扩展类的Java序列化

Question

在java序列化类Mp3player扩展电子设备实现序列化在这个代码中超级类电子设备是不实现序列化的。

 import java.io.*;
 class ElectronicDevice { 
      ElectronicDevice() 
      {
           System.out.print("ed "); 
      }
  }
 class Mp3player extends ElectronicDevice implements Serializable {
       Mp3player() 
       { 
          System.out.print("mp "); 
       }
 }
class MiniPlayer extends Mp3player {
     MiniPlayer()
     { 
         System.out.print("mini "); 
     }
     public static void main(String[] args) {
          MiniPlayer m = new MiniPlayer();
          try {
                 FileOutputStream fos = new FileOutputStream("dev.txt");
             ObjectOutputStream os = new ObjectOutputStream(fos);
                 os.writeObject(m); os.close();

                 FileInputStream fis = new FileInputStream("dev.txt");
                 ObjectInputStream is = new ObjectInputStream(fis);
                 MiniPlayer m2 = (MiniPlayer) is.readObject(); 
                 is.close();
                 System.out.println();
          } catch (Exception x) {
                System.out.print("x "); 
          }
     }
  }

Answer 1

这是超类序列化的规则：

如果你是一个可序列化的类，但是你的超类是不可序列化的，那么你从该超类中继承的任何实例变量都将被重置为它们在对象的原始构造过程中给出的值。这是因为非序列化的类构造函数将运行。

因此，如果向ElectronicDevice添加一些实例变量，请注意超类的状态将不会序列化。（除非超类实现可序列化）

Answer 2

因为超类没有实现可序列化的内容，所以超类不会序列化。只有子类的内容会序列化。当您反序列化时，超类的默认构造函数将被执行，超类的字段将被初始化，就像您调用默认构造函数一样。

下面的例子说明了这一点。

public class SerializationTest {

    public static class Base {
        private String name;

        public Base() {
            this.name = "johnDow";
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }
    }

    public static class Sub extends Base implements Serializable {
        private static final long serialVersionUID = 1L;
        private String age;

        public String getAge() {
            return age;
        }

        public void setAge(String age) {
            this.age = age;
        }
    }

    public static void main(String[] args) throws Exception {
        ByteArrayOutputStream byteArrayOS = new ByteArrayOutputStream();
        ObjectOutputStream out = new ObjectOutputStream(byteArrayOS);
        Sub s = new Sub();
        s.setName("name");
        s.setAge("10");
        out.writeObject(s);
        ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(byteArrayOS.toByteArray()));
        Sub d = (Sub) ois.readObject();
        System.out.println(d.getName() + "-" + d.getAge());
    }
}

打印出来的是

johnDow-10

Answer 3

否。在序列化过程中，只会写出和恢复可序列化对象的字段。

根据javadocs

在反序列化过程中，非序列化类的字段将使用类的公共或受保护的no-arg构造函数初始化。

其中，可序列化子类的字段将从流中恢复。

请查看此示例
此处电子设备不可序列化，其中asMp3player可序列化。观察序列化过程中受尊重类的字段行为。

import java.io.*;
class ElectronicDevice  { 
  public int i = 0;
  protected ElectronicDevice() 
  {
       System.out.println("ed "); 
  }
}
class Mp3player extends ElectronicDevice implements Serializable {
   int j =0;
   Mp3player() 
   { 
       System.out.println("mp "); 
   }
}
class MiniPlayer extends Mp3player {
  MiniPlayer()
  { 
      System.out.println("mini "); 
  }
 public static void main(String[] args) {
      MiniPlayer m = new MiniPlayer();
      m.i = 30;
      m.j = 40;
      try {
             System.out.println("i value before serialization: "+m.i);//prints 30
             System.out.println("i value before serialization: "+m.j);//prints 40
             FileOutputStream fos = new FileOutputStream("dev.txt");
             ObjectOutputStream os = new ObjectOutputStream(fos);
             os.writeObject(m); os.close();

             FileInputStream fis = new FileInputStream("dev.txt");
             ObjectInputStream is = new ObjectInputStream(fis);
             MiniPlayer m2 = (MiniPlayer) is.readObject(); 
             is.close();
             System.out.println("i value after serialization: "+m2.i);//prints o
             System.out.println("j value after serialization: "+m2.j);//prints 40
             System.out.println();
        } catch (Exception x) {
            x.printStackTrace();
            System.out.print("x "); 
       }
   }
 }