Is there java.util.concurrent equivalent for WeakHashMap?

Question

Can the following piece of code be rewritten w/o using Collections.synchronizedMap() yet maintaining correctness at concurrency?

Collections.sy         


        

Answer 1

If you are using Java 7 and above, this use case is solved in a thread-safe manner with ClassValue https://docs.oracle.com/javase/7/docs/api/java/lang/ClassValue.html If you require the use of remove, think carefully about concurrency and read the doc thoroughly.

If you are using Java 6 or below. No, you have to synchronize a WeakHashMap.

Answer 2

Guava's CacheBuilder class allows you to do this easily.

CacheBuilder.newBuilder().weakKeys().build()

Note that this changes key equality semantics to be == instead of .equals() which will not matter in your case of using Class instances but is a potential pitfall.

Answer 3

Cafeine is a popular competitor of Guava cache.

- keys automatically wrapped in weak references
- values automatically wrapped in weak or soft references

usage:

LoadingCache<Key, Graph> graphs = Caffeine.newBuilder()
 .weakKeys()
 .weakValues()
 .build(key -> createExpensiveGraph(key));

Answer 4

Does wrapping the WeakHashMap in a synchronized map still work correctly for what you want to do, since the garbage collector can modify the weakreferences directly at anytime, bypassing the synchronized map wrapper? I think WeakHashMap only truly works in a single threaded model.

Answer 5

I don't believe there is. In fact the javadoc suggests using Collections.synchronizedMap()

"Like most collection classes, this class is not synchronized. A synchronized WeakHashMap may be constructed using the Collections.synchronizedMap method."

Answer 6

Does wrapping the WeakHashMap in a synchronized map still work correctly for what you want to do, since the garbage collector can modify the weakreferences directly at anytime, bypassing the synchronized map wrapper? I think WeakHashMap only truly works in a single threaded model.

As mentioned above, the documentation for WeakHashMap at https://docs.oracle.com/javase/7/docs/api/java/util/WeakHashMap.html specifically says:

"A synchronized WeakHashMap may be constructed using the Collections.synchronizedMap method"

Which implies to me that this technique must work in tandem with the garbage collector's behavior (unless the documentation is buggy!)