core/src/main/java/org/nanohttpd/protocols/http/response/Response.java
public static Response newFixedLengthResponse(IStatus status, String mimeType, String txt) {
        ContentType contentType = new ContentType(mimeType);
        if (txt == null) {
            return newFixedLengthResponse(status, mimeType, new ByteArrayInputStream(new byte[0]), 0);
        } else {
            byte[] bytes;
            try {
                CharsetEncoder newEncoder = Charset.forName(contentType.getEncoding()).newEncoder();
                if (!newEncoder.canEncode(txt)) {
                    contentType = contentType.tryUTF8();
                }
                bytes = txt.getBytes(contentType.getEncoding());
            } catch (UnsupportedEncodingException e) {
                NanoHTTPD.LOG.log(Level.SEVERE, "encoding problem, responding nothing", e);
                bytes = new byte[0];
            }
            return newFixedLengthResponse(status, contentType.getContentTypeHeader(), new ByteArrayInputStream(bytes), bytes.length);
        }
    }
public static Response newFixedLengthResponse(IStatus status, String mimeType, String txt) {
        ContentType contentType = new ContentType(mimeType);
        if (txt == null) {
            return newFixedLengthResponse(status, mimeType, new ByteArrayInputStream(new byte[0]), 0);
        } else {
            byte[] bytes;
            try {
                CharsetEncoder newEncoder = Charset.forName(contentType.getEncoding()).newEncoder();
                if (!newEncoder.canEncode(txt)) {
                    contentType = contentType.tryUTF8();
                }
                bytes = txt.getBytes(contentType.getEncoding());
            } catch (UnsupportedEncodingException e) {
                NanoHTTPD.LOG.log(Level.SEVERE, "encoding problem, responding nothing", e);
                bytes = new byte[0];
            }
            return newFixedLengthResponse(status, contentType.getContentTypeHeader(), new ByteArrayInputStream(bytes), bytes.length);
        }
    }
Create a text response with known length.