try {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
OutputStream zos = new GZIPOutputStream(baos);
OutputStreamWriter writer = new OutputStreamWriter(zos);
writer.write(value);
writer.close();
zos.close();
baos.close();
return baos.toByteArray();
} catch (IOException e) {
throw new RuntimeException(e);
}

Concerning features: I’d find it extremely useful if you could add support for GZIP compressing JSON into the writer. It must be configurable (based on the Accept-Encoding request header). Support for gzip compression is built into at least 1.4, so there is no additional dependency. But using the GZIPOutputStreamWriter directly is just plain ugly. Converting my JSON to an bytearray and back to a compressed string takes about 12 line of client code… I’d add that support using factory methods, eg.
new JSONWriter().write(”usuall stuff”)
JSONWriter.validating().write(”validated”)
JSONWRiter.compressing().write(”compressed”)
JSONWriter.validating().compressing().write(”both validated and compressed!”)

I’m sure you can come up with a nice API, I really like what I’ve seen so far.