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LowMemoryListener for Garbage Collector

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      Name: tb29552 Date: 02/22/99


      Problem/Motivation:
      I found that it is very complicated to write a generic
      object cache. I used Proxy Objects (holding the
      "primary key" data, e.g. a file name used to reload
      e.g. an image) with Weak/SoftReferences to the large
      data (e.g. the image/blob). SoftReferences didn't
      really work because they were too fast reclaimed
      by the virtual machine and I was unable to specify
      the order (which object should be reclaimed first).
      So I found a solution, where I still use the proxys,
      but use a dynamically sized list that holds hard references
      to the most used images/blobs. The garbage collector now
      may delete all weak referenced images in the proxys, but
      not the e.g. 100 most used ones, because the list holds
      hard references to them.

      My intention was to allow the list to dynamically grow
      (lots of memory free) or shrink (memory needed by other
      parts of the application). That lead me into big trouble.

      The only way I know is freeMemory()/totalMemory(). I set up
      a thread that read these values and resized the cache list.
      That lead me into OutOfMemoryErrors, because I found out that
      there were peaks between the points my thread asked for the
      values. So I tried catching OutOfMemoryError, so that the
      catch handler shrinks my list. That doesn't work either,
      because I ran into problems when there were other lines
      in my code that needed only a few bytes, but failed. But I
      could not forsee the memory peaks (I reserved >3 MB, but that
      was not sufficient for the peaks!).

      -> Currently I see no efficient way, to communicate with the garbage
      collector.

      Solution:
      I'd like to aks the garbage collector, if he recommends
      me to be careful with memory or if he says "well, 2 MB, no problem,
      lots of memory there for you, maybe I'll collect a little bit".
      I also like to register some class in my program as a
      "LowMemoryListener" that has the chance of freeing memory
      _before_ the garbage collector issues a OutOfMemoryError.
      And I would like to get some statistical information from
      the garbage collector, because there is no way to get the
      information in my program that a human can see when using
      java -verbosegc.
      Maybe there could also be a Listener that is informed whenever
      the garbage collector ran and grab that information or recalc
      the size of self made caches.
      (Review ID: 54528)
      ======================================================================

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              Assignee:
              mr Mark Reinhold
              Reporter:
              tbell Tim Bell
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