Java's builtin ThreadLocal has a serious flaw: it can take an arbitrarily long amount of time to dereference the things you had stored in it, even once the ThreadLocal instance itself is no longer referenced. This is because there is single, master map stored for each thread, which all ThreadLocals share, and that master map only periodically purges "stale" entries.
While not technically a memory leak, because eventually the memory will be reclaimed, it can take a long time and you can easily hit OutOfMemoryError because from the GC's standpoint the stale entries are not reclaimable.
This class works around that, by only enrolling WeakReference values into the ThreadLocal, and separately holding a hard reference to each stored value. When you call
close(), these hard references are cleared and then GC is freely able to reclaim space by objects stored in it.
We can not rely on
ThreadLocal.remove()as it only removes the value for the caller thread, whereas
close()takes care of all threads. You should not call
close()until all threads are done using the instance.
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