彻底理解Java深克隆和浅克隆的原理及实现

一、为什么要克隆？

答案是：克隆的对象可能包含一些已经修改过的属性,保留着你想克隆对象的值，而new出来的对象的属性全是一个新的对象，对应的属性没有值，所以我们还要重新给这个对象赋值。即当需要一个新的对象来保存当前对象的“状态”就靠clone方法了。那么我把这个对象的临时属性一个一个的赋值给我新new的对象不也行嘛？可以是可以，但是一来麻烦不说，二来，大家通过上面的源码都发现了clone是一个native方法，就是快啊，在底层实现的。

二、如何实现克隆

分三步：

对象的类实现Cloneable接口；
覆盖Object类的clone()方法（覆盖clone()方法，访问修饰符设为public，默认是protected）；
在clone()方法中调用super.clone()；

三、两种不同的克隆方法，浅克隆(ShallowClone)和深克隆(DeepClone)。

浅克隆是指拷贝对象时仅仅拷贝对象本身（包括对象中的基本变量），而不拷贝对象包含的引用指向的对象。

深克隆不仅拷贝对象本身，而且拷贝对象包含的引用指向的所有对象。举例来说更加清楚：

浅克隆

package cn.zhm.day4;public class Student implements Cloneable {private int age;private String name;public Student(int age, String name) {this.age = age;this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}@Overridepublic String toString() {return "Student [age=" + age + ", name=" + name + "]";}@Overridepublic Object clone() throws CloneNotSupportedException {// TODO Auto-generated method stubreturn super.clone();}/*** @param args* @throws CloneNotSupportedException*/public static void main(String[] args) throws CloneNotSupportedException {Student student1 = new Student(20, "张三");Student student2 = (Student) student1.clone();student2.setAge(22);// 注意修改student2的age值 但是没有影响 student1的值System.out.println("student1：" + student1.getName() + "-->"+ student1.getAge());System.out.println("student2：" + student2.getName() + "-->"+ student2.getAge());}
}
运行结果：
student1：张三-->20
student2：张三-->22

引入深克隆，浅入克隆导致的问题

package cn.zhm.day4;class Teacher implements Cloneable {private String name;private Student student;public String getName() {return name;}public void setName(String name) {this.name = name;}public Student getStudent() {return student;}public void setStudent(Student student) {this.student = student;}@Overridepublic String toString() {return "Teacher [name=" + name + ", student=" + student + "]";}@Overridepublic Object clone() throws CloneNotSupportedException {// TODO Auto-generated method stubreturn super.clone();}public static void main(String[] args) throws CloneNotSupportedException {Student s1 = new Student();s1.setAge(20);s1.setName("张三");Teacher teacher1 = new Teacher();teacher1.setName("小赵老师");teacher1.setStudent(s1);//为什么会出现以下结果, Teacher中的clone方法Teacher teacher2 = (Teacher)teacher1.clone();Student s2 = teacher2.getStudent();s2.setName("李四");s2.setAge(30);System.out.println("teacher1:"+teacher1);System.out.println("teacher2:"+teacher2);}}

运行结果：

teacher1:Teacher [name=小赵老师, student=Student [age=30, name=李四]]
teacher2:Teacher [name=小赵老师, student=Student [age=30, name=李四]

深克隆

要克隆的类和类中所有非基本数据类型的属性对应的类

必需在clone()方法注意点

package cn.zhm.day4;class Teacher implements Cloneable {private String name;private Student student;public String getName() {return name;}public void setName(String name) {this.name = name;}public Student getStudent() {return student;}public void setStudent(Student student) {this.student = student;}@Overridepublic String toString() {return "Teacher [name=" + name + ", student=" + student + "]";}@Overridepublic Object clone() throws CloneNotSupportedException {// TODO Auto-generated method stub//注意以下代码Teacher teacher = (Teacher)super.clone();teacher.setStudent((Student)teacher.getStudent().clone());return teacher;}public static void main(String[] args) throws CloneNotSupportedException {Student s1 = new Student();s1.setAge(20);s1.setName("张三");Teacher teacher1 = new Teacher();teacher1.setName("小赵老师");teacher1.setStudent(s1);Teacher teacher2 = (Teacher)teacher1.clone();teacher2.setName("小明老师");Student s2 = teacher2.getStudent();s2.setName("李四");s2.setAge(30);System.out.println("teacher1:"+teacher1);System.out.println("teacher2:"+teacher2);}}

运行结果：

teacher1:Teacher [name=小赵老师, student=Student [age=20, name=张三]]

teacher2:Teacher [name=小明老师, student=Student [age=30, name=李四]]

好好理解上面代码一定可以掌握浅克隆(ShallowClone)和深克隆(DeepClone)。