protected int actualReadableBytes() {
        return internalBuffer().readableBytes();
    }
Returns the actual number of readable bytes in the internal cumulative
buffer of this decoder. You usually do not need to rely on this value
to write a decoder. Use it only when you must use it at your own risk.
This method is a shortcut to {@link #internalBuffer() internalBuffer().readableBytes()}.
src/main/java/com/alipay/remoting/codec/AbstractBatchDecoder.java