How to change MidiChannel volume?

Question

iam trying to change the midi vol using this func

public static void setVolume(int value) {
    int CHANGE_VOLUME = 7;
    midivol = value;
    try {
        if (synthesizer.getDefaultSoundbank() == null) {
                ShortMessage volumeMessage = new ShortMessage();
                for (int i = 0; i < 16; i++) {
                    volumeMessage.setMessage(ShortMessage.CONTROL_CHANGE, i, CHANGE_VOLUME, midivol);
                    volumeMessage.setMessage(ShortMessage.CONTROL_CHANGE, i, 39, midivol);
                    MidiSystem.getReceiver().send(volumeMessage, -1);
                }
        } else {
            MidiChannel[] channels = synthesizer.getChannels();
            for (int c = 0; channels != null && c < channels.length; c++) {
                channels[c].controlChange(CHANGE_VOLUME, midivol);
                channels[c].controlChange(39, midivol);
            }
        }
    } catch (Exception e) {
            e.printStackTrace();
        }
}

and it wont change so iam asking what is the problem there and what is the range for the value is it from 0-127?

and why there is two of volumemessage and channels[c]

volumeMessage.setMessage(ShortMessage.CONTROL_CHANGE, i, CHANGE_VOLUME, midivol);
                volumeMessage.setMessage(ShortMessage.CONTROL_CHANGE, i, 39, midivol);

channels[c].controlChange(CHANGE_VOLUME, midivol);
            channels[c].controlChange(39, midivol);