As explained in the Javadoc for ByteBuffer, byte buffers can be direct or non-direct. Direct buffers may reside outside the Java heap which eliminates the need for copying for native I/O operations. That makes direct buffers particularly useful for receiving and sending data over a socket, but they’re also more expensive to create and release, which leads to the idea of pooling buffers.

PooledDataBuffer is an extension of DataBuffer that helps with reference counting which is essential for byte buffer pooling. How does it work? When a PooledDataBuffer is allocated the reference count is at 1. Calls to retain() increment the count, while calls to release() decrement it. As long as the count is above 0, the buffer is guaranteed not to be released. When the count is decreased to 0, the pooled buffer can be released, which in practice could mean the reserved memory for the buffer is returned to the memory pool.

Note that instead of operating on PooledDataBuffer directly, in most cases it’s better to use the convenience methods in DataBufferUtils that apply release or retain to a DataBuffer only if it is an instance of PooledDataBuffer.