Scala example source code file (Symbol.scala)

This example Scala source code file (Symbol.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

any, k, option, serializable, string, symbol, uniquenesscache, v, weakhashmap, weakreference

The Symbol.scala Scala example source code

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

package scala

/** This class provides a simple way to get unique objects for equal strings.
 *  Since symbols are interned, they can be compared using reference equality.
 *  Instances of `Symbol` can be created easily with Scala's built-in quote
 *  mechanism.
 *
 *  For instance, the [[http://scala-lang.org/#_top Scala]] term `'mysym` will
 *  invoke the constructor of the `Symbol` class in the following way:
 *  `Symbol("mysym")`.
 *
 *  @author  Martin Odersky, Iulian Dragos
 *  @version 1.8
 */
final class Symbol private (val name: String) extends Serializable {
  /** Converts this symbol to a string.
   */
  override def toString(): String = "'" + name

  @throws(classOf[java.io.ObjectStreamException])
  private def readResolve(): Any = Symbol.apply(name)
  override def hashCode = name.hashCode()
  override def equals(other: Any) = this eq other.asInstanceOf[AnyRef]
}

object Symbol extends UniquenessCache[String, Symbol] {
  override def apply(name: String): Symbol = super.apply(name)
  protected def valueFromKey(name: String): Symbol = new Symbol(name)
  protected def keyFromValue(sym: Symbol): Option[String] = Some(sym.name)
}

/** This is private so it won't appear in the library API, but
  * abstracted to offer some hope of reusability.  */
private[scala] abstract class UniquenessCache[K, V >: Null]
{
  import java.lang.ref.WeakReference
  import java.util.WeakHashMap
  import java.util.concurrent.locks.ReentrantReadWriteLock

  private val rwl = new ReentrantReadWriteLock()
  private val rlock = rwl.readLock
  private val wlock = rwl.writeLock
  private val map = new WeakHashMap[K, WeakReference[V]]

  protected def valueFromKey(k: K): V
  protected def keyFromValue(v: V): Option[K]

  def apply(name: K): V = {
    def cached(): V = {
      rlock.lock
      try {
        val reference = map get name
        if (reference == null) null
        else reference.get  // will be null if we were gc-ed
      }
      finally rlock.unlock
    }
    def updateCache(): V = {
      wlock.lock
      try {
        val res = cached()
        if (res != null) res
        else {
          // If we don't remove the old String key from the map, we can
          // wind up with one String as the key and a different String as
          // as the name field in the Symbol, which can lead to surprising
          // GC behavior and duplicate Symbols. See SI-6706.
          map remove name
          val sym = valueFromKey(name)
          map.put(name, new WeakReference(sym))
          sym
        }
      }
      finally wlock.unlock
    }

    val res = cached()
    if (res == null) updateCache()
    else res
  }
  def unapply(other: V): Option[K] = keyFromValue(other)
}