Using Java's ReferenceQueue

Question

Do SoftReference and WeakReference really only help when created as instance variables? Is there any benefit to using them in method scope?

Answer 1

One common thing to do is to create maps of soft references.

Map> cache = new HashMap<>();
Set thingsIAmCurrentlyGetting = new HashSet();
Object mutex = new Object();

BigThing getThing(String key) {
  synchronized(mutex) {
    while(thingsIAmCurrentlyGetting.contains(key)) {
      mutex.wait();
    }
    SoftReference ref = cache.get(key);
    BigThing bigThing = ref == null ? null : ref.get();
    if(bigThing != null) return bigThing;
    thingsIAmCurrentlyGetting.add(key);
  }

  BigThing bigThing = getBigThing(key); // this may take a while to run.

  synchronized(mutex) {
    cache.put(key, bigThing);
    thingsIAmCurrentlyGetting.remove(key);
    mutex.notifyAll();
  }

  return bigThing;
}

I'm showing my old school here - the new java packages probably have much neater ways to do this.