java.nio.*Buffer read/write atomicity

Zhong Yu zhong.j.yu at
Wed Dec 19 23:23:07 UTC 2012

Users are unlikely to expect multi-byte atomicity on a ByteBuffer.

However they are more likely to expect int-width atomicity on an
IntBuffer view of a ByteBuffer; such atomicity is unfortunately not
guaranteed either.

Zhong Yu

On Wed, Dec 19, 2012 at 11:48 AM, Aleksey Shipilev
<aleksey.shipilev at> wrote:
> Hi guys,
> I wanted to cross-check what's the expected behavior of Buffers with
> respect to atomicity? Don't confuse the atomic operations (a la
> j.u.c.atomic.*) and the read/write atomicity. Here's the concrete
> example, should the assert always be true?
>   ByteBuffer buf = ByteBuffer.allocate(100);
>   (publish $buf to both threads properly)
>   (start both threads)
>   Thread 1:
>    buf.putInt(0, 42); // at position 0
>   Thread 2:
>    int i = buf.getInt(0); // at position 0
>    Assert.assertTrue( (i == 0) || (i == 42) )
> Javadoc is silent about that, except for noting Buffers are not supposed
> to be used from the multiple threads without synchronization. I would
> anyway advocate to follow the atomicity behavior of plain fields and
> arrays, and make these reads/writes atomic under the race.
> The apparent reason for at least BB to fail to be atomic is that we
> read/write larger values byte-by-byte. Luckily, it appears to be easy to
> fix (for a given endianness, we can just throw in the Unsafe call).
> Before going out and submitting the RFE, I wanted to crosscheck if
> somebody has strong feelings about this.
> Thanks,
> Aleksey.

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