CODE::

import processing.serial.*;

Serial myPort;
int val;

void setup() {
size(200, 200);
}

void draw() {
if(!mousePressed) {
if (myPort!=null)
myPort.stop();
myPort=null;
} else {
if (myPort==null)
myPort=new Serial(this, Serial.list()[0], 9600);

}
}