<p>I need a byte buffer class in Java for single-threaded use. I should be able to insert data at the back of the buffer and read data at the front, with an amortized cost of O(1). The buffer should resize when it's full, rather than throw an exception or something.</p>

<p>I could write one myself, but I'd be very surprised if this didn't exist yet in a standard Java package, and if it doesn't, I'd expect it to exist in some well-tested public library.</p>

<p>What would you recommend?</p>

Not sure if it is "the best", but you have a nice example of
Circular Byte buffer here
.
Those
Java Utilities - OstermillerUtils
classes are under
GPL license
.
This Circular Byte Buffer implements the circular buffer producer/consumer model for bytes. Filling and emptying the buffer is done with standard Java InputStreams and OutputStreams.
Using this class is a simpler alternative to using a PipedInputStream and a PipedOutputStream.
PipedInputStreams and PipedOutputStreams don't support the mark operation, don't allow you to control buffer sizes that they use, and have a more complicated API that requires a instantiating two classes and connecting them.