUID = 1L;
	public String name;
	public int age;
	public Teacher teacher;
	public Student(String name,int age,Teacher t){
		this.name = name;
		this.age = age;
		this.teacher = t;
	}
	public Object deepClone()throws Exception{
		ByteArrayOutputStream bos=new ByteArrayOutputStream();
		ObjectOutputStream oos=new ObjectOutputStream(bos);
		oos.writeObject(this);//从流里读出来
		ByteArrayInputStream bis=new ByteArrayInputStream(bos.toByteArray());
		ObjectInputStream ois=new ObjectInputStream(bis);
		return(ois.readObject());
	}
}
public class TestClone {
	public static void main(String[] args) throws Exception {
		Teacher teacher = new Teacher("Job",50);
		Student stu1 = new Student("Kobe",24,teacher);
		Student stu2 =  (Student)stu1.deepClone();
		System.out.println("stu1 == stu2?-->" + (stu1 == stu2));
		System.out.println("stu1[name = "+ stu1.name + ",age = " + stu1.age 
				+ "],stu2[name = " + stu2.name + ",age = " + stu2.age + "]");
		System.out.println("stu1.teacher == stu2.teacher?-->" 
				+ (stu1.teacher == stu2.teacher));
		/**
		 * 此处改变stu2引用对象的值，不会改变stu1引用对象的值
		 * 
		 */
		stu2.teacher.name = "Mike";
		stu2.teacher.age = 60;
		System.out.println("stu1[name = " + stu1.teacher.name 
				+ ",age = " + stu1.teacher.age + "]");
	}
}

从结果可以看出改变stu2引用对象属性的值，stu1中的引用对象属性值并没有改变，这种方法也可以实现深拷贝（前提是所有引用对象都必须可序列化，否则，就需要仔细考察那些不可串行化的对象可否设成transient，从而将之排除在复制过程之外）