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Java Virtual Machine Questions

  1. What is strong soft weak and phantom references

What is strong soft weak and phantom references

Going from strongest to weakest, the different levels of memory resilience reflect the life cycle of an object.

strong when there is a reference in scope it will not be garbaged collected.

soft always eligible for garbage collection, it resists GC cycle until memory is available and there is no risk of OutOfMemoryError (in that case, it can be removed).

weak always eligible for garbage collection, it depends on the jvm if retain it or not.

phantom This reference type differs from the other types defined in scala.ref Package because it isn't meant to be used to access the object, but as a signal that the object has already been finalized, and the garbage collector is ready to reclaim its memory.

import scala.ref.SoftReference
import scala.ref.WeakReference
import scala.ref.PhantomReference
import scala.ref.ReferenceQueue

case class A(a: Int)


val strongReference = A(1)
var anotherStrongReference = A(1)


val softReference = SoftReference(A(4))

softReference.get match {
    case Some(A(value)) => println("Enough memory so not garbage collected")
    case None =>  println("Near running out of memory so was garbage collected")
}

val weakReference = WeakReference(A(4))

weakReference.get match {
    case Some(A(value)) => println("Garbage collection has not yet run so still active")
    case None =>  println("Garbage collection ran and has garbage collected")
}


val referenceQueue = new ReferenceQueue()
val phantomReference = new PhantomReference(A(2), referenceQueue)
// Later on another point
val reference = referenceQueue.remove
// Now, clear up any thing you want