Synchronize block not synchronizing a Java array list

Question

I have a Java ArrayList, that is being used by 500+ threads simultaneously. For some reason, the synchronization block is not ensuring synchronization on a Java Array List. I known that ArrayLists are non thread-safe, i.e., they are not synchronized. However, I thought that by wrapping the list into a synchronized block I would achieve that. Unfortunately, in rare (but in some situations) two or more threads are entering the synchronized block simultaneously, which is giving me non-deterministic behaviour. Am I missing something? How can I guarantee that my array list (or any other list collection) are completely thread safe throughout 500+ simultaneous threads operating in the array.

There is a related question, (Correct way to synchronize ArrayList in java), but I did not understood its answer. Should I create a Synchronized collection at every "run" of my threads???

Sample:

Thread 1

synchronized (_myList) {
   Iterator it = _myList.iterator();
   ...
}

Thread 2

synchronized (_myList) {
   Iterator it = _myList.iterator();
   ...
}

Thread n

synchronized (_myList) {
   Iterator it = _myList.iterator();
   ...
}

You're synchronizing on _myList but then iterating over _unackedSentQueue... why? If two instances have different _myList references but the same _unackedSentQueue references, that will cause the problem you're seeing. Ideally, you should post a short but complete program demonstrating the problem. Fundamentally you should probably consider using a list implementation designed for this though... — Jon Skeet
– Jon Skeet, Commented Aug 17, 2014 at 7:56
This doesn't answer your question, but you could use Vector if you need a synchronized variant of arraylist. — kviiri
– kviiri, Commented Aug 17, 2014 at 7:57
@kviiri: No, that only synchronizes each individual operation - it doesn't help if you're trying to synchronize a whole sequence of operations (like iterating). — Jon Skeet
– Jon Skeet, Commented Aug 17, 2014 at 7:57
If this list is used across all 500 threads you may make a similar global object lock Object _unackedSentQueueLock = new Object() then use that for your synchronized locks. Alternatively you could always just synchronize with the list your actually using _unackedSentQueue — ug_
– ug_, Commented Aug 17, 2014 at 7:59

Sagar D · Accepted Answer · 2014-08-17 08:16:57Z

Option 1 :

Use a custom lock : Object lock = new Object();

and synchronize things over this lock like :

synchronized(lock) { //operations on array list }

Option 2:

Use java.util.Collections.synchronizedList()

Refer the following code :

import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;

    public class SynchronizedListExample {

        public static void main(String[] args) {

            List<String> syncList = Collections.synchronizedList(new ArrayList<String>());

            syncList.add("one");
            syncList.add("two");
            syncList.add("three");

            // when iterating over a synchronized list, we need to synchronize access to the synchronized list
            synchronized (syncList) {
                Iterator<String> iterator = syncList.iterator();
                while (iterator.hasNext()) {
                    System.out.println("item: " + iterator.next());
                }
            }

        }

    }

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