If you are using Jackson for JSON parsing, it can automatically convert byte[] to/from Base64 encoded Strings via data-binding.

Or, if you want low-level access, both JsonParser and JsonGenerator have binary access methods (writeBinary, readBinary) to do the same at level of JSON token stream.

For automatic approach, consider POJO like:

public class Message {
  public String mimetype;
  public byte[] value;
}

and to create JSON, you could do:

Message msg = ...;
String jsonStr = new ObjectMapper().writeValueAsString(msg);

or, more commonly would write it out with:

OutputStream out = ...;
new ObjectMapper().writeValue(out, msg);

The problem with using the ObjectMapper directly is that if you alter the configuration of the mapper it can lead to problems. However, if you do not change the underlying config you should be safe anyway (more reading here).

But if you use the ObjectReader and ObjectWriter you are totally safe, even if you actually do alter the configuration of the mapper. This is possible since the reader/writer are immutable and it is therefore not possible to change the underlying state.

So, to read/write to bytes the following approach works fine:

ObjectMapper om = new ObjectMapper();
JsonNode oldNode = om.createObjectNode();

// Writing...
// Create an immutable writer (in this case using the default settings)
final ObjectWriter writer = om.writer();

// Use the writer for thread safe access.
final byte[] bytes = writer.writeValueAsBytes(oldNode);


// Reading...
// Create an immutable reader
final ObjectReader reader = om.reader();

// Use the reader for thread safe access
final JsonNode newNode = reader.readTree(new ByteArrayInputStream(bytes));

So, basically you can use the same principles (with the byte streams) but if you need to be sure that you are using thread safe access to the mapper you should access the reading/writing via the ObjectReader and ObjectWriter.

The writeValueAsBytes is described like this in the JavaDoc:

Functionally equivalent to calling writeValue(Writer,Object) with java.io.ByteArrayOutputStream and getting bytes, but more efficient.

For the reading you can simply use the readTree(InputStream) version.

Get the bytes of the string:

obj.toString().getBytes(theCharset);

Here is a good example of base64 encoding byte arrays. It gets more complicated when you throw unicode characters in the mix to send things like PDF documents. After encoding a byte array the encoded string can be used as a JSON property value.

Apache commons offers good utilities:

 byte[] bytes = getByteArr();
 String base64String = Base64.encodeBase64String(bytes);
 byte[] backToBytes = Base64.decodeBase64(base64String);

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Base64_encoding_and_decoding

Java server side example:

public String getUnsecureContentBase64(String url)
        throws ClientProtocolException, IOException {

            //getUnsecureContent will generate some byte[]
    byte[] result = getUnsecureContent(url);

            // use apache org.apache.commons.codec.binary.Base64
            // if you're sending back as a http request result you may have to
            // org.apache.commons.httpclient.util.URIUtil.encodeQuery
    return Base64.encodeBase64String(result);
}

JavaScript decode:

//decode URL encoding if encoded before returning result
var uriEncodedString = decodeURIComponent(response);

var byteArr = base64DecToArr(uriEncodedString);

//from mozilla
function b64ToUint6 (nChr) {

  return nChr > 64 && nChr < 91 ?
      nChr - 65
    : nChr > 96 && nChr < 123 ?
      nChr - 71
    : nChr > 47 && nChr < 58 ?
      nChr + 4
    : nChr === 43 ?
      62
    : nChr === 47 ?
      63
    :
      0;

}

function base64DecToArr (sBase64, nBlocksSize) {

  var
    sB64Enc = sBase64.replace(/[^A-Za-z0-9\+\/]/g, ""), nInLen = sB64Enc.length,
    nOutLen = nBlocksSize ? Math.ceil((nInLen * 3 + 1 >> 2) / nBlocksSize) * nBlocksSize : nInLen * 3 + 1 >> 2, taBytes = new Uint8Array(nOutLen);

  for (var nMod3, nMod4, nUint24 = 0, nOutIdx = 0, nInIdx = 0; nInIdx < nInLen; nInIdx++) {
    nMod4 = nInIdx & 3;
    nUint24 |= b64ToUint6(sB64Enc.charCodeAt(nInIdx)) << 18 - 6 * nMod4;
    if (nMod4 === 3 || nInLen - nInIdx === 1) {
      for (nMod3 = 0; nMod3 < 3 && nOutIdx < nOutLen; nMod3++, nOutIdx++) {
        taBytes[nOutIdx] = nUint24 >>> (16 >>> nMod3 & 24) & 255;
      }
      nUint24 = 0;

    }
  }

  return taBytes;
}

This can be done by writing your own custom serializer and deserializer.

http://www.baeldung.com/jackson-custom-serialization

http://www.baeldung.com/jackson-deserialization

Here's an example similar to what you are doing...

Holder class

public static class Holder {
    private Bytes bytes;
    private String otherStuff;

    public Bytes getBytes() {
        return bytes;
    }

    public void setBytes(Bytes bytes) {
        this.bytes = bytes;
    }

    public String getOtherStuff() {
        return otherStuff;
    }

    public void setOtherStuff(String otherStuff) {
        this.otherStuff = otherStuff;
    }

}

Bytes class

Notice the annotations for the custom serializer...

@JsonSerialize(using = BytesSerializer.class)
@JsonDeserialize(using = BytesDeserializer.class)
public static class Bytes {
    private byte[] bytes;

    public Bytes(byte[] bytes) {
        this.bytes = bytes;
    }

    public byte[] getBytes() {
        return bytes;
    }
}

Serializer

This will serialize a "Bytes" object as a base64 string...

public static class BytesSerializer extends StdSerializer<Bytes> {

    private static final long serialVersionUID = -5510353102817291511L;

    public BytesSerializer() {
        super(Bytes.class);
    }

    @Override
    public void serialize(Bytes value, JsonGenerator gen, SerializerProvider provider) throws IOException {
        gen.writeString(Base64.encode(value.getBytes()));
    }
}

Deserializer

public static class BytesDeserializer extends StdDeserializer<Bytes> {

    private static final long serialVersionUID = 1514703510863497028L;

    public BytesDeserializer() {
        super(Bytes.class);
    }

    @Override
    public Bytes deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        JsonNode node = p.getCodec().readTree(p);
        String base64 = node.asText();
        return new Bytes(Base64.decode(base64));
    }
}

Main method

A simple test method...

public static void main(String[] args) throws Exception {
    ObjectMapper mapper = new ObjectMapper();

    Holder holder = new Holder();
    holder.setOtherStuff("[OTHER STUFF]");
    holder.setBytes(new Bytes(new byte[] { 1, 2, 3, 4, 5 }));

    String json = mapper.writeValueAsString(holder);

    System.out.println(json);
    Holder deserialised = mapper.readValue(json, Holder.class);

    System.out.println(Arrays.toString(deserialised.getBytes().getBytes()));
}

Output

{"bytes":"AQIDBAU=","otherStuff":"[OTHER STUFF]"}
[1, 2, 3, 4, 5]

Run it yourself

All of the classes above are "static" because I'd wrapped them into one big class called "Stack".

If you want to run this, create a new class (called anything you want) and paste all the code here into it...

Jackson will convert byte[] into Base64-encoded binary data. This is the safe way to pass binary content. Otherwise there is no way to know which character encoding might be used by contained data, so trying to build a String out of it would be risky and error-prone.

So the simplest way would be to have receiver base64 decode contents back into binary data.

You could alternatively add custom serializer to turn into other representations (hex, base85), but it really depends on what the goal is.

Since you are not specifying no CharSet on converting the Json array string to bytes. Simply use :

   arr = new JSONArray(new String(bArr));