Sunday, January 01, 2017

Java ByteBuffer Gotcha

I had a weird bug in some Java code in jSuneido that took me a while to figure out. I briefly thought I'd found a bug in the ByteBuffer implementation, although I realized that was a low probability. (Especially given the confusing nature of ByteBuffer.) In the end it makes sense, although it perhaps could be documented better.

Here's the scenario - you slice or duplicate a ByteBuffer. This makes a new ByteBuffer instance that shares its data with the original. Then you compact the original buffer. Note - this does not modify the data that you are interested in. However, it does move it.

ByteBuffer buf = ByteBuffer.allocate(8192);
for (int i = 0; i <= 300; ++i)
    buf.put((byte) i);
for (int i = 0; i < 12; ++i)
ByteBuffer slice = buf.duplicate();

This will print 0 and then 12, i.e. the slice has changed, even though you didn't explicitly modify the buffer.

In retrospect it's obvious - compact does alter the buffer, which is shared with the slice. So the contents of the slice "changes".

I'd be tempted to add a warning to the documentation for compact that it will "invalidate" any existing slices or duplicates, the same way that C++ documentation points out which operations invalidate iterators. But maybe it should be obvious.

I'm mostly posting this because my searches for the problem didn't find anything. Perhaps this will help someone in the future find the problem quicker.

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