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Scala example source code file (SyncVar.scala)

This example Scala source code file (SyncVar.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

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

The Scala SyncVar.scala source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2011, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala.concurrent

/** 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: A = _

  def get = synchronized {
    while (!isDefined) wait()
    value
  }
  
  /** Waits `timeout` millis. If `timeout <= 0` just returns 0. If the system clock
   *  went backward, it will return 0, so it never returns negative results.
   */
  private def waitMeasuringElapsed(timeout: Long): Long = if (timeout <= 0) 0 else {
    val start = System.currentTimeMillis
    wait(timeout)
    val elapsed = System.currentTimeMillis - start
    if (elapsed < 0) 0 else 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
    }
    if (isDefined) Some(value)
    else None
  }
  
  def take() = synchronized {
    try get
    finally unset()
  }

  def set(x: A) = synchronized {
    value = x
    isDefined = true
    notifyAll()
  }

  def put(x: A) = synchronized {
    while (isDefined) wait()
    set(x)
  }
  
  def isSet: Boolean = synchronized {
    isDefined
  }

  def unset(): Unit = synchronized {
    isDefined = false
    notifyAll()
  }
}

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