The GC delivers cleared references to the reference processing thread via the pending list, and the reference processing thread delivers them to their associated ReferenceQueue (if any). That delivery occurs as the reference processing thread gets opportunities to run, so might not be instantly.

There's a need to find out whether all cleared references associated with a queue have been delivered to that queue. This shows up in testing fairly often, with various at best indirect and not entirely reliable solutions currently being used.

We have private Reference.waitForReferenceProcessing(), which blocks if there are cleared but not yet enqueued references, waiting for the reference processing thread to process some of them. It's available to corelibs through SharedSecrets. (It's used by java.nio.Bits.reserveMemory().)

We could make it available to tests via WhiteBox.