how to add feedback between client and server in Kotlin? - kotlin

Question

I am writing a simple client - server in Kotlin. There was a problem sending data from the server to the client.
Server: without the "writer" everything works fine and the message comes from the client
Client: without "reader" everything works fine and the server accepts the message.
As soon as I try to add "feedback" to the server and client, everything immediately stops working.
I understand that streams need to be closed, but I cannot understand at what point exactly. I tried many options - it does not work
Server:
import java.net.ServerSocket
import java.util.*
fun main() {
val server = ServerSocket(9000)
val client = server.accept()
val reader = Scanner(client.getInputStream())
val writer = client.getOutputStream()
print(reader.nextLine())
writer.write("Yes".toByteArray())
writer.flush()
}
Client
import java.net.Socket
import java.util.*
fun main() {
val client = Socket("192.168.1.8", 9000)
var writer = client.getOutputStream()
var reader = Scanner(client.getInputStream())
writer.write("Hi".toByteArray())
writer.flush()
print(reader.nextLine())
}

Answer 1

It looks like you're missing newlines in both messages, so the calls to reader.nextLine() never return, because the 'next line' hasn't been completely received. When using nextLine() you need to think of the communication protocol as "newline delimited messages".
Note that this also means you can't have newlines in your messages. If you need more complex messages you may want to consider base 64 encoding the message content.
Working with low level socket protocols can be very challenging, using a higher level protocol like HTTP adds complexity but can also be simpler in some ways.
If you need to use sockets you're likely better off reading a byte at a time and implementing a state machine to process bytes to output, though this will be much more complex than calling nextLine().
The following code works for me:
Server:
import java.net.ServerSocket
import java.util.*
fun main() {
val server = ServerSocket(9000)
val client = server.accept()
val reader = Scanner(client.getInputStream())
val writer = client.getOutputStream()
print(reader.nextLine())
writer.write("Yes\n".toByteArray()) // <<<<<< Added \n
writer.flush()
}
Client:
import java.net.Socket
import java.util.*
fun main() {
val client = Socket("127.0.0.1", 9000) // << Also changed to localhost IP.
var writer = client.getOutputStream()
var reader = Scanner(client.getInputStream())
writer.write("Hi\n".toByteArray()) // <<<<< Added \n
writer.flush()
print(reader.nextLine())
}