Sometimes constraints are questionable

[Martin Buchholz]

I wrote an earlier version of this grow logic, and then it was
transplanted into other classes.

We strongly suspect that the VM will throw OOME when we try to
allocate an array beyond MAX_ARRAY_SIZE, so are reluctant to do so,
but we also consider the VM behavior a bug that may eventually get
fixed (or is already a non-issue with a different VM).  We are trying
for good behavior with both sorts of VM.

On Sat, May 30, 2020 at 7:32 AM Jim Laskey <james.laskey at oracle.com> wrote:
>
> I'm working through https://bugs.openjdk.java.net/browse/JDK-8230744 <https://bugs.openjdk.java.net/browse/JDK-8230744> Several classes throw OutOfMemoryError without message .
>
> I'm wondering why hugeCapacity in src/jdk.zipfs/share/classes/jdk/nio/zipfs/ByteArrayChannel.java is defined as
>
>     /**
>      * The maximum size of array to allocate.
>      * Some VMs reserve some header words in an array.
>      * Attempts to allocate larger arrays may result in
>      * OutOfMemoryError: Requested array size exceeds VM limit
>      */
>     private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
>
>     /**
>      * Increases the capacity to ensure that it can hold at least the
>      * number of elements specified by the minimum capacity argument.
>      *
>      * @param minCapacity the desired minimum capacity
>      */
>     private void grow(int minCapacity) {
>         // overflow-conscious code
>         int oldCapacity = buf.length;
>         int newCapacity = oldCapacity << 1;
>         if (newCapacity - minCapacity < 0)
>             newCapacity = minCapacity;
>         if (newCapacity - MAX_ARRAY_SIZE > 0)
>             newCapacity = hugeCapacity(minCapacity);
>         buf = Arrays.copyOf(buf, newCapacity);
>     }
>
>     private static int hugeCapacity(int minCapacity) {
>         if (minCapacity < 0) // overflow
>             throw new OutOfMemoryError();
>         return (minCapacity > MAX_ARRAY_SIZE) ?
>             Integer.MAX_VALUE :
>             MAX_ARRAY_SIZE;
>     }
>
> It just seems that it's pushing the inevitable off to Arrays.copyOf.  Shouldn't it be:
>
>     private static int hugeCapacity(int minCapacity) {
>         if (minCapacity < 0 || minCapacity > MAX_ARRAY_SIZE) {
>             throw
>                 new OutOfMemoryError("ByteArrayChannel exceeds maximum size: " +
>                                       MAX_ARRAY_SIZE);
>         }
>
>         return MAX_ARRAY_SIZE;
>     }
>
> Real question: is there some hidden purpose behind this kind of logic?
>
>
> Cheers,
>
> -- Jim
>
>