Java中的线程。 “ AWT-EventQueue-0” java.util.ConcurrentModificationException

Question

I have a timer, it makes every 0.02 seconds of action. The actions there checking on the condition, if it is carried out - call Thread for playing music. The problem is that while the melody is playing, this condition is again checked. In result Exception: Exception in thread "AWT-EventQueue-0" java.util.ConcurrentModificationException

- Background.java
....
 Thread eventMusic = new Thread(new EventMusic());
... check different conditions ... (every 0.02s)
 if everything is ok  -> eventMusic.start();

- EventMusic.java
package Shooter2Dv23082013;
import javazoom.jl.decoder.JavaLayerException;
import javazoom.jl.player.Player;
import java.io.FileInputStream;
import java.io.FileNotFoundException;

public class EventMusic implements Runnable{
    @Override
    public void run() {

        try {
            Player p = new Player(new FileInputStream(getClass().getClassLoader().getResource("res/nameSong.mp3").getPath()));
            p.play();
        } catch (JavaLayerException e) {
            e.printStackTrace();
        } catch (FileNotFoundException e) {
            e.printStackTrace();}


    }
}

The question is.

1. Create multiple threads? How to create threads for each condition (ie, a lot of threads, but I do not know how many of them will, after work - it will closed)

2. Frankly, it begs the observer template, if all conditions are met - notify () thread that wants to play a song, for different occasions.

Answer 1

You haven't shown enough code to pin-point the problem but one of your threads have tried to modify a collection that one of your other threads is iterating over currently.

The JavaDocs for ConcurrentModificationException say

This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible. For example, it is generally permissible for one thread to a Collection while another thread is over it.

If a thread is allowed to continue iterating over a collection which is simultaneously being modified the program behaviour would become non-deterministic and it would fail at an undetermined time leading into hard to find bugs. Hence, this exception is thrown by the Iterator and the program is said to have failed fast rather than failing later. Iterators that do this are known as fail-fast iterators.

Answer 2

first, your problem is probably in your check-function.

You could use some kind of lock to prevent a condition-check while playing music:

ReentrantLock lock = new ReentrantLock();


Thread eventMusic = new Thread(new EventMusic());
lock.lock();
... check different conditions ... (every 0.02s)
if everything is ok  -> eventMusic.start();
lock.unlock();


public class EventMusic implements Runnable{
@Override
public void run() {

    try {
        lock.lock();
        Player p = new Player(new FileInputStream(getClass().getClassLoader().getResource("res/nameSong.mp3").getPath()));
        p.play();
    } catch (JavaLayerException e) {
        e.printStackTrace();
    } catch (FileNotFoundException e) {
        e.printStackTrace();}
    finally{
       lock.unlock();
    }

 }
}

You should place the lock somewhere so both classes can access it