Cloneable接口与Serializable接口都是定义接口而没有任何的方法。Cloneable可以实现对象的克隆复制，Serializable主要是对象序列化的接口定义。很多时候我们涉及到对象的复制，我们不可能都去使用setter去实现，这样编写代码的效率太低。JDK提供的Cloneable接口正是为了解决对象复制的问题而存在。Cloneable结合Serializable接口可以实现JVM对象的深度复制。
Java JDK定义的接口
本文涉及到的两个接口都是功能定义接口没有实际的方法声明：
java.lang.Cloneable
代码语言：javascript复制/*
* Copyright (c) 1995, 2004, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*/
package java.lang;
/**
* A class implements the <code>Cloneable</code> interface to
* indicate to the {@link java.lang.Object#clone()} method that it
* is legal for that method to make a
* field-for-field copy of instances of that class.
* <p>
* Invoking Object's clone method on an instance that does not implement the
* <code>Cloneable</code> interface results in the exception
* <code>CloneNotSupportedException</code> being thrown.
* <p>
* By convention, classes that implement this interface should override
* <tt>Object.clone</tt> (which is protected) with a public method.
* See {@link java.lang.Object#clone()} for details on overriding this
* method.
* <p>
* Note that this interface does <i>not</i> contain the <tt>clone</tt> method.
* Therefore, it is not possible to clone an object merely by virtue of the
* fact that it implements this interface. Even if the clone method is invoked
* reflectively, there is no guarantee that it will succeed.
*
* @author unascribed
* @see java.lang.CloneNotSupportedException
* @see java.lang.Object#clone()
* @since JDK1.0
*/
public interface Cloneable {
}Cloneable接口是一个空接口，仅用于标记对象，Cloneable接口里面是没有clone()方法，的clone()方法是Object类里面的方法！默认实现是一个Native方法
代码语言：javascript复制protected native Object clone() throws CloneNotSupportedException;如果对象implement Cloneable接口的话，需要覆盖clone方法（因为Object类的clone方法是protected，需要覆盖为public）
代码语言：javascript复制public Object clone() throws CloneNotSupportedException{
return super.clone();
}Object类里的clone()方法仅仅用于浅拷贝（拷贝基本成员属性，对于引用类型仅返回指向改地址的引用）
java.io.Serializable
代码语言：javascript复制/*
* Copyright (c) 1996, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*/
package java.io;
/**
* Serializability of a class is enabled by the class implementing the
* java.io.Serializable interface. Classes that do not implement this
* interface will not have any of their state serialized or
* deserialized. All subtypes of a serializable class are themselves
* serializable. The serialization interface has no methods or fields
* and serves only to identify the semantics of being serializable. <p>
*
* To allow subtypes of non-serializable classes to be serialized, the
* subtype may assume responsibility for saving and restoring the
* state of the supertype's public, protected, and (if accessible)
* package fields. The subtype may assume this responsibility only if
* the class it extends has an accessible no-arg constructor to
* initialize the class's state. It is an error to declare a class
* Serializable if this is not the case. The error will be detected at
* runtime. <p>
*
* During deserialization, the fields of non-serializable classes will
* be initialized using the public or protected no-arg constructor of
* the class. A no-arg constructor must be accessible to the subclass
* that is serializable. The fields of serializable subclasses will
* be restored from the stream. <p>
*
* When traversing a graph, an object may be encountered that does not
* support the Serializable interface. In this case the
* NotSerializableException will be thrown and will identify the class
* of the non-serializable object. <p>
*
* Classes that require special handling during the serialization and
* deserialization process must implement special methods with these exact
* signatures:
*
* <PRE>
* private void writeObject(java.io.ObjectOutputStream out)
* throws IOException
* private void readObject(java.io.ObjectInputStream in)
* throws IOException, ClassNotFoundException;
* private void readObjectNoData()
* throws ObjectStreamException;
* </PRE>
*
* <p>The writeObject method is responsible for writing the state of the
* object for its particular class so that the corresponding
* readObject method can restore it. The default mechanism for saving
* the Object's fields can be invoked by calling
* out.defaultWriteObject. The method does not need to concern
* itself with the state belonging to its superclasses or subclasses.
* State is saved by writing the individual fields to the
* ObjectOutputStream using the writeObject method or by using the
* methods for primitive data types supported by DataOutput.
*
* <p>The readObject method is responsible for reading from the stream and
* restoring the classes fields. It may call in.defaultReadObject to invoke
* the default mechanism for restoring the object's non-static and
* non-transient fields. The defaultReadObject method uses information in
* the stream to assign the fields of the object saved in the stream with the
* correspondingly named fields in the current object. This handles the case
* when the class has evolved to add new fields. The method does not need to
* concern itself with the state belonging to its superclasses or subclasses.
* State is saved by writing the individual fields to the
* ObjectOutputStream using the writeObject method or by using the
* methods for primitive data types supported by DataOutput.
*
* <p>The readObjectNoData method is responsible for initializing the state of
* the object for its particular class in the event that the serialization
* stream does not list the given class as a superclass of the object being
* deserialized. This may occur in cases where the receiving party uses a
* different version of the deserialized instance's class than the sending
* party, and the receiver's version extends classes that are not extended by
* the sender's version. This may also occur if the serialization stream has
* been tampered; hence, readObjectNoData is useful for initializing
* deserialized objects properly despite a "hostile" or incomplete source
* stream.
*
* <p>Serializable classes that need to designate an alternative object to be
* used when writing an object to the stream should implement this
* special method with the exact signature:
*
* <PRE>
* ANY-ACCESS-MODIFIER Object writeReplace() throws ObjectStreamException;
* </PRE><p>
*
* This writeReplace method is invoked by serialization if the method
* exists and it would be accessible from a method defined within the
* class of the object being serialized. Thus, the method can have private,
* protected and package-private access. Subclass access to this method
* follows java accessibility rules. <p>
*
* Classes that need to designate a replacement when an instance of it
* is read from the stream should implement this special method with the
* exact signature.
*
* <PRE>
* ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException;
* </PRE><p>
*
* This readResolve method follows the same invocation rules and
* accessibility rules as writeReplace.<p>
*
* The serialization runtime associates with each serializable class a version
* number, called a serialVersionUID, which is used during deserialization to
* verify that the sender and receiver of a serialized object have loaded
* classes for that object that are compatible with respect to serialization.
* If the receiver has loaded a class for the object that has a different
* serialVersionUID than that of the corresponding sender's class, then
* deserialization will result in an {@link InvalidClassException}. A
* serializable class can declare its own serialVersionUID explicitly by
* declaring a field named <code>"serialVersionUID"</code> that must be static,
* final, and of type <code>long</code>:
*
* <PRE>
* ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;
* </PRE>
*
* If a serializable class does not explicitly declare a serialVersionUID, then
* the serialization runtime will calculate a default serialVersionUID value
* for that class based on various aspects of the class, as described in the
* Java(TM) Object Serialization Specification. However, it is <em>strongly
* recommended</em> that all serializable classes explicitly declare
* serialVersionUID values, since the default serialVersionUID computation is
* highly sensitive to class details that may vary depending on compiler
* implementations, and can thus result in unexpected
* <code>InvalidClassException</code>s during deserialization. Therefore, to
* guarantee a consistent serialVersionUID value across different java compiler
* implementations, a serializable class must declare an explicit
* serialVersionUID value. It is also strongly advised that explicit
* serialVersionUID declarations use the <code>private</code> modifier where
* possible, since such declarations apply only to the immediately declaring
* class--serialVersionUID fields are not useful as inherited members. Array
* classes cannot declare an explicit serialVersionUID, so they always have
* the default computed value, but the requirement for matching
* serialVersionUID values is waived for array classes.
*
* @author unascribed
* @see java.io.ObjectOutputStream
* @see java.io.ObjectInputStream
* @see java.io.ObjectOutput
* @see java.io.ObjectInput
* @see java.io.Externalizable
* @since JDK1.1
*/
public interface Serializable {
}使用Cloneable接口对Java对象进行克隆
实现克隆的步骤：
1.让该类实现java.lang.Cloneable接口；
2. 重写（override）Object类的clone()方法。
普通对象的克隆是克隆属性的值，而不是引用。
代码语言：javascript复制public class Person implements Cloneable {
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
protected Object clone() throws CloneNotSupportedException {
// TODO Auto-generated method stub
return super.clone();
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
return "Person [id=" id ", name=" name "]";
}
public static void main(String[] args) throws CloneNotSupportedException {
Person person = new Person();
person.setId(1);
person.setName("boonya");
System.out.println(person);
Person clonePerson = (Person) person.clone();
System.out.println("==" (person == clonePerson));
person.setId(2);
System.out.println(clonePerson);
}
}运行时输出：
代码语言：javascript复制Person [id=1, name=boonya]
==false
Person [id=1, name=boonya]同样的，我们在测试一个：
代码语言：javascript复制import java.util.Date;
public class User implements Cloneable {
private String username;
private String password;
private Date birthdate;
public User(String username, String password, Date birthdate) {
super();
this.username = username;
this.password = password;
this.birthdate = birthdate;
}
/**
* @return the username
*/
public String getUsername() {
return username;
}
/**
* @param username the username to set
*/
public void setUsername(String username) {
this.username = username;
}
/**
* @return the password
*/
public String getPassword() {
return password;
}
/**
* @param password the password to set
*/
public void setPassword(String password) {
this.password = password;
}
/**
* @return the birthdate
*/
public Date getBirthdate() {
return birthdate;
}
/**
* @param birthdate the birthdate to set
*/
public void setBirthdate(Date birthdate) {
this.birthdate = birthdate;
}
/* (non-Javadoc)
* @see java.lang.Object#clone()
*/
@Override
protected Object clone() throws CloneNotSupportedException {
// TODO Auto-generated method stub
return super.clone();
}
/* (non-Javadoc)
* @see java.lang.Object#equals(java.lang.Object)
*/
public boolean equals(User user) {
return toString().equals(user.toString());
}
/* (non-Javadoc)
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
return "User [username=" username ", password=" password
", birthdate=" birthdate "]";
}
public static void main(String[] args) throws CloneNotSupportedException {
User u1 = new User("Kent", "123456", new Date());
User u2 = u1;
User u3 = (User) u1.clone();
System.out.println(u1 == u2); // true
System.out.println(u1.equals(u2)); // true
System.out.println(u1 == u3); // false
System.out.println(u1.equals(u3)); // true
}
}u1和u2指向同一个引用，而u1和u3只是值相同。注意重写Object的clone()方法时需要实现接口，否则报错，说明必须实现java.lang.Cloneable接口：
代码语言：javascript复制Exception in thread "main" java.lang.CloneNotSupportedException: PUser
at java.lang.Object.clone(Native Method)
at PUser.clone(PUser.java:53)
at PUser.main(PUser.java:61)对象实现Cloneable和Serializable接口实现对象深度克隆
先来看下如何简单地实现Serializable接口：
代码语言：javascript复制import java.io.Serializable;
public class Student implements Serializable {
// getter
// setter
/**
*
*/
private static final long serialVersionUID = -2467119047141875151L;
}一般地对象克隆可能存在问题，来看下如下代码：
代码语言：javascript复制import java.util.Date;
public class PUser implements Cloneable {
private User user;
private Boolean editable;
public PUser(User user, Boolean editable) {
this.user = user;
this.editable = editable;
}
/**
* @return the user
*/
public User getUser() {
return user;
}
/**
* @param user
* the user to set
*/
public void setUser(User user) {
this.user = user;
}
/**
* @return the editable
*/
public Boolean getEditable() {
return editable;
}
/**
* @param editable
* the editable to set
*/
public void setEditable(Boolean editable) {
this.editable = editable;
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#clone()
*/
@Override
protected Object clone() throws CloneNotSupportedException {
// TODO Auto-generated method stub
return super.clone();
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#equals(java.lang.Object)
*/
public boolean equals(PUser pUser) {
// TODO Auto-generated method stub
return toString().equals(pUser.toString());
}
/*
* (non-Javadoc)
*
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
return "PUser [user=" user ", editable=" editable "]";
}
public static void main(String[] args) throws CloneNotSupportedException {
PUser a1 = new PUser(new User("Kent", "123456", new Date()), true);
PUser a2 = a1;
PUser a3 = (PUser) a1.clone();
System.out.println(a1 == a2); // true
System.out.println(a1.equals(a2)); // true
System.out.println(a1 == a3); // false
System.out.println(a1.equals(a3)); // true
System.out.println(a1.getUser() == a3.getUser()); // true ! It's not our expected!!!!!
System.out.println(a1.getUser().equals(a3.getUser())); // true
}
}a1和a3获取用户的比较应该是false才符合预期，这就是对象浅层复制的问题。实现对象的深度克隆需要借助java.io.Serializable接口。
下面来看一个序列化的测试：
代码语言：javascript复制import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
public class DeepClone implements Serializable {
// 序列化 ID 在 Eclipse 下提供了两种生成策略
// 一个是固定的 1L，一个是随机生成一个不重复的 long 类型数据（实际上是使用 JDK 工具生成）
// 如果没有特殊需求，就是用默认的 1L 就可以
static final long serialVersionUID = 1L;
String name;
int num;
static int staticNum;
// transient关键字是不能被虚拟机默认序列化的，如果想序列化需要重写private void
// writeObject(ObjectOutputStream s)和private void
// readObject(ObjectInputStream s)
transient String pwd;
transient int num0;
DeepClone(String name, int num, int staticNum, String pwd, int num0) {
this.name = name;
this.num = num;
this.staticNum = staticNum;
this.pwd = pwd;
this.num0 = num0;
}
public String toString() {
return "name=" name ",num=" num ",staticNum=" staticNum
",pwd=" pwd ",num0=" num0;
}
public static void main(String[] args) {
ObjectOutputStream objectos = null;
DeepClone old = new DeepClone("str", 12, 1, "123456", 8);
try {
objectos = new ObjectOutputStream(new FileOutputStream("test.dat"));
objectos.writeObject(old);
objectos.flush();
objectos.close();
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
ObjectInputStream objectin;
DeepClone mts = null;
try {
objectin = new ObjectInputStream(new FileInputStream("test.dat"));
mts = (DeepClone) objectin.readObject();
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println(old);
System.out.println(mts);
System.out.println(old==mts);
// 注意，没有覆盖equals()方法，Object类的equals()默认是地址的比较
System.out.println(mts.equals(old));
}
}输出：
代码语言：javascript复制name=str,num=12,staticNum=1,pwd=123456,num0=8
name=str,num=12,staticNum=1,pwd=null,num0=0
false
false上面基于对象流的操作很好地实现了对象的序列化处理。将基于序列化的流操作提出为一个工具类：
代码语言：javascript复制import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
public class CloneUtil {
@SuppressWarnings("unchecked")
public static <T> T cloneTo(T src) throws RuntimeException {
ByteArrayOutputStream memoryBuffer = new ByteArrayOutputStream();
ObjectOutputStream out = null;
ObjectInputStream in = null;
T dist = null;
try {
out = new ObjectOutputStream(memoryBuffer);
out.writeObject(src);
out.flush();
in = new ObjectInputStream(new ByteArrayInputStream(
memoryBuffer.toByteArray()));
dist = (T) in.readObject();
} catch (Exception e) {
throw new RuntimeException(e);
} finally {
if (out != null)
try {
out.close();
out = null;
} catch (IOException e) {
throw new RuntimeException(e);
}
if (in != null)
try {
in.close();
in = null;
} catch (IOException e) {
throw new RuntimeException(e);
}
}
return dist;
}
}此时我们在通过工具类去复制对象–User、PUser都需要实现Serilizable接口：
代码语言：javascript复制import java.io.Serializable;
import java.util.Date;
public class User implements Cloneable,Serializable{
/**
*
*/
private static final long serialVersionUID = -7040740302952460639L;
private String username;
private String password;
private Date birthdate;
public User(String username, String password, Date birthdate) {
super();
this.username = username;
this.password = password;
this.birthdate = birthdate;
}
/**
* @return the username
*/
public String getUsername() {
return username;
}
/**
* @param username the username to set
*/
public void setUsername(String username) {
this.username = username;
}
/**
* @return the password
*/
public String getPassword() {
return password;
}
/**
* @param password the password to set
*/
public void setPassword(String password) {
this.password = password;
}
/**
* @return the birthdate
*/
public Date getBirthdate() {
return birthdate;
}
/**
* @param birthdate the birthdate to set
*/
public void setBirthdate(Date birthdate) {
this.birthdate = birthdate;
}
/* (non-Javadoc)
* @see java.lang.Object#clone()
*/
@Override
protected Object clone() throws CloneNotSupportedException {
// TODO Auto-generated method stub
return super.clone();
}
/* (non-Javadoc)
* @see java.lang.Object#equals(java.lang.Object)
*/
public boolean equals(User user) {
return toString().equals(user.toString());
}
/* (non-Javadoc)
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
return "User [username=" username ", password=" password
", birthdate=" birthdate "]";
}
public static void main(String[] args) throws CloneNotSupportedException {
User u1 = new User("Kent", "123456", new Date());
User u2 = u1;
User u3 = (User) u1.clone();
System.out.println(u1 == u2); // true
System.out.println(u1.equals(u2)); // true
System.out.println(u1 == u3); // false It's our expected!!!
System.out.println(u1.equals(u3)); // true
}
}输出：
代码语言：javascript复制true
true
false
true
false
true这样就符合我们的预期了。
参考资料
Java对象克隆（Clone）及Cloneable接口、Serializable接口的深入探讨
Java中的Serializable接口transient关键字，及字节、字符、对象IO
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