Class CloseableThreadLocal<T>

  • All Implemented Interfaces:
    Closeable, AutoCloseable

    public class CloseableThreadLocal<T>
    extends Object
    implements Closeable
    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.

    NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.
    • Constructor Detail

      • CloseableThreadLocal

        public CloseableThreadLocal()
    • Method Detail

      • initialValue

        protected T initialValue()
      • get

        public T get()
      • set

        public void set​(T object)