HashMap with weak values

Question

I\'m implementing a cache for Objects stored persistently. The idea is:

• Method getObjectFromPersistence(long id); ///Takes about 3 seconds
• M

Answer 1

I had the need to store tagged weak objects and figured instead of using WeakHashMap, I could just use WeakHashMap instead.

This is Kotlin, but should apply to Java equally:

abstract class InstanceFactory {
    @Volatile
    private var instances: MutableMap = WeakHashMap()

    protected fun getOrCreate(tag: String = SINGLETON, creator: () -> T): T =
        findByTag(tag)?.let {
            it
        } ?: synchronized(this) {
            findByTag(tag)?.let {
                it
            } ?: run {
                creator().also {
                    instances[it] = tag
                }
            }
        }

    private fun findByTag(tag: String): T? = instances.entries.find { it.value == tag }?.key

    companion object {
        const val SINGLETON = "singleton"
    }
}

This can be used as follows:

class Thing(private val dependency: Dep) { ... }

class ThingFactory(private val dependency: Dep) : InstanceFactory() {

    createInstance(tag: String): Thing = getOrCreate(tag) { Thing(dependency) }

}

Simple singletons can be done like this:

object ThingFactory {
    getInstance(dependency: Dependency): Thing = getOrCreate { Thing(dependency) }
}