public void writeTo(Output output, T message)
                        throws IOException
                {
                    try
                    {
                        BigInteger value = (BigInteger) f.get(message);
                        if (value != null)
                            output.writeByteArray(number, value.toByteArray(),
                                    false);
                    }
                    catch (IllegalAccessException e)
                    {
                        throw new RuntimeException(e);
                    }
                    catch (IllegalArgumentException e)
                    {
                        throw new RuntimeException(e);
                    }
                }
Field factory via plain old reflection api.

@author David Yu
@created Jul 7, 2011
protostuff-runtime/src/main/java/io/protostuff/runtime/RuntimeReflectionFieldFactory.java