服务器和客户端之间的套接字 object 不同

Question

Are there any ways to check if the same Socket is used between the server-side and client-side?

• What I did.

1. Create DateServer.java and DateClient.java
2. Run DateServer.java
3. Run DateClient.java
4. Check both hashCode on the console. The hashCode is different. I thought it was the same.

• DateServer.java

package net;
    
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Date;

public class DateServer {
    
    public static final int LISTENING_PORT = 32007;
    
    public static void main(String[] args) {
        ServerSocket listener;
        Socket connection;
        try {
            listener = new ServerSocket(LISTENING_PORT);
    
            while (true) {
                connection = listener.accept();
                System.out.println(connection.hashCode());
                
                sendDate(connection);
            }
        } catch (Exception e) {
            return;
        }
    }
    
    private static void sendDate(Socket client) {
        try {
            Date now = new Date();
            PrintWriter outgoing = new PrintWriter(client.getOutputStream());
            outgoing.println(now.toString());
            outgoing.flush();
            client.close();
        } catch (Exception e) {
            System.out.println(e);
        }
    }
}

• DateClient.java

package net;
    
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.Socket;
    
public class DateClient {
    
    public static final int LISTENING_PORT = 32007;
    
    public static void main(String[] args) {
    
        String hostName = "localhost";
        Socket connection;
        BufferedReader incoming;
    
        try {
            connection = new Socket(hostName, LISTENING_PORT);
            System.out.println(connection.hashCode());
                
            incoming = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String lineFromServer = incoming.readLine();
            System.out.println();
            System.out.println(lineFromServer);
            System.out.println();
            incoming.close();
        } catch (Exception e) {
            System.out.println(e);
        }
    }
}

• Output on server side.

  1804094807

• Output on client side.

  1826771953

  Sun Mar 07 18:16:05 JST 2021

Answer 1

The hashCode is different. I thought it was the same

This assumption is wrong. And I don't know where it comes from. But hashCode just returns a local identifier associated with a local object.

Answer 2

As already stated by Steffen, the hashcodes of the server-side and client-side sockets is just based on the memory address of the two objects in two different JVMs.

What will be equal is:

• connection.getLocalSocketAddress() of the client
• connection.getRemoteSocketAddres() of the server

and viceversa. That includes the result of the hashCode() and toString() methods, since InetSockeAddress overrides all three equals , hashCode and toString methods.

Answer 3

It depends on hashcode implementation, as you can see in case of Socket the hashcode is unique to the instance that was created within that process.

Please take a look on this example

Hashcode is the same only because It is driven by state of Person, and we had to implement It. However the identity hashcode of person objects differ.

public static void main(String[] args) {
    class Person {
        private int age;
        private int cash;

        public Person(int age, int cash) {
            this.age = age;
            this.cash = cash;
        }

        @Override
        public int hashCode() {
            return (31 * age) + cash;
        }

        public int defaultHashCodeImplementation() {
            return super.hashCode();
        }
    }

    Person p1 = new Person(30, 100);
    System.out.println("p1 hashcode: " + p1.hashCode());
    System.out.println("p1 identity hashcode: " + p1.defaultHashCodeImplementation());

    Person p2 = new Person(30, 100);
    System.out.println("p2 hashcode: " + p2.hashCode());
    System.out.println("p2 identity hashcode: " + p2.defaultHashCodeImplementation());
}

Output

p1 hashcode: 1030
p1 object identity hashcode: 1712669532
p2 hashcode: 1030
p2 object identity hashcode: 1225373914