Bugs in java.util.ArrayList, java.util.Hashtable and java.io.ByteArrayOutputStream

Osvaldo Doederlein opinali at gmail.com
Tue Mar 9 20:02:23 UTC 2010

Should we really consider this a VM bug? I'm not sure that it's a good idea
to allocate a single object which size exceeds 4Gb (for a byte[] - due to
the object header and array size field) - even on a 64-bit VM. An array with
2^32 elements is impossible, the maximum allowed by the size field is 2^32-1
which will be just as bad as 2^32-N for any other tiny positive N, for
algorithms that love arrays of [base-2-] "round" sizes.

And then if this bug is fixed, it may have slightly different variations.
For a long[] or double[] array, the allocation for the maximum size would
exceed 32Gb, so it exceeds the maximum heap size for 64-bit HotSpot with
CompressedOops. (Ok this is an artificial issue because we won't like have a
100% free heap, so the only impediment for "new long[2^32-1]" would be the
array header.)

My suggestion: impose some fixed N (maybe 64, or 0x100, ...), limiting
arrays to 2^32-N (for ANY element type). The artificial restriction should
be large enough to fit the object header of any vendor's JVM, plus the
per-object overhead of any reasonable heap structure. This limit could be
added to the spec, so the implementation is not a bug anymore :) and it
would be a portable limit. Otherwise, some app may work reliably on HotSpot
if it relies on the fact that 2^32-5 positions are possible, but may break
on some other vendor's JVM where perhaps the implementation limit is 2^32-13
or something else.


2010/3/9 Martin Buchholz <martinrb at google.com>

> On Tue, Mar 9, 2010 at 03:59, Ulf Zibis <Ulf.Zibis at gmx.de> wrote:
> > In PriorityQueue:
> >
> > let's result newCapacity in 0xFFFF.FFFC  =-4
> > then "if (newCapacity - MAX_ARRAY_SIZE > 0)" ---> false
> > then Arrays.copyOf(queue, newCapacity) --->
> ArrayIndexOutOfBoundsException
> How could newCapacity ever become -4?
> Since growth is by 50%.  But even 100% looks safe...
> > Am I wrong ?
> >
> > 2.) Why don't you prefer a system-wide constant for MAX_ARRAY_SIZE ???
> This should never become a public API - it's a bug in the VM.
> I prefer the duplication of code to creating a new external dependency.
> Martin
> > -Ulf
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