Question

Edit: I was able to fix it by making changing val to lazy val in the MessageParser class. I forgot that I had previously tested it using def instead of val. Can someone make it clear why this change fixes it?

So, I am currently writing an IRC Server. I decided to use Scala’s Combinator Parser library to help me parse the messages. I’ve been able to correctly parse a message through a test program, but when I attempted to incorporate my parser into an echo server I already wrote I receive the following error message when I make a connection to my server:

Connected to the target VM, address: '127.0.0.1:55567', transport: 'socket'
Exception in thread "main" java.lang.ExceptionInInitializerError
    at IRCServer.main(IRCServer.scala)
Caused by: java.lang.NullPointerException
    at messages.MessageParser.<init>(MessageParser.scala:11)
    at net.Connection.<init>(Connection.scala:14)
    at net.Server.start(Server.scala:14)
    at IRCServer$.<init>(IRCServer.scala:12)
    at IRCServer$.<clinit>(IRCServer.scala)
    ... 1 more
Disconnected from the target VM, address: '127.0.0.1:55567', transport: 'socket'

The Connection class handles a listener Socket created from a ServerSocket

class Connection(socket: Socket) extends Thread {
    private val out = new PrintStream(socket.getOutputStream)
    private val in  = new BufferedReader(new InputStreamReader(socket.getInputStream))
    private val parser = new MessageParser
    override def run(): Unit = {
        var line = ""
        while({(line = in.readLine); line != null}) {
            Console.println("received: " + line)
            parser.parseLine(line.trim)
            out.println("out: " + line)
        }
    }

}

And the following is my MessageParser:

class MessageParser extends JavaTokenParsers {
    def parseLine(line :CharSequence) = {
        parseAll(message, line)
    }

    val message: Parser[Any] = opt(":"~prefix)~command~opt(params) ^^ (x=> {println("message: "+x)})
    val prefix: Parser[Any] = nick~"!"~user~"@"~host | servername ^^ (x=> {println("prefix: " +x)})
    val nick: Parser[Any] = letter~rep(letter | wholeNumber | special) ^^ (x=> {println("nick: " +x)})
    val special: Parser[Any] = "-" | "[" | "]" | "\\" | "`" | "^" | "{" | "}" ^^ (x=> {println("special: " +x)})
    val user: Parser[Any] = """[^\s@]+""".r ^^ (x=> {println("user: " +x)})
    val host: Parser[Any] = """[\w\.]+\w+""".r ^^ (x=> {println("host: " +x)})
    val servername: Parser[Any] = host ^^ (x=> {println("servername: " +x)})
    val command: Parser[Any] = """([A-Za-z]+)|([0-9]{3})""".r ^^ (x=> {println("command: " +x)})
    val params: Parser[Any] = rep(param)~opt(":"~tail) ^^ (x=> {println("params: " +x)})
    val param: Parser[Any] = """[^:][\S]*""".r
    val tail: Parser[Any] = """.*$""".r ^^ (x=> {println("tail: " +x)})
    val letter: Parser[Any] = """[A-Za-z]""".r ^^ (x=> {println("letter: " +x)})
}

I’m not quite sure what could be causing this. Hopefully I’m just being blind to something small.

Answer 1

lazy val values are populated as-needed; val values are populated in the order you specify. With a parser, earlier entries refer to later ones which don’t exist yet. So they’d better be lazy val or def (which one depends on the parser; the packrat parser likes lazy val, while the others usually assume def, but I’m not sure that they require it).