scala.Symbol.scala Maven / Gradle / Ivy
/*
* Scala (https://www.scala-lang.org)
*
* Copyright EPFL and Lightbend, Inc.
*
* Licensed under Apache License 2.0
* (http://www.apache.org/licenses/LICENSE-2.0).
*
* See the NOTICE file distributed with this work for
* additional information regarding copyright ownership.
*/
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 Scala term `'mysym` will
* invoke the constructor of the `Symbol` class in the following way:
* `Symbol("mysym")`.
*/
final class Symbol private (val name: String) extends Serializable {
/** Converts this symbol to a string.
*/
override def toString(): String = "Symbol(" + 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[this] val rwl = new ReentrantReadWriteLock()
private[this] val rlock = rwl.readLock
private[this] val wlock = rwl.writeLock
private[this] 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
// the name field in the Symbol, which can lead to surprising GC
// behavior and duplicate Symbols. See scala/bug#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)
}