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 { //实现Cloneable接口，将clone重定义为public，Cloneable只是标记接口，没有具体方法，唯一目的就是用instanceof进行类型检查 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。