Java必备技能：clone浅克隆与深克隆

介绍

一直以来只知道Java有clone方法，该方法属于Object的，对于什么是浅克隆与深克隆就比较模糊了，现在就来补充学习一下。

概念

浅拷贝（浅克隆）复制出来的对象的所有变量都含有与原来的对象相同的值，而所有的对其他对象的引用仍然指向原来的对象。

深拷贝（深克隆）复制出来的所有变量都含有与原来的对象相同的值，那些引用其他对象的变量将指向复制出来的新对象，而不再是原有的那些被引用的对象。换言之，深复制把要复制的对象所引用的对象都复制了一遍。

实现

Student.java

public class Student implements Cloneable {
 private String name;
 private int age;
 private Teacher teacher;
 public Student(String name, int age, Teacher teacher) {
 this.name = name;
 this.age = age;
 this.teacher = teacher;
 }
}

Teacher.java

public class Teacher implements Cloneable {
 private String name;
 private String course;
 public Teacher(String name, String course) {
 this.name = name;
 this.course = course;
 }
}

Student类中包含有name，age和Teacher对象。

浅克隆

克隆对象实现Cloneable接口（该接口是一个标记接口），在克隆的方法里面调用super.clone(),就会返回克隆后的对象。

public class Student implements Cloneable {
 private String name;
 private int age;
 private Teacher teacher;
 public Student(String name, int age, Teacher teacher) {
 this.name = name;
 this.age = age;
 this.teacher = teacher;
 }
 public Student clone() {
 Student student = null;
 try {
 student = (Student) super.clone();
 } catch (CloneNotSupportedException e) {
 e.printStackTrace();
 }
 return student;
 }
}

测试

 public static void main(String args[]) throws IOException, ClassNotFoundException {
 Teacher teacher = new Teacher("王老师", "英语");
 Student student = new Student("小明", 11, teacher);
 Student clone = student.clone();
 clone.setName("小强");
 clone.setAge(20);
 clone.getTeacher().setName("李老师");
 System.out.println(student.getName() + " " + student.getAge());
 System.out.println(clone.getName() + " " + clone.getAge());
 System.out.println(clone.getTeacher() == student.getTeacher());
 }

输出：

小明 11
小强 20

true复制代码从上面结果可知，克隆出来的Student对象里的name和age是新的，但是teacher是和原来的共享的，这就是浅克隆。

深克隆

Student.java

public class Student implements Cloneable {
 private String name;
 private int age;
 private Teacher teacher;
 public Student(String name, int age, Teacher teacher) {
 this.name = name;
 this.age = age;
 this.teacher = teacher;
 }
 public Student clone() {
 Student student = null;
 try {
 student = (Student) super.clone();
 Teacher teacher = this.teacher.clone();
 student.setTeacher(teacher);
 } catch (CloneNotSupportedException e) {
 e.printStackTrace();
 }
 return student;
 }
}

Teacher.java

public class Teacher implements Cloneable {
 private String name;
 private String course;
 public Teacher(String name, String course) {
 this.name = name;
 this.course = course;
 }
 public Teacher clone() {
 Teacher clone = null;
 try {
 clone = (Teacher) super.clone();
 } catch (CloneNotSupportedException e) {
 e.printStackTrace();
 }
 return clone;
 }
}

要实现深克隆的话，克隆对象里面的对象类型也必须实现Cloneable接口并调用clone()。

测试：

public static void main(String args[]) throws IOException, ClassNotFoundException {
 Teacher teacher = new Teacher("王老师", "英语");
 Student student = new Student("小明", 11, teacher);
 Student clone = student.clone();
 clone.setName("小强");
 clone.setAge(20);
 clone.getTeacher().setName("李老师");
 System.out.println(student.getName() + " " + student.getAge());
 System.out.println(clone.getName() + " " + clone.getAge());
 System.out.println(clone.getTeacher() == student.getTeacher());
}

输出：

小明 11
小强 20

false复制代码这时，两个对象的中的Teacher就不是同一个对象了，实现了深克隆，但是如果要克隆的对象继承链比较长的话要实现深克隆，就必须逐层地实现Cloneable，这个过程是比较麻烦的，不过还有一种方法可以简便地实现深克隆。

serializable克隆

大家知道，Java可以把对象序列化写进一个流里面，反之也可以把对象从序列化流里面读取出来，但这一进一出，这个对象就不再是原来的对象了，就达到了克隆的要求。

public class Student implements Serializable {
 private String name;
 private int age;
 private Teacher teacher;
 public Student(String name, int age, Teacher teacher) {
 this.name = name;
 this.age = age;
 this.teacher = teacher;
 }
 public Student serializableClone() throws IOException, ClassNotFoundException {
 Student clone;
 ByteArrayOutputStream bo = new ByteArrayOutputStream();
 ObjectOutputStream oo = new ObjectOutputStream(bo);
 oo.writeObject(this);
 ByteArrayInputStream bi = new ByteArrayInputStream(bo.toByteArray());
 ObjectInputStream oi = new ObjectInputStream(bi);
 clone = (Student) oi.readObject();
 return clone;
 }
}

通过把对象写进ByteArrayOutputStream里，再把它读取出来。注意这个过程中所有涉及的对象都必须实现Serializable接口，由于涉及IO操作，这种方式的效率会比前面的低。

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