Scala example source code file (rwlock.scala)

This example Scala source code file (rwlock.scala) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Scala tags/keywords

lock, lock, notifyall, reading, reading, readwritelock, readwritelock, readwritelock1, readwritelock3, readwritelock4, readwritelock5, signal, signal, writing

The Scala rwlock.scala source code

package examples.pilib

/**
* From Pi to Scala: Semaphores, monitors, read/write locks.
* Readers/writers locks.
*/
object rwlock {

  import scala.concurrent.pilib._

  class Signal extends Chan[unit] {
    def send = write(())
    def receive = read
  }

  class CountLock {
    private val busy = new Signal
    def get = busy.send
    def release = busy.receive
    spawn < release >
  }

  /** A binary semaphore
  */
  class Lock {
    private val busy = new Signal;
    private val free = new Signal;
    def get = busy.send;
    def release = free.send;
    spawn < (while (true) {
      choice (
        busy * (x => free.receive),
        free * (x => ())
      )
    }) >
  }

  /** A monitor a la Java
  */
  class JavaMonitor {

    private val lock = new Lock

    private var waiting: List[Signal] = Nil

    def Wait = {
      val s = new Signal
      waiting = s :: waiting
      lock.release
      s.receive
      lock.get
    }

    def Notify =
      if (!waiting.isEmpty) {
        waiting.head.send
        waiting = waiting.tail
      }

    def NotifyAll =
      while (!waiting.isEmpty) {
        waiting.head.send
        waiting = waiting.tail
      }

    def await(cond: => boolean): unit =
      while (false == cond) (Wait)
  }

  /*
  class Buffer[a](size: Int) extends JavaMonitor with {
    var in = 0, out = 0, n = 0;
    val elems = new Array[a](size);
    def put(x: a) = synchronized {
      await(n < size);
      elems(out) = x;
      out = (out + 1) % size;
    }
    def get: a = synchronized {
      await(n > 0);
      val x = elems(in);
      in = (in + 1) % size;
      x
    }
  }
  */

  /** A readers/writers lock. */
  trait ReadWriteLock {
    def startRead: unit
    def startWrite: unit
    def endRead: unit
    def endWrite: unit
  }

  /**
  * A readers/writers lock, using monitor abstractions.
  */
  class ReadWriteLock1 extends JavaMonitor with ReadWriteLock {

    private var nactive: int = 0
    private var nwriters: int = 0

    def status =
      System.out.println(nactive + " active, " + nwriters + " writers");

    def startRead = synchronized {
      await(nwriters == 0)
      nactive = nactive + 1
      status
    }

    def startWrite = synchronized {
      nwriters = nwriters + 1
      await(nactive == 0)
      nactive = 1
      status
    }

    def endRead = synchronized {
      nactive = nactive - 1
      if (nactive == 0) NotifyAll
      status
    }

    def endWrite = synchronized {
      nwriters = nwriters - 1
      nactive = 0
      NotifyAll
      status
    }
  }

  /** A readers/writers lock, using semaphores
   */
  class ReadWriteLock2 extends ReadWriteLock {

    private var rc: int = 0  // reading readers
    private var wc: int = 0  // writing writers
    private var rwc: int = 0 // waiting readers
    private var wwc: int = 0 // waiting writers
    private val mutex = new Lock
    private val rsem = new Lock
    private val wsem = new Lock

    def startRead = {
      mutex.get;
      if (wwc > 0 || wc > 0) {
        rwc = rwc + 1;
        mutex.release;
        rsem.get;
        rwc = rwc - 1
      }
      rc = rc + 1;
      if (rwc > 0) rsem.release;
      mutex.release
    }

    def startWrite = {
      mutex.get;
      if (rc > 0 || wc > 0) {
        wwc = wwc + 1;
        mutex.release;
        wsem.get;
        wwc = wwc - 1
      }
      wc = wc + 1;
      mutex.release
    }

    def endRead = {
      mutex.get;
      rc = rc - 1;
      if (rc == 0 && wwc > 0) wsem.release;
      mutex.release
    }

    def endWrite = {
      mutex.get;
      wc = wc - 1;
      if (rwc > 0)
        rsem.release
      else if (wwc > 0) wsem.release;
      mutex.release
    }
  }

  /** A readers/writers lock, using channels, without priortities
  */
  class ReadWriteLock3 extends ReadWriteLock {

    private val sr = new Signal
    private val er = new Signal
    private val sw = new Signal
    private val ew = new Signal

    def startRead = sr.send
    def startWrite = sw.send
    def endRead = er.send
    def endWrite = ew.send

    private def rwlock: unit = choice (
      sr * (x => reading(1)),
      sw * (x => { ew.receive; rwlock })
    )

    private def reading(n: int): unit = choice (
      sr * (x => reading(n+1)),
      er * (x => if (n == 1) rwlock else reading(n-1))
    )

    spawn < rwlock >
  }

  /** Same, with sequencing
  */
  class ReadWriteLock4 extends ReadWriteLock {

    private val rwlock = new ReadWriteLock3

    private val sr = new Signal
    private val ww = new Signal
    private val sw = new Signal

    def startRead = sr.send
    def startWrite = { ww.send; sw.send }
    def endRead = rwlock.endRead
    def endWrite = rwlock.endWrite

    private def queue: unit = choice (
      sr * (x => { rwlock.startRead ; queue }),
      ww * (x => { rwlock.startWrite; sw.receive; queue })
    )

    spawn < queue >;
  }

  /** Readwritelock where writers always have priority over readers
  */
  class ReadWriteLock5 extends ReadWriteLock {

    private val sr = new Signal
    private val er = new Signal
    private val ww = new Signal
    private val sw = new Signal
    private val ew = new Signal

    def startRead = sr.send
    def startWrite = { ww.send; sw.send }
    def endRead = er.send
    def endWrite = ew.send

    private def Reading(nr: int, nw: int): unit = 
      if (nr == 0 && nw == 0)
        choice (
          sr * (x => Reading(1, 0)),
          ww * (x => Reading(0, 1))
        )
      else if (nr == 0 && nw != 0) {
        sw.receive;
        Writing(nw);
      }
      else if (nr != 0 && nw == 0)
        choice (
          sr * (x => Reading(nr + 1, 0)),
          er * (x => Reading(nr - 1, 0)),
          ww * (x => Reading(nr, 1))        
        )
      else if (nr != 0 && nw != 0)
        choice (
          ww * (x => Reading(nr, nw + 1)),
          er * (x => Reading(nr - 1, nw))
        );

    private def Writing(nw: int): unit  = choice (
      ew * (x => Reading(0, nw - 1)),
      ww * (x => Writing(nw + 1))
    );

    spawn < Reading(0, 0) >;

  }

  /**
   * Main function.
   */
  def main(args: Array[String]): unit = {
    val random = new java.util.Random()

    def reader(i: int, rwlock: ReadWriteLock): unit = {
      Thread.sleep(1 + random.nextInt(100))
      System.err.println("Reader " + i + " wants to read.")
      rwlock.startRead
      System.err.println("Reader " + i + " is reading.")
      Thread.sleep(1 + random.nextInt(100))
      rwlock.endRead
      System.err.println("Reader " + i + " has read.")
      reader(i, rwlock)
    }

    def writer(i: int, rwlock: ReadWriteLock): unit = {
      Thread.sleep(1 + random.nextInt(100))
      System.err.println("Writer " + i + " wants to write.")
      rwlock.startWrite
      System.err.println("Writer " + i + " is writing.")
      Thread.sleep(1 + random.nextInt(100))
      rwlock.endWrite
      System.err.println("Writer " + i + " has written.")
      writer(i, rwlock)
    }

    val n = try { Integer.parseInt(args(0)) } catch { case _ => 0 }
    if (n < 1 || 5 < n) {
      Console.println("Usage: scala examples.pilib.rwlock <n> (n=1..5)")
      exit
    }
    val rwlock = n match {
      case 1 => new ReadWriteLock1
      case 2 => new ReadWriteLock2
      case 3 => new ReadWriteLock3
      case 4 => new ReadWriteLock4
      case 5 => new ReadWriteLock5
    }
    List.range(0, 5) foreach (i => spawn < reader(i, rwlock) >)
    List.range(0, 5) foreach (i => spawn < writer(i, rwlock) >)
  }

}