java.nio.*Buffer read/write atomicity (original) (raw)
David Holmes david.holmes at oracle.com
Wed Dec 19 20:37:46 UTC 2012
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Aleksey,
If multiple threads have to synchronize access to the buffer then the reads/writes do not need to be atomic. Atomicity is only needed when data races are allowed.
David
On 20/12/2012 3:48 AM, Aleksey Shipilev 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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