今天遇到一道面试题，询问深拷贝的两种方法。主要就是clone方法和序列化方法。今天就来分析一下这两种方式如何实现深拷贝。如果想跳过解析的朋友，直奔“重点来了！”寻找答案。
clone方法
例1：我们不妨建立一个Exam对象
考试类Exam.java文件
public class Exam implements Cloneable {
private int examId;
private String examName;
public Exam() {
}
public Exam(int examId, String examName) {
this.examId = examId;
this.examName = examName;
}
public int getExamId() {
return examId;
}
public void setExamId(int examId) {
this.examId = examId;
}
public String getExamName() {
return examName;
}
public void setExamName(String examName) {
this.examName = examName;
}
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
测试类Main.java
public class Main {
public static void main(String[] args) throws CloneNotSupportedException {
Exam exam = new Exam(1, "语文考试");
Exam cloneExam = (Exam) exam.clone();
System.out.println(cloneExam != exam);
System.out.println(cloneExam.equals(exam));
}
}
控制台输出：
true
false
我们确实拷贝出了另一个对象。equals没有覆写，所以调用的是java.lang.Object中的以下方法：
public boolean equals(Object obj) {
return (this == obj);
}
例2：假如我们给考试加个监考老师
老师类Teacher.java，不实现Cloneable接口
public class Teacher {
private String name;
public Teacher(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
把老师对象作为属性新增到考试类Exam.java中（设置监考老师）
public class Exam implements Cloneable {
private int examId;
private String examName;
private Teacher teacher;
public Exam() {
}
public Exam(int examId, String examName) {
this.examId = examId;
this.examName = examName;
}
public int getExamId() {
return examId;
}
public void setExamId(int examId) {
this.examId = examId;
}
public String getExamName() {
return examName;
}
public void setExamName(String examName) {
this.examName = examName;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
@Override
public String toString() {
return "Exam{" +
"examId=" + examId +
", examName=\'" + examName + \'\\\'\' +
", teacher=" + teacher +
\'}\';
}
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
改写测试类Main.java
public class Main {
public static void main(String[] args) throws CloneNotSupportedException {
Exam exam = new Exam(1, "语文考试");
Teacher teacher = new Teacher("马老师");
exam.setTeacher(teacher);
Exam cloneExam = (Exam) exam.clone();
System.out.println(cloneExam != exam);
System.out.println(cloneExam.equals(exam));
cloneExam.getTeacher().setName("Lily");
System.out.println(exam.toString());
System.out.println(cloneExam.toString());
}
}
控制台输出：
true
false
Exam{examId=1, examName=\'语文考试\', teacher=Teacher{name=\'Lily\'}}
Exam{examId=1, examName=\'语文考试\', teacher=Teacher{name=\'Lily\'}}
相信眼尖的朋友已经发现端倪了，详细的分析可见下文“clone方法的存在问题”
clone方法总结：
调用clone方法的前提：
- 类
Exam需要继承java.lang.Cloneable接口。否则代码在运行时报错。
解释：
调用exam.clone()的对象类Exam需要继承Cloneable接口，否则会在代码运行时抛出CloneNotSupportedException异常
- 类
Exam需要覆写父类的clone()方法。否则代码在编译时报错。
解释：
因为clone()在java.lang.Object中是protected访问控制。如果不覆写，exam.clone()这句代码无法编译通过。
clone方法的存在问题：
我们从上述例2中结果中发现，我原本只想将克隆出来的考试的监考老师改为 Lily ,但是把原考试对象的监考老师也修改了，这就十分尴尬了。
阅读
java.lang.Object中的clone()方法上的英文注释时有这样一段话：
*** this method creates a new instance of the class of this object and initializes all its fields with exactly the contents of the corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned. Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation. ***
翻译为：
该方法创建该对象类的新实例，并使用该对象相应字段的内容完全初始化其所有字段，就像通过赋值一样； 字段的内容本身不会被克隆。 因此，此方法执行此对象的“浅复制”，而不是“深复制”操作。
重点来了！使用clone方式实现“深拷贝”
覆写考试类Exam.java的clone()方法
@Override
protected Object clone() throws CloneNotSupportedException {
Exam exam = (Exam) super.clone();
if (teacher != null) {
Teacher teacher = (Teacher) this.teacher.clone();
exam.setTeacher(teacher);
}
return exam;
}
解析
用上述方法，取代return super.clone()的默认实现。同时因为这里调用了teacher.clone()，所以类Teacher也要实现Cloneable接口,覆写clone()方法。
改写老师类Teacher.java
public class Teacher implements Cloneable{
private String name;
public Teacher() {
}
public Teacher(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Teacher{" +
"name=\'" + name + \'\\\'\' +
\'}\';
}
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
控制台输出：
true
false
Exam{examId=1, examName=\'语文考试\', teacher=Teacher{name=\'马老师\'}}
Exam{examId=1, examName=\'语文考试\', teacher=Teacher{name=\'Lily\'}}
序列化方法
每个对象覆写
Cloneable方法也是够麻烦的，接下来的介绍的序列化方法更为简洁。
原理：对象->字节数组（拷贝）->对象
提到序列化，就不得不提到
java.lang.Serializable,建议好好阅读一下类上的注释。
静态的序列化“深拷贝”方法（简易版）
public class Util {
private Util() {}
public static Object deepCopy(Object exam) throws IOException, ClassNotFoundException {
ByteArrayOutputStream bs = new ByteArrayOutputStream();
ObjectOutputStream os = new ObjectOutputStream(bs);
os.writeObject(exam);
ByteArrayInputStream bis = new ByteArrayInputStream(bs.toByteArray());
ObjectInputStream ois = new ObjectInputStream(bis);
return ois.readObject();
}
}
例1：考试类（无对象成员变量）
考试类对象Exam.java实现Serializable接口
public class Exam implements Serializable {
private int examId;
private String examName;
public Exam() {
}
public Exam(int examId, String examName) {
this.examId = examId;
this.examName = examName;
}
public int getExamId() {
return examId;
}
public void setExamId(int examId) {
this.examId = examId;
}
public String getExamName() {
return examName;
}
public void setExamName(String examName) {
this.examName = examName;
}
}
测试类Main.java
public class Main {
public static void main(String[] args) throws IOException, ClassNotFoundException {
Exam exam = new Exam(1, "语文考试");
Exam copyExam = (Exam) Util.deepCopy(exam);
System.out.println(copyExam != exam);
System.out.println(copyExam.equals(exam));
}
}
控制台输出:
true
false
例2：考试类（含对象成员变量）
老师类Teacher.java
public class Teacher implements Serializable {
private String name;
public Teacher() {
}
public Teacher(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Teacher{" +
"name=\'" + name + \'\\\'\' +
\'}\';
}
}
改写Exam.java，新增成员变量teacher
public class Exam implements Serializable {
private int examId;
private String examName;
private Teacher teacher;
public Exam() {
}
public Exam(int examId, String examName) {
this.examId = examId;
this.examName = examName;
}
public int getExamId() {
return examId;
}
public void setExamId(int examId) {
this.examId = examId;
}
public String getExamName() {
return examName;
}
public void setExamName(String examName) {
this.examName = examName;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
@Override
public String toString() {
return "Exam{" +
"examId=" + examId +
", examName=\'" + examName + \'\\\'\' +
", teacher=" + teacher +
\'}\';
}
}
改写测试类Main.java
public class Main {
public static void main(String[] args) throws IOException, ClassNotFoundException {
Exam exam = new Exam(1, "语文考试");
Teacher teacher = new Teacher("马老师");
exam.setTeacher(teacher);
Exam copyExam = (Exam) Util.deepCopy(exam);
System.out.println(copyExam != exam);
System.out.println(copyExam.equals(exam));
copyExam.getTeacher().setName("Lily");
System.out.println(exam);
System.out.println(copyExam);
}
}
控制台输出：
true
false
Exam{examId=1, examName=\'语文考试\', teacher=Teacher{name=\'马老师\'}}
Exam{examId=1, examName=\'语文考试\', teacher=Teacher{name=\'Lily\'}}
序列化方法总结
调用deepCopy方法的前提：
- 类
Exam需要实现java.lang.Serializable接口。否则代码在运行时报错。
解释：
对象类Exam需要实现java.lang.Serializable接口，否则会在代码执行到os.writeObject(exam)时抛出NotSerializableException异常。
Exam中的成员变量类Teacher也需要实现java.lang.Serializable接口。否则在运行时报错。
解释：
当类Exam中包含了成员变量Teacher时，如果只有Exam实现java.lang.Serializable接口，但是Teacher没有实现java.lang.Serializable接口，那么代码执行到os.writeObject(exam)时还是会**抛出NotSerializableException异常。
重点来了！使用泛型实现序列化“深拷贝”方法
public class Util {
private Util() {}
@SuppressWarnings("unchecked")
public static <T extends Serializable> T deepCopy(T obj) {
T cloneObj = null;
try {
//写入字节流
ByteArrayOutputStream out = new ByteArrayOutputStream();
ObjectOutputStream obs = new ObjectOutputStream(out);
obs.writeObject(obj);
obs.close();
//分配内存，写入原始对象，生成新对象
ByteArrayInputStream ios = new ByteArrayInputStream(out.toByteArray());
ObjectInputStream ois = new ObjectInputStream(ios);
//返回生成的新对象
cloneObj = (T) ois.readObject();
ois.close();
} catch (Exception e) {
e.printStackTrace();
}
return cloneObj;
}
}
使用该方法可以在代码编译期检查出没有实现
java.lang.Serializable接口的对象。
总结
- clone()方法要求目标类及其成员变量类都需要实现
java.lang.Cloneable接口，并且覆写java.lang.Object的clone()方法。 - 序列化方法通过静态方法实现，其目标类及其成员变量类都需要实现
java.lang.Serializable接口。