Java中的深拷贝(深复制)和浅拷贝(浅复制)

系统管理员 2022-06-15 03:14 288阅读 0赞

转自：[http://www.cnblogs.com/shuaiwhu/archive/2010/12/14/2065088.html][http_www.cnblogs.com_shuaiwhu_archive_2010_12_14_2065088.html]

深拷贝(深复制)和浅拷贝(浅复制)是两个比较通用的概念，尤其在C++语言中，若不弄懂，则会在delete的时候出问题，但是我们在这幸好用的是Java。虽然java自动管理对象的回收，但对于深拷贝(深复制)和浅拷贝(浅复制)，我们还是要给予足够的重视，因为有时这两个概念往往会给我们带来不小的困惑。

浅拷贝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。深拷贝不仅拷贝对象本身，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：对象A1中包含对B1的引用，B1中包含对C1的引用。浅拷贝A1得到A2，A2 中依然包含对B1的引用，B1中依然包含对C1的引用。深拷贝则是对浅拷贝的递归，深拷贝A1得到A2，A2中包含对B2（B1的copy）的引用，B2 中包含对C2（C1的copy）的引用。

若不对clone()方法进行改写，则调用此方法得到的对象即为浅拷贝，下面我们着重谈一下深拷贝。

运行下面的程序，看一看浅拷贝：

class Professor0 implements Cloneable {
        String name;
        int age;
     
        Professor0(String name, int age) {
            this.name = name;
            this.age = age;
        }
     
        public Object clone() throws CloneNotSupportedException {
            return super.clone();
        }
    }
     
    class Student0 implements Cloneable {
        String name;// 常量对象。
        int age;
        Professor0 p;// 学生1和学生2的引用值都是一样的。
     
        Student0(String name, int age, Professor0 p) {
            this.name = name;
            this.age = age;
            this.p = p;
        }
     
        public Object clone() {
            Student0 o = null;
            try {
                o = (Student0) super.clone();
            } catch (CloneNotSupportedException e) {
                System.out.println(e.toString());
            }
     
            return o;
        }
    }
     
    public class ShallowCopy {
        public static void main(String[] args) {
            Professor0 p = new Professor0("wangwu", 50);
            Student0 s1 = new Student0("zhangsan", 18, p);
            Student0 s2 = (Student0) s1.clone();
            s2.p.name = "lisi";
            s2.p.age = 30;
            s2.name = "z";
            s2.age = 45;
            System.out.println("学生s1的姓名：" + s1.name + "\n学生s1教授的姓名：" + s1.p.name + "," + "\n学生s1教授的年纪" + s1.p.age);// 学生1的教授
        }
    }

s2变了，但s1也变了，证明s1的p和s2的p指向的是同一个对象。这在我们有的实际需求中，却不是这样，因而我们需要深拷贝：

class Professor implements Cloneable {
        String name;
        int age;
     
        Professor(String name, int age) {
            this.name = name;
            this.age = age;
        }
     
        public Object clone() {
            Object o = null;
            try {
                o = super.clone();
            } catch (CloneNotSupportedException e) {
                System.out.println(e.toString());
            }
            return o;
        }
    }
     
    class Student implements Cloneable {
        String name;
        int age;
        Professor p;
     
        Student(String name, int age, Professor p) {
            this.name = name;
            this.age = age;
            this.p = p;
        }
     
        public Object clone() {
            Student o = null;
            try {
                o = (Student) super.clone();
            } catch (CloneNotSupportedException e) {
                System.out.println(e.toString());
            }
            o.p = (Professor) p.clone();
            return o;
        }
    }
     
    public class DeepCopy {
        public static void main(String args[]) {
            long t1 = System.currentTimeMillis();
            Professor p = new Professor("wangwu", 50);
            Student s1 = new Student("zhangsan", 18, p);
            Student s2 = (Student) s1.clone();
            s2.p.name = "lisi";
            s2.p.age = 30;
            System.out.println("name=" + s1.p.name + "," + "age=" + s1.p.age);// 学生1的教授不改变。
            long t2 = System.currentTimeMillis();
            System.out.println(t2-t1);
        }
    }

当然我们还有一种深拷贝方法，就是将对象串行化：

import java.io.*;
    //Serialization is time-consuming
    class Professor2 implements Serializable {
        /**
         * 
         */
        private static final long serialVersionUID = 1L;
        String name;
        int age;
     
        Professor2(String name, int age) {
            this.name = name;
            this.age = age;
        }
    }
     
    class Student2 implements Serializable {
        /**
         * 
         */
        private static final long serialVersionUID = 1L;
        String name;// 常量对象。
        int age;
        Professor2 p;// 学生1和学生2的引用值都是一样的。
     
        Student2(String name, int age, Professor2 p) {
            this.name = name;
            this.age = age;
            this.p = p;
        }
     
        public Object deepClone() throws IOException, OptionalDataException,
                ClassNotFoundException {
            // 将对象写到流里
            ByteArrayOutputStream bo = new ByteArrayOutputStream();
            ObjectOutputStream oo = new ObjectOutputStream(bo);
            oo.writeObject(this);
            // 从流里读出来
            ByteArrayInputStream bi = new ByteArrayInputStream(bo.toByteArray());
            ObjectInputStream oi = new ObjectInputStream(bi);
            return (oi.readObject());
        }
     
    }
     
    public class DeepCopy2 {
     
        /**
         * @param args
         */
        public static void main(String[] args) throws OptionalDataException,
                IOException, ClassNotFoundException {
            long t1 = System.currentTimeMillis();
            Professor2 p = new Professor2("wangwu", 50);
            Student2 s1 = new Student2("zhangsan", 18, p);
            Student2 s2 = (Student2) s1.deepClone();
            s2.p.name = "lisi";
            s2.p.age = 30;
            System.out.println("name=" + s1.p.name + "," + "age=" + s1.p.age); // 学生1的教授不改变。
            long t2 = System.currentTimeMillis();
            System.out.println(t2-t1);
        }
     
    }

但是串行化却很耗时，在一些框架中，我们便可以感受到，它们往往将对象进行串行化后进行传递，耗时较多。

[http_www.cnblogs.com_shuaiwhu_archive_2010_12_14_2065088.html]: http://www.cnblogs.com/shuaiwhu/archive/2010/12/14/2065088.html