浅谈关于java中的深浅拷贝

一.浅拷贝(shallow copy)

1.如何实现浅拷贝?

Object类是所有类的直接或间接父类,Object中存在clone方法,如下

protected native Object clone() throws CloneNotSupportedException;

如果想要使一个类的对象能够调用clone方法 ,则需要实现Cloneable接口, 并重写 clone方法:

public class Student implements Cloneable{private int sno ;private String name;//getter ,setter 省略
@Overridepublic Object clone() throws CloneNotSupportedException {Student s = null;try{s = (Student)super.clone();}catch (Exception e){e.printStackTrace();}return s;}}

现在测试clone方法:

@Testpublic void test04() throws CloneNotSupportedException {//创建Student对象Student s1 = new Student();s1.setSno(1);s1.setName("Rye");//通过clone 拷贝一个对象Student s2 = (Student)s1.clone();System.out.println("s1:"+s1);System.out.println("s2:"+s2);System.out.println("s1 == s2 ? ==> "+(s1 == s2));}

按照预期,克隆出的对象s2中的字段值应该与s1相同,但与s1对应的对象不在同一块内存空间,结果如下:

s1:Student{sno=1, name='Rye'}
s2:Student{sno=1, name='Rye'}
s1 == s2 ? ==> false

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此时如果修改 s1中的sno为2,那么会不会影响到s2中的sno呢?

//修改s1中的sno
s1.setSno(2);

结果如下:

s1:Student{sno=2, name='Rye'}
s2:Student{sno=1, name='Rye'}

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此时看似已经完成了 copy, s1 与 s2有着自己不同的值,但如果为Student中新增了Teacher类型的成员变量,结果还是跟上面一样吗?让我们改造下代码:

public class Teacher {private int tno;private String name;//getter setter省略...  
}

public class Student  implements Cloneable{private int sno ;private String name;private Teacher teacher;//getter ,setter ,toString 省略...
@Overridepublic Object clone() throws CloneNotSupportedException {Student s = null;try{s = (Student)super.clone();}catch (Exception e){e.printStackTrace();}return s;}    
}

此时测试代码如下:

    @Testpublic void test02() throws CloneNotSupportedException {Student student1 = new Student();student1.setSno(1);student1.setName("Rye");Teacher teacher = new Teacher();teacher.setTno(1);teacher.setName("LinTong");student1.setTeacher(teacher);Student student2 = (Student)student1.clone();System.out.println("student1:"+student1);System.out.println("student2:"+student2);System.out.println("student1 == student2 ? ==> "+ (student1 ==student2));System.out.println("student1.teacher == student2.teacher ? ==> "+ (student1.getTeacher() ==student2.getTeacher()));}

运行结果如下:

student1:Student{sno=1, name='Rye', teacher=Teacher{tno=1, name='LinTong'}}
student2:Student{sno=1, name='Rye', teacher=Teacher{tno=1, name='LinTong'}}
student1 == student2 ? ==> false
student1.teacher == student2.teacher ? ==> true

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由此可见,此时经过clone生成的student2, 与 student1.二者中的teacher字段, 指向同一块内存空间;

那么可能会有人问,这会有什么影响吗?

我们拷贝的目的,更多的时候是希望获得全新并且值相同的对象,操作原始对象或拷贝的新对象,对彼此之间互不影响;

此时我们修改student1中teacher的tno ,如下:

//修改teacher中的 tno值为2
student1.getTeacher().setTno(2);

再次运行test:

student1:Student{sno=1, name='Rye', teacher=Teacher{tno=2, name='LinTong'}}
student2:Student{sno=1, name='Rye', teacher=Teacher{tno=2, name='LinTong'}}
student1 == student2 ? ==> false
student1.teacher == student2.teacher ? ==> true

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此时发现,student2中的teacher的tno ,也跟着变化了.

变化的原因是:通过student1执行clone时,基本类型会完全copy一份到student2对应对象内存空间中, 但是对于Teacher对象仅仅是copy了一份Teacher的引用而已.

而student1 与 student2的引用指向的是同一块堆内存,因此不论是通过student1或是student2修改teacher 都会影响另外一个;

通过图会更直观一些:

2.浅拷贝中引用类型的变量拷贝的是对象的引用 , 可通过如下思路解决:

Teacher类中也覆写clone方法:

    @Overrideprotected Object clone() {Teacher teacher = null;try {teacher = (Teacher)super.clone();} catch (CloneNotSupportedException e) {e.printStackTrace();}return teacher;}

修改Student中的clone方法,如下:

    @Overridepublic Object clone() {Student s = null;try{s = (Student)super.clone();Teacher t = (Teacher)this.teacher.clone();s.setTeacher(t);}catch (Exception e){e.printStackTrace();}return s;}

此时再次运行test:

student1:Student{sno=1, name='Rye', teacher=Teacher{tno=2, name='LinTong'}}
student2:Student{sno=1, name='Rye', teacher=Teacher{tno=1, name='LinTong'}}
student1 == student2 ? ==> false
student1.teacher == student2.teacher ? ==> false

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由此可见,在copy Student的同时将Teacher也进行了修改,如图:

目前来看是满足了我们的需求,但是如果Teacher类中,同样也有别的引用类型的成员变量呢?

那么就同样需要一直覆写clone方法,如果这个关系不是特多还可以接受,如果引用关系很复杂就会显得代码繁琐;

此时应该使用序列化完成深度拷贝;

二.深拷贝(deep copy)

使用序列化完成深拷贝

深拷贝是利用对象流,将对象序列化,再反序列化得出新的对象. 因此首先需要实现序列化接口,如下:

public class Student implements Serializable{private static final long serialVersionUID = -2232725257771333130L;private int sno ;private String name;private Teacher teacher;
　　//getter ,setter,toString()省略...
}

Teacher也要实现序列化接口:

public class Teacher implements Serializable{private static final long serialVersionUID = 4477679176385287943L;private int tno;private String name;

　//getter ,setter,toString()省略...

工具方法:

　　//工具方法public Object cloneObject(Object object) throws IOException, ClassNotFoundException {//将对象序列化ByteArrayOutputStream outputStream = new ByteArrayOutputStream();ObjectOutputStream objectOutputStream = new ObjectOutputStream(outputStream);objectOutputStream.writeObject(object);//将字节反序列化ByteArrayInputStream inputStream = new ByteArrayInputStream(outputStream.toByteArray());ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);Object obj = objectInputStream.readObject();return obj;}

测试类:

　　 public void test05() throws IOException, ClassNotFoundException {Student student1 = new Student();student1.setSno(1);student1.setName("Rye");Teacher teacher = new Teacher();teacher.setTno(1);teacher.setName("LinTong");student1.setTeacher(teacher);Student student2 = (Student)cloneObject(student1);//修改teacher中的 tno值为2student1.getTeacher().setTno(2);System.out.println("student1:"+student1);System.out.println("student2:"+student2);System.out.println("student1 == student2 ? ==> "+ (student1 ==student2));System.out.println("student1.teacher == student2.teacher ? ==> "+ (student1.getTeacher() ==student2.getTeacher()));}

如果Teacher类或者Student类没有实现序列化接口,则执行时会报异常,如下:

java.io.NotSerializableException: com.example.test.Teacher

在都实现了Serializable接口的情况下,运行结果如下:

student1:Student{sno=1, name='Rye', teacher=Teacher{tno=2, name='LinTong'}}
student2:Student{sno=1, name='Rye', teacher=Teacher{tno=1, name='LinTong'}}
student1 == student2 ? ==> false
student1.teacher == student2.teacher ? ==> false

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由此通过对象流的方式,成功完成了深度拷贝;

三.重写clone方法与通过序列化两种拷贝方式比较:

clone方法:

优点:速度快,效率高

缺点:在对象引用比较深时,使用此方式比较繁琐

通过序列化:

优点:非常简便的就可以完成深度copy

缺点:由于序列化的过程需要跟磁盘打交道,因此效率会低于clone方式

如何抉择?

实际开发中,根据两种方式的优缺点进行选择即可!

转载于:https://www.cnblogs.com/lzzRye/p/9459465.html