如何使用线程将对象添加到列表并同时使用该列表而不会获得 ConcurrentModificationException？

Question

The title tells it all, I have a Timer t1 which periodically adds objects to a List. Then I have an other Timer t2 iterating on the list at constant intervals to display its content.

I totaly understand why i'm getting these exceptions but I can't come up with the right answer to avoid it. Iv'e tried running my operations in synchronized(list){} and such things but nothing makes it. My algorithm apears to be working just fine despite these exceptions but i would like to make it clean nontheless.

Any idea?

Edit: I'm using java.util.Timer;

Answer 1

All access to the List much be wrapped by a synchronized block like so:

import java.util.ArrayList;
import java.util.List;
import java.util.Timer;
import java.util.TimerTask;

public final class Answer {
    private static final int FIVE_SECONDS = 5 * 1000;
    private static final int TWO_SECONDS = 2 * 1000;

    public static void main(String... args) {
        // this list contains the values that the producer will add to and the consumer will pull from
        final List<Long> theList = new ArrayList<>();

        // consumes all values found in theList
        new Timer("consumer").scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                final List<Long> copy;
                synchronized (theList) {
                    // we make a copy and clear theArray as quickly as possible in 
                    // order to not block the producer for too long
                    copy = new ArrayList<>(theList);
                    theList.clear();
                }

                System.out.println("copy = " + copy);
            }
        }, 1, FIVE_SECONDS);

        // adds the current time in ms to theList every 2 seconds
        new Timer("producer").scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                synchronized (theList) {
                    theList.add(System.currentTimeMillis());
                }
            }
        }, 1, TWO_SECONDS);
    }
}

That being said using a BlockingQueue will do this for you.

Answer 2

https://www.baeldung.com/java-mutex也许看看 mutex-structures，这些可能会派上用场