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Standard library for the Scala Programming Language
There is a newer version: 2.11.2
/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala.sys

import scala.collection.{ mutable, Iterator }
import scala.collection.JavaConverters._
import java.security.AccessControlException
import scala.language.implicitConversions


/** A bidirectional map wrapping the java System properties.
 *  Changes to System properties will be immediately visible in the map,
 *  and modifications made to the map will be immediately applied to the
 *  System properties.  If a security manager is in place which prevents
 *  the properties from being read or written, the AccessControlException
 *  will be caught and discarded.
 *
 *  @author Paul Phillips
 *  @version 2.9
 *  @since   2.9
 */
class SystemProperties
extends mutable.AbstractMap[String, String]
   with mutable.Map[String, String] {

  override def empty = new SystemProperties
  override def default(key: String): String = null

  def iterator: Iterator[(String, String)] =
    wrapAccess(System.getProperties().asScala.iterator) getOrElse Iterator.empty
  def get(key: String) =
    wrapAccess(Option(System.getProperty(key))) flatMap (x => x)
  override def contains(key: String) =
    wrapAccess(super.contains(key)) exists (x => x)

  def -= (key: String): this.type = { wrapAccess(System.clearProperty(key)) ; this }
  def += (kv: (String, String)): this.type = { wrapAccess(System.setProperty(kv._1, kv._2)) ; this }

  def wrapAccess[T](body: => T): Option[T] =
    try Some(body) catch { case _: AccessControlException => None }
}

/** The values in SystemProperties can be used to access and manipulate
 *  designated system properties.  See `scala.sys.Prop` for particulars.
 *  @example {{{
 *    if (!headless.isSet) headless.enable()
 *  }}}
 */
object SystemProperties {
  /** An unenforceable, advisory only place to do some synchronization when
   *  mutating system properties.
   */
  def exclusively[T](body: => T) = this synchronized body

  implicit def systemPropertiesToCompanion(p: SystemProperties): SystemProperties.type = this
  private lazy val propertyHelp = mutable.Map[String, String]()
  private def addHelp[P <: Prop[_]](p: P, helpText: String): P = {
    propertyHelp(p.key) = helpText
    p
  }
  private def str(key: String, helpText: String) = addHelp(Prop[String](key), helpText)
  private def bool(key: String, helpText: String): BooleanProp = addHelp[BooleanProp](
    if (key startsWith "java.") BooleanProp.valueIsTrue(key) else BooleanProp.keyExists(key),
    helpText
  )
  def help(key: String) = propertyHelp.getOrElse(key, "")

  // Todo: bring some sanity to the intersection of system properties aka "mutable
  // state shared by everyone and everything" and the reality that there is no other
  // mechanism for accomplishing some things on the jvm.
  lazy val headless            = bool("java.awt.headless", "system should not utilize a display device")
  lazy val preferIPv4Stack     = bool("java.net.preferIPv4Stack", "system should prefer IPv4 sockets")
  lazy val preferIPv6Addresses = bool("java.net.preferIPv6Addresses", "system should prefer IPv6 addresses")
  lazy val noTraceSupression   = bool("scala.control.noTraceSuppression", "scala should not suppress any stack trace creation")
}