Java 深拷贝与浅拷贝的分析

时间:2022-09-23 16:34:59

在正式的进入主题之前,我们先来了解下深拷贝和前拷贝的概念:

浅拷贝

会创建一个新对象,这个对象有着原始对象属性值的一份精确拷贝,如果属性是基本类型,拷贝的是基本类型的值;如果属性是内存地址,拷贝的就是内存地址,因此如果一个对象改变了这个地址就会影响到另一个对象;

深拷贝:

不仅要复制对象的所有非引用成员变量值,还要为引用类型的成员变量创建新的实例,并且初始化为形式参数实例值;

了解完概念之后,我们来测试下普通的对象赋值操作属于深拷贝还是浅拷贝:

测试代码:

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public class DepthCopy {
  public static void main(String[] args) {
    Copy first = new Copy("hzw", 24);
    Copy second = first;
    second.name = "shanxi";
    System.out.println(first.name);//输出shanxi
  }
}
class Copy
{
  public String name;
  public int age;
  public Copy(String name,int age) {
    this.name = name;
    this.age = age;
  }
}

可以发现,在second将name属性值修改为shanxi之后,first的name属性值也变成了shanxi,这点就可以看出普通的对象赋值属于浅拷贝;

明白了对象之间赋值是浅拷贝之后,接下来我们来看看克隆到底是深拷贝还是浅拷贝,测试代码是让上面的Copy对象实现Cloneable接口里面的clone方法:

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public class DepthCopy {
  public static void main(String[] args) {
    Copy first = new Copy("hzw", 24);
    Copy second = null;
    try {
      second = (Copy) first.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    second.name = "shanxi";
    System.out.println(first.name);//输出: hzw
    System.out.println(first);//输出: com.hzw.day33.Copy@7f39ebdb
    System.out.println(second);//输出: com.hzw.day33.Copy@33abb81e
  }
}
class Copy implements Cloneable
{
  public String name;
  public int age;
  public Copy(String name,int age) {
    this.name = name;
    this.age = age;
  }
  @Override
  protected Object clone() throws CloneNotSupportedException {
    return super.clone();
  }
}

可以看出原先创建出的对象first和克隆创建出的对象second是两个实例,因此对于second中name属性的修改并不会影响first中的name属性;但是,我们并不能单纯的认为克隆就是深拷贝的,比如下面这个例子:

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public class DepthCopy {
  public static void main(String[] args) {
    Student student = new Student(95);
    Copy first = new Copy("hzw", 24,student);
    Copy second = null;
    try {
      second = (Copy) first.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    second.name = "shanxi";
    second.student.score = 60;
    System.out.println(first == second);//false
    System.out.println(first.student == second.student);//true
    System.out.println(first.student.score);//60
  }
}
class Copy implements Cloneable
{
  public String name;
  public int age;
  public Student student;
  public Copy(String name,int age,Student student) {
    this.name = name;
    this.age = age;
    this.student = student;
  }
  @Override
  protected Object clone() throws CloneNotSupportedException {
    return super.clone();
  }
}
class Student 
{
  public int score;
  public Student(int score) {
    this.score = score;
  }
}

看到没有呢?我们通过克隆的方式创建了second,很明显发现first和second是两个实例,因为first == second输出为false,但是first和second里面的student对象却是一样的,通过second修改了student的score值之后,first里面student的score也发生了改变,这也就是说first和second里面的student是相同的,这也就说明了克隆是浅拷贝的,我们要想实现克隆的深拷贝,必须让Copy对象里面的Student对象也要实现Cloneable接口里面的clone方法,并且在Copy里面的克隆方法返回Student的一个克隆即可,这样就可以保证Student的唯一啦,修改之后的代码如下:

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public class DepthCopy {
  public static void main(String[] args) {
    Student student = new Student(95);
    Copy first = new Copy("hzw", 24,student);
    Copy second = null;
    try {
      second = (Copy) first.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    second.name = "shanxi";
    second.student.score = 60;
    System.out.println(first == second);//false
    System.out.println(first.student == second.student);//false
    System.out.println(first.student.score);//95
    System.out.println(second.student.score);//60
  }
}
class Copy implements Cloneable
{
  public String name;
  public int age;
  public Student student;
  public Copy(String name,int age,Student student) {
    this.name = name;
    this.age = age;
    this.student = student;
  }
  @Override
  protected Object clone() throws CloneNotSupportedException {
    Copy copy = (Copy)super.clone();
    copy.student = (Student) student.clone();
    return copy;
  }
}
class Student implements Cloneable
{
  public int score;
  public Student(int score) {
    this.score = score;
  }
  @Override
  protected Object clone() throws CloneNotSupportedException {
    return super.clone();
  }
}

可以看到此时first和second和first.student和second.student都不是相同的,因此我们修改second的student的score之后并没有影响到first里的student的score值,达到了深拷贝的目的;

但是,仔细一想问题就出来了,假如我们上面例子的Student类中也存在引用类型的属性,比如College类,那么我们必须让College类实现Cloneable接口,然后在Student类里面的clone方法里面调用College类的clone方法,在Copy类的clone方法中调用Student类的clone方法,发现没有了,这个过程好复杂,必须让类中的有关引用类型全部实现Cloneable接口,感觉好麻烦是不是,好的,接下来就该牛人登场了;

解决深拷贝问题最好的方式就是采用序列化方式,这样各种类均不用实现Cloneable接口的,直接序列化反序列化就可以啦,我们来见识下吧。

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import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
 
public class DepthCopy {
  public static void main(String[] args) {
    College school = new College("nongda");
    Student student = new Student(95, school);
    Copy copy = new Copy("hzw",23, student);
    Copy another = null;//表示反序列化出来的类实例
    //进行序列化操作
    try {
      FileOutputStream fos = new FileOutputStream(new File("d:/copy.txt"));
      ObjectOutputStream oos = new ObjectOutputStream(fos);
      oos.writeObject(copy);
    } catch (Exception e) {
      e.printStackTrace();
    }
    //进行反序列化操作
    FileInputStream fis;
    try {
      fis = new FileInputStream(new File("d:/copy.txt"));
      ObjectInputStream ois = new ObjectInputStream(fis);
      another = (Copy) ois.readObject();
    } catch (Exception e) {
      e.printStackTrace();
    }
    System.out.println(copy == another);//false
    System.out.println(copy.student == another.student);//false
    System.out.println(copy.student.school == another.student.school);//false
    another.student.school.schoolName = "wuda";
    System.out.println(copy.student.school.schoolName);//nongda
  }
}
class Copy implements Serializable
{
  public String name;
  public int age;
  public Student student;
  public Copy(String name,int age,Student student) {
    this.name = name;
    this.age = age;
    this.student = student;
  }
}
class Student implements Serializable
{
  public int score;
  public College school;
  public Student(int score,College school) {
    this.score = score;
    this.school = school;
  }
}
class College implements Serializable
{
  public String schoolName;
  public College(String schoolName) {
    this.schoolName = schoolName;
  }
}

从输出就可以看出来,反序列化之后生成的对象完全就是对原对象的一份拷贝,除了属性值相同之外并不和原对象有任何关系,因此当我们修改反序列化生成对象的schoolName为"wuda"的时候并没有修改原来实例的schoolName值,还是输出"nongda",因此达到了真正的深拷贝效果,但是要想实现序列化,所有的有关类都必须实现Serializable接口,这总也比既实现Cloneable接口又实现clone方法更方便吧。

以上就是对Java 深拷贝和浅拷贝的详细讲解,有需要的可以参考下。