Concurrent Modification Exception trying to add objects on OSC event

Answers

• MenteCode

  July 2015 edited July 2015

  Ah, the classic multi-threading CM Exception.

  There are definitely more effective ways of getting around this problem that I haven't thought of, but the easiest option would be to replace the type of the variable asteroids to a CopyOnWriteArrayList, since I'm assuming asteroids is a regular ArrayList.

  You can use this thread-safe ArrayList by placing this at the top of your code:

  import java.util.concurrent.CopyOnWriteArrayList;
  
• GoToLoop

  July 2015 edited July 2015

  I'm assuming has to do with the fact that the OSCEvent() is another Thread...

  • You're dead on it! >-)
  • Indeed, as @MenteCode tipped ya, CopyOnWriteArrayList() is the easiest approach.
  • Although it's the slowest too! Since every single access to it clones its internal array. YIKES!
  • I'm afraid that your volatile isBusy doesn't cut for protecting asteroids against ConcurrentModificationException at all! :-S
  • If you choose to stick w/ the regular ArrayList container, you're gonna need synchronized in all places that its size() is modified.
  • Some of those methods include add(), addAll(), remove(), clear(), etc.
  • The only place you've originally tried out synchronized @ line 24 wasn't so necessary for no change in size() happens there!
  • Instead, you're gonna need synchronized blocks for the add() & remove() parts:

  ---

  void oscEvent(OscMessage osc) {
    if (osc.addrPattern().equals("/peter/rhythm"))  synchronized (asteroids) {
      asteroids.add( new Asteroid(0) );
    }
  }
  

  ---

  synchronized (asteroids) {
    for (int a = asteroids.size(), len = a; a-- != 0;)  if (asteroids.get(a).dead()) {
      asteroids.set(a, asteroids.get(--len));
      asteroids.remove(len);
    }
  }
  

  ---

  And a similar online example as well: :bz
  http://studio.ProcessingTogether.com/sp/pad/export/ro.9K-kjhuONcJDZ/latest
• ablanton

  February 2016

  Ok I'm having the same issue trying to bring in OSC data then call a class. I tried both import java.util.concurrent.CopyOnWriteArrayList; and synchronized to no avail. I'm not sure my implementation of synchronized was correct though.

  Any pointers in the right directions would be super helpful!

  Here is a basic example:

  import java.util.concurrent.CopyOnWriteArrayList;
  import oscP5.*;
  
  OscP5 oscP5;
  float inx, iny;
  ArrayList<Ball> balls = new ArrayList();
  
  void setup() {
    size(600, 600);
    smooth();
    oscP5 = new OscP5(this, 11111);
  }
  
  void draw() {
    background(230);
    for (Ball b: balls)  b.display();
  }
  
  void adder(float inxx,float inyy) {
    balls.add( new Ball(inxx,inyy) );
    println(inxx,inyy);
  }
  
  void oscEvent(OscMessage theOscMessage) {
    float inyy = 0;
    float inxx = 0;
  
    if (theOscMessage.checkAddrPattern("/x")==true) {
        inxx = theOscMessage.get(0).floatValue();
        inyy = theOscMessage.get(1).floatValue();
        adder(inxx, inyy);
        return;
    }
  }
  
  class Ball{
    float x, y, x1, x2, x3, y1, y2, y3;
    float diameter;
    boolean on = false;
  
    Ball(float inx, float iny) {   
    x=inx;
    y=iny;
    on = true;
    diameter = 1;
    }
  
    void display() {
      if (on == true) {
        noFill();
        strokeWeight(4);
        stroke(225);
        diameter += 0.5;
        quad(
          x+(-diameter),y+(-diameter),
          x+diameter*x,y+(-diameter)*y,
          x+(-diameter)*x,y+diameter*y,
          x+diameter*x,y+diameter*y
          );
      }
    }
  }
  

  The overall idea being that i am bringing data in via OSC and then passing that data to the ball class to make an instance each time data is passed in.
• koogs

  February 2016

  Set a flag in oscevent(), add stuff at end of draw if flag is set?
• GoToLoop

  February 2016 edited February 2016

  // forum.Processing.org/two/discussion/11634/
  // concurrent-modification-exception-trying-to-add-objects-on-osc-event
  
  // GoToLoop (2016-Feb-08)
  
  import oscP5.*;
  import java.util.List;
  
  final List<Ball> balls = new ArrayList<Ball>();
  
  void setup() {
    size(600, 600, FX2D);
    smooth(4);
  
    rectMode(CORNER);
    strokeWeight(1.5);
    stroke(0);
    fill(#FFFF00);
  
    balls.add(new Ball(random(width - Ball.DIAM), random(height - Ball.DIAM)));
  
    new OscP5(this, 11111);
  }
  
  void draw() {
    background(0350);
  
    synchronized (balls) {
      for (final Ball b : balls)  b.display();
    }
  }
  
  void oscEvent(final OscMessage osc) {
    if (osc.checkAddrPattern("/x")) {
      final float x = osc.get(0).floatValue();
      final float y = osc.get(1).floatValue();
      final Ball  b = new Ball(x, y);
  
      synchronized (balls) {
        balls.add(b);
      }
    }
  }
  
  class Ball {
    static final int DIAM = 30;
    float x, y;
  
    Ball(float xx, float yy) {
      x = xx;
      y = yy;
    }
  
    void display() {
      rect(x, y, DIAM, DIAM);
    }
  }
  
• ablanton

  February 2016

  DUDE!!! Rock and Roll :D:D:D that totally works. And thanks for the quick reply.

  I now see the implementation of the synchronized in the draw loop and the osc receiver. And basically that will synchronize any writing to or removing from the array? Very cool! Thanks.
• benpurdy

  February 2017

  If you need to mutate a lot of different lists/state in response to OSC messages you can also put the OSC messages into a list and then synchronize on that list to take care of this issue across the whole sketch.

  import oscP5.*;
  import netP5.*;
  
  OscP5 oscP5;
  ArrayList oscMessageQueue = new ArrayList();
  
  void setup() {
    oscP5 = new OscP5(this, 12000);
  }
  
  void draw() {
    
    // take care of the OSC messages once per frame
    dispatchOSCMessages();
    
    // Do the rest of your rendering here, it won't be bothered by OSC messages showing up.
  }
  
  // Handle all the enqueued OSC messages at once.
  void dispatchOSCMessages() {
    synchronized(oscMessageQueue) {
      for(OscMessage message : oscMessageQueue) {
        // Do things here that require modifying lists or other actions
        // which should never happen half-way through the draw function.
      }
      
      // clear the list of messages.
      oscMessageQueue.clear();
    }
  }
  
  // Add the OSC messages to the queue to be handled later.
  void oscEvent(OscMessage message) {
    synchronized(oscMessageQueue) {
      oscMessageQueue.add(message);
    }
  }