Thursday, November 27, 2008

Burnt by ByteBuffer

In my last post about this, I had "solved" my ByteBuffer problem rather crudely, by setting the position back to zero. But I wasn't really happy with that - I figured I should know where the position was getting modified.

Strangely, the bug only showed up when I had debugging code in place. (Which is why the bug didn't show up until I put in debugging code to track down a different problem!) That told me that it was probably the debugging code itself that was changing the ByteBuffer position.

I started putting in asserts to check the position. The result baffled me at first. Here's what I had:
int pos = buf.position();
assert(pos == buf.position());
Since mark() saves the position and reset() restores the position, I figured this should never fail. But it did! It turns out, what was happening was this:
method1:                         method2:
... buf.mark();
method2(); ...
... buf.reset();
The problem was that the nested mark() in method2 was overwriting the first mark(). So the outer reset() was restoring to the nested position, not to its own mark.

A classic example of why mutable state can cause problems. So not only does ByteBuffer burden you with this extra baggage, but it's also error prone baggage. Granted, it's my own fault for using it improperly.

The fix was easy. I quit using mark() and reset() and instead did:
   int pos = buf.position();
That solved the problem.

I almost got burnt another way. I had used buf.rewind() to set the position back to zero. When I read the documentation more closely I found out that rewind() also clears the position saved by mark(). So if a nested method had called rewind() that would also have "broken" my use of mark/reset.

Oh well, I found the problem and fixed it, and now I know to be more careful. On to the next bug!

PS. It's annoying how Blogger loses the line spacing after any kind of block tag (like pre).

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