一、序列化概念

将对象在内存中的状态保存下来，在需要的时候获取。

• 序列化：将对象转换为字节序列，以便在网络传输或存储。
• 反序列化：将字节序列转换为对象。

二、序列化特点

• 类必须实现Serializable接口，父类未实现Serializable接口则父类不参与序列化，父类实现Serializable接口后子类不需要显式实现Serializable接口；
• 类的静态变量不参与序列化；
• transient关键字修饰的变量不参与序列化；
• 序列化对象的成员变量serialVersionUID和反序列化的对象的成员变量serialVersionUID值必须一致；
• 对象的实例变量引用其他对象，引用的对象也将参与序列化

三、序列化步骤

1. 创建OutputStream；
   OutputStream outputStream = new FileOutputStream("object.txt");
2. 创建ObjectOutputStream；
   ObjectOutputStream objectOutputStream = new ObjectOutputStream(outputStream);
3. 调用ObjectOutputStream的writeObject方法进行序列化；
   objectOutputStream.writeObject(obj);
4. 关闭输出流；
   objectOutputStream.close();

四、反序列化步骤

1. 创建InputStream；
   InputStream inputStream = new FileInputStream("object.txt");
2. 创建ObjectInputStream；
   ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);
3. 调用ObjectInputStream的readObject方法进行反序列化；
   Object obj = objectInputStream.readObject();
4. 关闭输出流；
   objectInputStream.close();

五、序列化实例

5.1 实例一:类必须实现Serializable接口

• Serializable接口是一个标记接口，本身并没有定义方法，但是类必须实现Serializable接口才能进行序列化，否则进行序列化时将抛出NotSerializableException异常。

public interface Serializable {
}

• 类User包含username和password两个成员变量，并未实现Serializable接口。

package com.zzuhkp.javanote.serializable;public class User{private String username;private String password;public User() {System.out.println("User no arg constructor");}public User(String username, String password) {System.out.println("User with arg constructor");this.username = username;this.password = password;}public String getUsername() {return username;}public void setUsername(String username) {this.username = username;}public String getPassword() {return password;}public void setPassword(String password) {this.password = password;}@Overridepublic String toString() {return "User{" +"username='" + username + '\'' +", password='" + password + '\'' +'}';}
}

• 测试类Main对User类中的成员变量赋值，进行序列化后然后反序列化打印User信息；

package com.zzuhkp.javanote.serializable;import java.io.*;public class Main {public static void main(String[] args) throws IOException, ClassNotFoundException {User user = new User("zzuhkp", "123456");writeObj(user);System.out.println(readObj());}public static void writeObj(Object obj) throws IOException {OutputStream outputStream = new FileOutputStream("object.txt");ObjectOutputStream objectOutputStream = new ObjectOutputStream(outputStream);objectOutputStream.writeObject(obj);objectOutputStream.close();}public static Object readObj() throws IOException, ClassNotFoundException {InputStream inputStream = new FileInputStream("object.txt");ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);Object obj = objectInputStream.readObject();objectInputStream.close();return obj;}}

• 程序运行后抛出java.io.NotSerializableException异常，如下所示；

User with arg constructor
Exception in thread "main" java.io.NotSerializableException: com.zzuhkp.javanote.serializable.Userat java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)at com.zzuhkp.javanote.serializable.Main.writeObj(Main.java:16)at com.zzuhkp.javanote.serializable.Main.main(Main.java:9)

5.2 实例二 未实现Serializable的父类不参与序列化

• 创建类CommonVO,其只有一个成员变量id，该类未实现Serializable接口；

package com.zzuhkp.javanote.serializable;public class CommonVO {private String id;public CommonVO(){System.out.println("CommonVO no arg constructor");}public CommonVO(String id) {System.out.println("CommonVO with arg constructor");this.id = id;}public String getId() {return id;}public void setId(String id) {this.id = id;}@Overridepublic String toString() {return "CommonVO{" +"id='" + id + '\'' +'}';}
}

• 使User类继承CommonVO类；

package com.zzuhkp.javanote.serializable;public class User extends CommonVO implements Serializable {//省略代码和实例一相同，未做改变
}

• 修改测试类如下所示：

package com.zzuhkp.javanote.serializable;import java.io.*;public class Main {public static void main(String[] args) throws IOException, ClassNotFoundException {User user = new User("zzuhkp", "123456");user.setId("123");writeObj(user);User user1= (User) readObj();System.out.println(user1.toString());System.out.println("id:"+user1.getId());}//省略代码和实例一相同，未做改变}

• 程序运行结果如下所示，可以看到反序列化并没有把前面设置的父类成员变量id的值打印出来，并且可以看到反序列化时程序调用了User的无参构造方法创建了User的实例；

CommonVO no arg constructor
User with arg constructor
CommonVO no arg constructor
User{username='zzuhkp', password='123456'}
id:null

5.3 实例三 类的静态变量和transient修饰的变量不参与序列化；

• 修改实例一中的User如下所示；

package com.zzuhkp.javanote.serializable;import java.io.Serializable;public class User  implements Serializable {public static String USER_TYPE="1";private transient String password;//省略代码和实例一中的User一致
}

• 修改测试类如下所示；

package com.zzuhkp.javanote.serializable;import java.io.*;public class Main {public static void main(String[] args) throws IOException, ClassNotFoundException {User user = new User("zzuhkp", "123456");writeObj(user);User.USER_TYPE="2";User user1= (User) readObj();System.out.println(user1.toString());System.out.println(User.USER_TYPE);}//省略代码和实例一中的Main类保持一致}

• 序列化后修改User类的静态变量USER_TYPE，程序运行，打印结果如下。反序列化后获取的静态变量的值和序列化之前并不保持一致，说明类的静态变量不参数序列化；transient修饰的password变量在反序列化后并未打印出序列化之前设置的值，说明transient修饰的成员变量也不参与序列化。

User with arg constructor
User{username='zzuhkp', password='null'}
2

5.4 实例四 序列化ID serialVersionUID必须一致

• 修改User类如下所示，注意serialVersionUID的值为1L。

package com.zzuhkp.javanote.serializable;import java.io.Serializable;public class User implements Serializable {private static final long serialVersionUID = 1L;private String username;private String password;public User() {System.out.println("User no arg constructor");}public User(String username, String password) {System.out.println("User with arg constructor");this.username = username;this.password = password;}public String getUsername() {return username;}public void setUsername(String username) {this.username = username;}public String getPassword() {return password;}public void setPassword(String password) {this.password = password;}@Overridepublic String toString() {return "User{" +"username='" + username + '\'' +", password='" + password + '\'' +'}';}
}

• 新建SocketClient类作为Socket连接的客户端，代码如下：

package com.zzuhkp.javanote.serializable;import java.io.*;
import java.net.Socket;public class SocketClient {public static void main(String[] args) throws IOException {Socket socket=new Socket("127.0.0.1",8080);User user = new User("zzuhkp", "123456");OutputStream outputStream = socket.getOutputStream();ObjectOutputStream objectOutputStream = new ObjectOutputStream(outputStream);objectOutputStream.writeObject(user);objectOutputStream.close();}
}

• 新建项目，并在新项目中新建User类,User类的serialVersionUID为2L，其他内容和原项目User保值一致，代码如下；

package com.zzuhkp.javanote.serializable;import java.io.Serializable;public class User  implements Serializable {private static final long serialVersionUID = 2L;//省略代码和实例四上面User内容一致
}

• 在新项目中新建SocketServer类作为Socket的服务端，代码如下：

package com.zzuhkp.javanote.serializable;import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;public class SocketServer {public static void main(String[] args) throws IOException, ClassNotFoundException {ServerSocket serverSocket=new ServerSocket(8080);Socket socket=serverSocket.accept();InputStream inputStream = socket.getInputStream();ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);Object obj = objectInputStream.readObject();objectInputStream.close();System.out.println(obj.toString());}
}

• 分别运行SocketServer和SocketClient，SocketServer程序运行报InvalidClassException异常，并提示 stream classdesc serialVersionUID = 1, local class serialVersionUID = 2，说明serialVersionUID必须保持一致才能序列化成功。

Exception in thread "main" java.io.InvalidClassException: com.zzuhkp.javanote.serializable.User; local class incompatible: stream classdesc serialVersionUID = 1, local class serialVersionUID = 2at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:687)at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1883)at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1749)at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2040)at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1571)at java.io.ObjectInputStream.readObject(ObjectInputStream.java:431)at com.zzuhkp.javanote.serializable.SocketServer.main(SocketServer.java:15)

六、序列化对象的writeObject方法和readObject方法

• 序列化时ObjectOutputStream将通过反射获取并尝试调用序列化对象的private void writeObject(ObjectOutputStream objectOutputStream)方法进行序列化，如果不存在该方法则ObjectOutputStream使用默认的序列化方式进行序列化。在writeObject方法中可以先调用objectOutputStream.defaultWriteObject()方法进行默认的序列化，然后调用ObjectOutputStream类的其他方法序列化；
• 反序列化时ObjectInputStream将通过反射获取并尝试调用序列化对象的private void readObject(ObjectInputStream objectInputStream)方法进行反序列化，如果不存在该方法则ObjectInputStream使用默认的反序列化方式进行反序列化。在readObject中可以先调用objectInputStream.defaultReadObject()方法进行默认的反序列化，然后调用ObjectInputStream类的其他方法反序列化；
• 修改User类如下所示：

package com.zzuhkp.javanote.serializable;import java.io.*;public class User implements Serializable {private static final long serialVersionUID = 1L;private String username;private transient String password;private void writeObject(ObjectOutputStream objectOutputStream) throws IOException, ClassNotFoundException {objectOutputStream.defaultWriteObject();objectOutputStream.writeObject(this.password);}private void readObject(ObjectInputStream objectInputStream) throws IOException, ClassNotFoundException {objectInputStream.defaultReadObject();this.password = (String) objectInputStream.readObject();}//省略代码和实例一中的User类代码相同
}

• 修改实例一中的Main类如下所示：

package com.zzuhkp.javanote.serializable;import java.io.*;public class Main {public static void main(String[] args) throws IOException, ClassNotFoundException {User user = new User("zzuhkp", "123456");writeObj(user);User user1= (User) readObj();System.out.println(user1.toString());}//省略代码和实例一中的Main类代码一致
}

• 运行结果如下，说明使用自定义的序列化方式对transient修饰的成员变量序列化已经生效。

User with arg constructor
User{username='zzuhkp', password='123456'}

七、序列化接口Externalizable

Externalizable接口继承Serializable接口，并声明了两个方法用于分别对对象进行序列化和反序列化，查看Externalizable的主要源码如下：

package java.io;import java.io.ObjectOutput;
import java.io.ObjectInput;public interface Externalizable extends java.io.Serializable {void writeExternal(ObjectOutput out) throws IOException;void readExternal(ObjectInput in) throws IOException, ClassNotFoundException;
}

一个类实现Externalizable接口，该类即可参与序列化，此时需要我们自己调用writeExternal方法进行序列化，调用readExternal方法进行反序列化，并且实现Serializable接口进行的默认序列化方式不会生效。

修改实例一中的User类如下：

package com.zzuhkp.javanote.serializable;import java.io.*;public class User implements Externalizable {private static final long serialVersionUID = 1L;private String username;private String password;@Overridepublic void writeExternal(ObjectOutput out) throws IOException {out.writeObject(this.username);}@Overridepublic void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {this.username = (String) in.readObject();}//省略代码和实例一中User的代码一致}

修改Main类的代码如下：

package com.zzuhkp.javanote.serializable;import java.io.*;public class Main {public static void main(String[] args) throws IOException, ClassNotFoundException {User user = new User("zzuhkp", "123456");writeObj(user);User user1= (User) readObj();System.out.println(user1.toString());}//省略代码和实例一中Main方法的代码一致}

在User类中只对成员变量username进行序列化和反序列化，运行结果如下所示,说明我们自定义的序列化和反序列化已经生效：

User with arg constructor
User no arg constructor
User{username='zzuhkp', password='null'}

八、序列化对象的writeReplace和readResolve方法

• 如果序列化对象包含Object writeReplace() throws ObjectStreamException方法，在序列化时将使用writeReplace方法的返回值作为序列化的对象替代原序列化对象；
• 如果序列化对象包含Object readResolve() throws ObjectStreamException方法，在反序列化时将使用readResolve方法的返回值作为反序列化的对象替代原反序列化对象。

修改实例一中的User类如下所示，注意添加了writeReplace()和 readResolve()方法。

package com.zzuhkp.javanote.serializable;import java.io.*;public class User implements Serializable {private static final long serialVersionUID = 1L;private String username;private String password;private Object writeReplace() throws ObjectStreamException{System.out.println("writeReplace() is called");User user=new User("writeReplace Name","123");return user;}private Object readResolve() throws ObjectStreamException{System.out.println("readResolve() is called");User user=new User("readResolve Name","456");return user;}public User() {System.out.println("User no arg constructor");}public User(String username, String password) {System.out.println("User with arg constructor");this.username = username;this.password = password;}public String getUsername() {return username;}public void setUsername(String username) {this.username = username;}public String getPassword() {return password;}public void setPassword(String password) {this.password = password;}@Overridepublic String toString() {return "User{" +"username='" + username + '\'' +", password='" + password + '\'' +'}';}}

测试类Main类代码如下:

package com.zzuhkp.javanote.serializable;import java.io.*;public class Main {public static void main(String[] args) throws IOException, ClassNotFoundException {User user = new User("zzuhkp", "123456");writeObj(user);User user1= (User) readObj();System.out.println(user1.toString());}public static void writeObj(Object obj) throws IOException {OutputStream outputStream = new FileOutputStream("object.txt");ObjectOutputStream objectOutputStream = new ObjectOutputStream(outputStream);objectOutputStream.writeObject(obj);objectOutputStream.close();}public static Object readObj() throws IOException, ClassNotFoundException {InputStream inputStream = new FileInputStream("object.txt");ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);Object obj = objectInputStream.readObject();objectInputStream.close();return obj;}}

运行结果如下，说明反序列化的对象已经被readResolve()方法的返回值替代。

User with arg constructor
writeReplace() is called
User with arg constructor
readResolve() is called
User with arg constructor
User{username='readResolve Name', password='456'}