10

Although the Java docs specify, that Weak references are primarily for canonicalizing mappings, you will find many, many, many people on the internet stating, that the WeakHashMap is perfect for storing object metadata during its lifetime. However, nobody bothers to make a comprehensible AND appropriate example.

Using WeakHashMap to add object some properties or store metadata sounds to me, like an arbitrary decision based on the will to just use the damn thing. In other words - a bad design. I understand, that there are situations where inheritance may not be available (final classes, interfaces), but what about composition? I can't think of one example where composition would not be an option. And certainly a better one, because it relies on a well-established principle, instead of a "language quirk".

So, is there a situation, where it would be better to use weak references instead of simple composition? If not, why does everyone on the internet seem to get it wrong?

Community
  • 1
Leprechaun
  • 240

1 Answers1

13

Let's say I need to associate some metadata with some data objects. This is a pretty common scenario. I don't control the data objects, and there's lots of them, and the API in question (callbacks, virtual methods, etc..) offers me those data objects, but without my metadata. So, I need to track some state for each of those data objects, namely the ones I've seen before and might see again. I'll call this metadata an adornment, which is a term sometimes used for that concept.

Simple composition provides for easy lookup from the adorning object (the one with metadata) to the other object (the one with data), with the presumption that since I control the metadata object design, I can put therein a reference directly to the data object.

However, unless you control the design of the data object and can change it, simple composition doesn't provide the reverse lookup (from data object to some metadata). In fact, unless you have a collection of the metadata objects, you can't even locate the associated metadata item given an object (data) item.

In addition to the problem of lookup in the direction going from the object to find its adorning metadata, there is also the problem of metadata lifetime.

Metadata that accumulates and is never released even when the data object is no longer used is a memory leak.

A weak referencing hashmap solves both problems. It allows locating the metadata given the data, and allows the metadata to be freed at some time after the data is freed.

Note further that a weak referencing hashmap allows not only (metadata) values to be reclaimed (gc'ed), but also the (data) keys to be reclaimed (gc'ed), since if the keys were kept that would also be a memory leak, and further, the values could never be released either.

Erik Eidt
  • 34,819