Scala example source code file (SyncVar.scala)

This example Scala source code file (SyncVar.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

a, boolean, concurrent, long, none, option, some, syncvar, unit, use

The SyncVar.scala Scala example source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala.concurrent

import java.util.concurrent.TimeUnit

/** A class to provide safe concurrent access to a mutable cell.
 *  All methods are synchronized.
 *
 *  @author  Martin Odersky
 *  @version 1.0, 10/03/2003
 */
class SyncVar[A] {
  private var isDefined: Boolean = false
  private var value: Option[A] = None

  def get: A = synchronized {
    while (!isDefined) wait()
    value.get
  }

  /** Waits `timeout` millis. If `timeout <= 0` just returns 0.
    * It never returns negative results.
    */
  private def waitMeasuringElapsed(timeout: Long): Long = if (timeout <= 0) 0 else {
    val start = System.nanoTime()
    wait(timeout)
    val elapsed = System.nanoTime() - start
    // nanoTime should be monotonic, but it's not possible to rely on that.
    // See http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6458294.
    if (elapsed < 0) 0 else TimeUnit.NANOSECONDS.toMillis(elapsed)
  }

  /** Waits for this SyncVar to become defined at least for
   *  `timeout` milliseconds (possibly more), and gets its
   *  value.
   *
   *  @param timeout     the amount of milliseconds to wait, 0 means forever
   *  @return            `None` if variable is undefined after `timeout`, `Some(value)` otherwise
   */
  def get(timeout: Long): Option[A] = synchronized {
    /* Defending against the system clock going backward
     * by counting time elapsed directly.  Loop required
     * to deal with spurious wakeups.
     */
    var rest = timeout
    while (!isDefined && rest > 0) {
      val elapsed = waitMeasuringElapsed(rest)
      rest -= elapsed
    }
    value
  }

  /** Waits for this SyncVar to become defined and returns
   *  the result */
  def take(): A = synchronized {
    try get
    finally unsetVal()
  }

  /** Waits for this SyncVar to become defined at least for
   *  `timeout` milliseconds (possibly more), and takes its
   *  value by first reading and then removing the value from
   *  the SyncVar.
   *
   *  @param timeout     the amount of milliseconds to wait, 0 means forever
   *  @return            the value or a throws an exception if the timeout occurs
   *  @throws NoSuchElementException on timeout
   */
  def take(timeout: Long): A = synchronized {
    try get(timeout).get
    finally unsetVal()
  }

  // TODO: this method should be private
  // [Heather] the reason why: it doesn't take into consideration
  // whether or not the SyncVar is already defined. So, set has been
  // deprecated in order to eventually be able to make "setting" private
  @deprecated("Use `put` instead, as `set` is potentionally error-prone", "2.10.0")
  // NOTE: Used by SBT 0.13.0-M2 and below
  def set(x: A): Unit = setVal(x)

  /** Places a value in the SyncVar. If the SyncVar already has a stored value,
   * it waits until another thread takes it */
  def put(x: A): Unit = synchronized {
    while (isDefined) wait()
    setVal(x)
  }

  /** Checks whether a value is stored in the synchronized variable */
  def isSet: Boolean = synchronized {
    isDefined
  }

  // TODO: this method should be private
  // [Heather] the reason why: it doesn't take into consideration
  // whether or not the SyncVar is already defined. So, unset has been
  // deprecated in order to eventually be able to make "unsetting" private
  @deprecated("Use `take` instead, as `unset` is potentionally error-prone", "2.10.0")
  // NOTE: Used by SBT 0.13.0-M2 and below
  def unset(): Unit = synchronized {
    isDefined = false
    value = None
    notifyAll()
  }

  // `setVal` exists so as to retroactively deprecate `set` without
  // deprecation warnings where we use `set` internally. The
  // implementation of `set` was moved to `setVal` to achieve this
  private def setVal(x: A): Unit = synchronized {
    isDefined = true
    value = Some(x)
    notifyAll()
  }

  // `unsetVal` exists so as to retroactively deprecate `unset` without
  // deprecation warnings where we use `unset` internally. The
  // implementation of `unset` was moved to `unsetVal` to achieve this
  private def unsetVal(): Unit = synchronized {
    isDefined = false
    value = None
    notifyAll()
  }

}