Java: Looking for mutable/re-referenceable weak reference implementation

Question

I am looking for a weak reference implementation similar to java.lang.ref.WeakReference, but which offers a set() method or some other way of re-reference the created weak reference object. Here is the example:

MutableWeakReference ref = new MutableWeakReference(someObject);
ref.set(anotherObject);

I need this to avoid object creation which, in my case slows down the execution time by an order of magnitude, because I am constantly changing the object to which my weak reference refers.

I tried to copy the code from JDK, but it seems impossible since java.lang.ref.Reference uses the sun.misc.Cleaner class which is internal. I also looked on Android implementation but it seems it depends on Dalvik VM for Garbage collection. I wonder if this is actually possible to implement without changing the JVM / environment.

Answer 1

Wouldn't it just be possible to encapsulate your references in a simple

class MyOwnReference<T> {
    public T ref;
    public void set(T o) { ref = o; }
}

and create WeakReference<MyOwnReference<WhatEver>>?

I wonder if this is actually possible to implement without changing the JVM / environment.

No, you probably can't "reimplement" the WeakReference. It is a JVM-supported class.

Are you sure it is the creation of WeakReference instances that slows it down? I wouldn't think doing

ref = new WeakReference(someObject);

instead of some

ref.set(anotherObject);

would be that much more expensive.

Answer 2

I am actually implementing an iterator of some kind. Whenever I advance to the next entry, I need to create a new WeakReference.

I'm puzzled why you would need to use a WeakReference at all in an iterator. The normal use-case for WeakReference is for long term references to objects. But an iterator is typically a short term object, and an iteration is typically a short term process. The fact that it / they use an ordinary (strong) reference to the target object in the short term shouldn't be a concern.

I have run some tests and it seems this is like 8-10 times slower.

Again, this suggests that you shouldn't be using a WeakReference at all.

Is there a particular reason why won't a regular reference work for you?