Function to help a Burp extension update a parameter

UpdateParameter.java

    static byte[] updateParameter(IExtensionHelpers helpers, IRequestInfo requestInfo, byte[] request, IParameter parameter, String value) {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        int bodyOffset = requestInfo.getBodyOffset();
        baos.write(request, bodyOffset, parameter.getValueStart() - bodyOffset);
        baos.write(value.getBytes(Charsets.ISO_8859_1), 0, value.length());
        baos.write(request, parameter.getValueEnd(), request.length - parameter.getValueEnd());
        byte[] newBody = baos.toByteArray();

        List<String> headers = requestInfo.getHeaders();
        for(int i = 0; i < headers.size(); i++) {
            if(headers.get(i).startsWith("Content-Length:")) {
                headers.set(i, String.format("Content-Length: %d", newBody.length));
            }
        }

        return helpers.buildHttpMessage(headers, newBody);
    }