3. @serializable final class Symbol private (val name: String) { override deftoString(): String = "'" + name @throws(classOf[java.io.ObjectStreamException]) private defreadResolve(): Any = Symbol.apply(name) } object Symbol extends UniquenessCache[String, Symbol] { protected defvalueFromKey(name: String): Symbol = new Symbol(name) protected defkeyFromValue(sym: Symbol): Option[String] = Some(sym.name) }
4. /** 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 valrwl = new ReentrantReadWriteLock() private valrlock = rwl.readLock private valwlock = rwl.writeLock private val map = new WeakHashMap[K, WeakReference[V]] protected defvalueFromKey(k: K): V protected defkeyFromValue(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 } defupdateCache(): V = { wlock.lock try { val res = cached() if (res != null) res else { valsym = valueFromKey(name) map.put(name, new WeakReference(sym)) sym } } finally wlock.unlock } val res = cached() if (res == null) updateCache() else res } defunapply(other: V): Option[K] = keyFromValue(other) }