public void writeTo(Writer out) throws IOException {
        if (this.writer != null) {
            throw new UnsupportedOperationException("writer not null");
        }
        out.write(buf, 0, count);
    }
Writes the contents of the buffer to another character stream.

@param out the output stream to write to
@throws IOException If an I/O error occurs.
src/main/java/com/alibaba/fastjson/serializer/SerializeWriter.java