java - Java 中与 Socket 一起使用时可序列化的外部类

Question

这里我有两个类 ClientToSendCalculable.java

    import java.io.*;
import java.net.*;

public class ClientToSendCalculable implements Serializable
{
    public ClientToSendCalculable(int port)
    {
        try
        (
            Socket s = new Socket("localhost", port);
            ObjectOutputStream writer = new ObjectOutputStream(s.getOutputStream());
            ObjectInputStream reader = new ObjectInputStream(s.getInputStream());
        )
        {
            System.out.println("Connection was established!");

            ClientToSendCalculable.Addition addition = new ClientToSendCalculable.Addition();
            addition.firstAddendum = 5;
            addition.secondAddendum = 7;

            writer.writeObject(addition);
            double res = (Double)reader.readObject();

            System.out.println(res);
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }

    private class Addition implements Calculable, Serializable
    {
        private double firstAddendum, secondAddendum;

        public double calc()
        {
            return firstAddendum + secondAddendum;
        }
    }

    public static void main(String[] args)
    {
        new ClientToSendCalculable(7777);
    }
}

和 ServerToGetCalculable.java

import java.io.*;
import java.net.*;

public class ServerToGetCalculable
{
    public ServerToGetCalculable(int port)
    {
        try
         (
             ServerSocket ss = new ServerSocket(port);
             Socket s = ss.accept();
             ObjectOutputStream writer = new ObjectOutputStream(s.getOutputStream());
             ObjectInputStream reader = new ObjectInputStream(s.getInputStream());
         )
        {
            System.out.println("Got a client!");

            Calculable calcObj = (Calculable)reader.readObject();
            double res = calcObj.calc();

            writer.writeObject(res);
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }

    public static void main(String[] args)
    {new ServerToGetCalculable(7777);}
}

我的问题是为什么 ClientToSendCalculable.java 应该实现 Serialized？我仅通过 Socket 发送其内部类 Addition.java，但我不单独发送 ClientToSendCalculable.java。这就是为什么在我看来，只要 Addition.java 是可序列化的就足够了，但如果我使 ClientToSendCalculable.java 不可序列化，我会在尝试执行这行代码时遇到异常:

writer.writeObject(addition);

Answer 1

Addition.java 是一个内部类(非静态) -> 它包含一个包含对封闭 ClientToSendCalculable 实例的引用的字段 -> 外部类的该字段必须被序列化 -> 它必须实现 Serialized 接口(interface)。