Scala chat application, blocking issue

Question

I'm writing a chat application in Scala, the problem is with the clients, the client reads from StdIn (which blocks) before sending the data to the echo server, so if multiple clients are connected then they don't receive data from the server until reading from StdIn has completed. I'm thinking that local IO, i.e reading from StdIn and reading/writing to the socket should be on separate threads but I can't think of a way to do this, below is the Client singleton code:

import java.net._
import scala.io._
import java.io._
import java.security._

object Client {

  var msgAcc = ""

  def main(args: Array[String]): Unit = {
    val conn = new ClientConnection(InetAddress.getByName(args(0)), args(1).toInt)
    val server = conn.connect()
    println("Enter a username")
    val user = new User(StdIn.readLine())
    println("Welcome to the chat " + user.username)
    sys.addShutdownHook(this.shutdown(conn, server))
    while (true) {
    val txMsg = StdIn.readLine()//should handle with another thread?
    if (txMsg != null) {
      conn.sendMsg(server, user, txMsg)
      val rxMsg = conn.getMsg(server)
      val parser = new JsonParser(rxMsg)
      val formattedMsg = parser.formatMsg(parser.toJson()) 
      println(formattedMsg)
      msgAcc = msgAcc + formattedMsg + "\n"
      }
    }
  }

  def shutdown(conn: ClientConnection, server: Socket): Unit = {
    conn.close(server)
    val fileWriter = new BufferedWriter(new FileWriter(new File("history.txt"), true))
    fileWriter.write(msgAcc) 
    fileWriter.close()
    println("Leaving chat, thanks for using")
  }

}

below is the ClientConnection class:

import javax.net.ssl.SSLSocket
import javax.net.ssl.SSLSocketFactory
import javax.net.SocketFactory
import java.net.Socket
import java.net.InetAddress
import java.net.InetSocketAddress
import java.security._
import java.io._
import scala.io._
import java.util.GregorianCalendar
import java.util.Calendar
import java.util.Date
import com.sun.net.ssl.internal.ssl.Provider
import scala.util.parsing.json._

class ClientConnection(host: InetAddress, port: Int) {

  def connect(): Socket = {
    Security.addProvider(new Provider())
    val sslFactory = SSLSocketFactory.getDefault()
    val sslSocket = sslFactory.createSocket(host, port).asInstanceOf[SSLSocket]
    sslSocket
   }

  def getMsg(server: Socket): String = new BufferedSource(server.getInputStream()).getLines().next()

  def sendMsg(server: Socket, user: User, msg: String): Unit = {
    val out = new PrintStream(server.getOutputStream())
    out.println(this.toMinifiedJson(user.username, msg))
    out.flush()
  }  

  private def toMinifiedJson(user: String, msg: String): String = {
    s"""{"time":"${this.getTime()}","username":"$user","msg":"$msg"}"""
  }

  private def getTime(): String = {
    val cal = Calendar.getInstance()
    cal.setTime(new Date())
    "(" + cal.get(Calendar.HOUR_OF_DAY) + ":" + cal.get(Calendar.MINUTE) + ":" + cal.get(Calendar.SECOND) + ")"
  }

  def close(server: Socket): Unit = server.close()
}

This is the Client singleton using a Thread to read from standard input:

import java.net._
import scala.io._
import java.io._
import java.security._
import java.util.NoSuchElementException

object Client {

  var msgAcc = ""

  def main(args: Array[String]): Unit = {
    val conn = new ClientConnection(InetAddress.getByName(args(0)), args(1).toInt)
    val server = conn.connect()
    println("Enter a username")
    val user = new User(StdIn.readLine())
    println("Welcome to the chat " + user.username)
    sys.addShutdownHook(this.shutdown(conn, server))
    new Thread(conn).start()
    while (true) {
    val tx = conn.tx
    if (tx != null) {
      conn.sendMsg(server, user, tx)
      val rxMsg = conn.getMsg(server)
      val parser = new JsonParser(rxMsg)
      val formattedMsg = parser.formatMsg(parser.toJson()) 
      println(formattedMsg)
      msgAcc = msgAcc + formattedMsg + "\n" 
      }
    }
  }

  def shutdown(conn: ClientConnection, server: Socket): Unit = {
    conn.close(server)
    val fileWriter = new BufferedWriter(new FileWriter(new File("history.txt"), true))
    fileWriter.write(msgAcc) 
    fileWriter.close()

This is the ClientConnection class extending Runnable:

import javax.net.ssl.SSLSocket
import javax.net.ssl.SSLSocketFactory
import javax.net.SocketFactory
import java.net.Socket
import java.net.InetAddress
import java.net.InetSocketAddress
import java.security._
import java.io._
import scala.io._
import java.util.GregorianCalendar
import java.util.Calendar
import java.util.Date
import com.sun.net.ssl.internal.ssl.Provider
import scala.util.parsing.json._

class ClientConnection(host: InetAddress, port: Int) extends Runnable {

  var tx: String = null

  override def run(): Unit = {
     tx = StdIn.readLine()
  }

  def connect(): Socket = {
    Security.addProvider(new Provider())
    val sslFactory = SSLSocketFactory.getDefault()
    val sslSocket = sslFactory.createSocket(host, port).asInstanceOf[SSLSocket]
    sslSocket
   }

  def getMsg(server: Socket): String = new BufferedSource(server.getInputStream()).getLines().next()

  def sendMsg(server: Socket, user: User, msg: String): Unit = {
    val out = new PrintStream(server.getOutputStream())
    out.println(this.toMinifiedJson(user.username, msg))
    out.flush()
  }  

  private def toMinifiedJson(user: String, msg: String): String = {
    s"""{"time":"${this.getTime()}","username":"$user","msg":"$msg"}"""
  }

  private def getTime(): String = {
    val cal = Calendar.getInstance()
    cal.setTime(new Date())
    "(" + cal.get(Calendar.HOUR_OF_DAY) + ":" + cal.get(Calendar.MINUTE) + ":" + cal.get(Calendar.SECOND) + ")"
  }

  def close(server: Socket): Unit = server.close()
}

Answer 1

So you've successfully moved the reading of the input to the Runnable, so it will run on another Thread, but now when we look at the logic on your main thread, we see that it will always send the message if it's not null. There are a few issues to this:

• You're not looping in the run method, so you're only going to receive a single message, and then your run method terminates - you want to wrap this in a while(true) or while(<some boolean indicating you're not done>) so you keep updating it.
• You still are printing out the messages from the server only after sending one to the server. You should decouple this so that sending a message to the server is done on the other thread entirely.

Something along the lines of this might solve it:

//This is your new run method in your Runnable
override def run(): Unit = {
    while(true) {
        tx = StdIn.readLine()
        conn.sendMsg(server, user, tx) //Note you'll need to pass those references in somehow
    }
}`

Then, in your main thread, just deal with getting messages and printing them out:

new Thread(conn).start()
while (true) {
    //note the lack of sending messages in here
    val rxMsg = conn.getMsg(server)
    val parser = new JsonParser(rxMsg)
    val formattedMsg = parser.formatMsg(parser.toJson()) 
    println(formattedMsg)
    msgAcc = msgAcc + formattedMsg + "\n" 
}

This way, the two behaviors are on different threads.