Java之浅拷贝与深拷贝
Posted 凉月缘
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----?浅拷贝
--- 概念
被复制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。简单说,浅拷贝就是只复制所考虑的对象,而不复制它所引用的对象
--- 实现方式
继承自java.lang.Object类的Cloneable接口,实现clone()方法
--- 举个栗子
package com.xxg; //浅拷贝 public class ShallowCopy { public static void main(String[] args) throws CloneNotSupportedException { Teacher teacher = new Teacher(); teacher.setName("Delacey"); teacher.setAge(29); Student student1 = new Student(); student1.setName("Dream"); student1.setAge(18); student1.setTeacher(teacher); Student student2 = (Student) student1.clone(); System.out.println("拷贝后"); System.out.println("student2: "+student2.getName()); System.out.println("student2: "+student2.getAge()); student1.setAge(20); System.out.println("属性值修改后"); System.out.println("student1: "+student1.getAge()); System.out.println("student2: "+student2.getAge()); System.out.println("student2.teacher_name: "+student2.getTeacher().getName()); System.out.println("student2.teacher_age: "+student2.getTeacher().getAge()); System.out.println("修改老师的信息后-------------"); // 修改老师的信息 teacher.setName("Jam"); System.out.println("student1: "+student1.getTeacher().getName()); System.out.println("student2: "+student2.getTeacher().getName()); } } class Teacher implements Cloneable{ private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override protected Object clone() throws CloneNotSupportedException { return super.clone(); } } class Student implements Cloneable{ private String name; private int age; private Teacher teacher; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public Teacher getTeacher() { return teacher; } public void setTeacher(Teacher teacher) { this.teacher = teacher; } @Override protected Object clone() throws CloneNotSupportedException { // TODO Auto-generated method stub return super.clone(); } }
--- 结果
--- 内存图
----?深拷贝
--- 概念
深拷贝是一个整个独立的对象拷贝,深拷贝会拷贝所有的属性,并拷贝属性指向的动态分配的内存。当对象和它所引用的对象一起拷贝时即发生深拷贝。深拷贝相比于浅拷贝速度较慢并且花销较大。简而言之,深拷贝把要复制的对象所引用的对象都复制了一遍。
--- 实现方式
一种是通过继承java.lang.Object类的Cloneable接口,重写clone方法;另外一种是通过实现对象序列化的方式,使对象实现Serializable接口,然后把对象(实际上只是对象的一个拷贝,原先的对象依然在JVM里面)写到一个流里,再从流里读出来,便可以重建对象。
--- 举个栗子
CloneTest2 深复制 public class CloneTest2 { public static void main(String[] args) throws Exception { Teacher teacher = new Teacher(); teacher.setName("Teacher Zhang"); teacher.setAge(40); Student2 student1 = new Student2(); student1.setName("ZhangSan"); student1.setAge(20); student1.setTeacher(teacher); Student2 student2 = (Student2) student1.clone(); System.out.println("拷贝得到的信息"); System.out.println(student2.getName()); System.out.println(student2.getAge()); System.out.println(student2.getTeacher().getName()); System.out.println(student2.getTeacher().getAge()); System.out.println("-------------"); // 修改老师的信息 teacher.setName("Teacher Zhang has changed"); System.out.println(student1.getTeacher().getName()); System.out.println(student2.getTeacher().getName()); // 两个引用student1和student2指向不同的两个对象 // 但是两个引用student1和student2中的两个teacher引用指向的是同一个对象 // 所以说明是浅拷贝 // 改为深复制之后,对teacher对象的修改只能影响第一个对象 } } class Teacher implements Cloneable { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public Object clone() throws CloneNotSupportedException { return super.clone(); } } class Student2 implements Cloneable { private String name; private int age; private Teacher teacher; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public Teacher getTeacher() { return teacher; } public void setTeacher(Teacher teacher) { this.teacher = teacher; } @Override public Object clone() throws CloneNotSupportedException { // 浅复制时: // Object object = super.clone(); // return object; // 改为深复制: Student2 student = (Student2) super.clone(); // 本来是浅复制,现在将Teacher对象复制一份并重新set进来 student.setTeacher((Teacher) student.getTeacher().clone()); return student; } }
--- 结果
拷贝得到的信息 ZhangSan 20 Teacher Zhang 40 ------------- Teacher Zhang has changed Teacher Zhang
--- 内存图
--- 举个栗子(序列化实现)
package com.xxg; import java.io.ByteArrayInputStream; import java.io.ByteArrayOutputStream; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; //序列化实现深拷贝 public class DeepCopyServiable { public static void main(String[] args) throws Exception { Teacher3 t = new Teacher3(); t.setName("Teacher Wang"); t.setAge(50); Student3 s1 = new Student3(); s1.setAge(20); s1.setName("ZhangSan"); s1.setTeacher(t); Student3 s2 = (Student3) s1.deepClone(); System.out.println("拷贝得到的信息:"); System.out.println(s2.getName()); System.out.println(s2.getAge()); System.out.println(s2.getTeacher().getName()); System.out.println(s2.getTeacher().getAge()); System.out.println("---------------------------"); // 将复制后的对象的老师信息修改一下: s2.getTeacher().setName("New Teacher Wang"); s2.getTeacher().setAge(28); System.out.println("修改了拷贝对象的教师后:"); System.out.println("拷贝对象的教师:"); System.out.println(s2.getTeacher().getName()); System.out.println(s2.getTeacher().getAge()); System.out.println("原来对象的教师:"); System.out.println(s1.getTeacher().getName()); System.out.println(s1.getTeacher().getAge()); } } class Teacher3 implements Serializable { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } } class Student3 implements Serializable { private String name; private int age; private Teacher3 teacher; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public Teacher3 getTeacher() { return teacher; } public void setTeacher(Teacher3 teacher) { this.teacher = teacher; } public Object deepClone() throws Exception { // 序列化 ByteArrayOutputStream bos = new ByteArrayOutputStream(); ObjectOutputStream oos = new ObjectOutputStream(bos); oos.writeObject(this); // 反序列化 ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray()); ObjectInputStream ois = new ObjectInputStream(bis); return ois.readObject(); } }
--- 结果
----?其他
?clone方法是Object的方法,为什么还要实现cloneable接口?
Cloneable接口是不包含任何方法的具体实现,仅仅作为一个识别标志,而且这个标志也仅仅是针对 Object类中clone()方法的,但是如果没有实现该接口,Object的clone()方法就会抛出ClassNotSupportedException异常
?为什么实现Serializable接口,同时需要一个serialVersionUID?
当一个类被序列化时,并且没有定义字段serialVersionUID,编译器eclipse会提示如下
在Eclipse中有两种生成方式:
一个是默认的1L;
private static final long serialVersionUID = 1L;
一个是根据类名、接口名、成员方法及属性等来生成一个64位的哈希字段,比如:
private static final long serialVersionUID = -932183802511122207L;
如果你没有考虑到兼容性的问题,就把它关掉,不过有这个功能是好的,只要任何类别实现了Serializable接口,如果没有加入serialVersionUID,Eclipse都会给你提示,这个serialVersionUID为了让该类别Serializable向后兼容。
如果你的对象序列化后存到硬盘上面后,你却更改了类的field(增加或减少或改名),当你反序列化时,就会出现异常,这样就会造成不兼容性的问题。
但当serialVersionUID相同时,它就会将不一样的field以type的缺省值Deserialize,这个可以避开不兼容性的问题。
参考资料:
java如果clone为什么要必须实现Cloneable接口?
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