Synchronize Java ArrayList

By default, Java ArrayList implementation is not synchronized (it is not thread-safe). This means if multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally.

How to synchronize?

We can achieve thread safety by wrapping the ArrayList using the Collections.synchronizedList method.

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

public class SynchronizedListExample {
    public static void main(String[] args) {
        List<String> list = Collections.synchronizedList(new ArrayList<String>());
        
        list.add("Java");
        list.add("Python");
        list.add("Ruby");
        
        synchronized (list) {
            for (String str : list) {
                System.out.println(str);
            }
        }
    }
}

Important: When iterating over the synchronized list, you must manually synchronize on the returned list to avoid non-deterministic behavior.

Alternative: CopyOnWriteArrayList

Another way to have a thread-safe list is using CopyOnWriteArrayList from java.util.concurrent package. It is a thread-safe variant of ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.

import java.util.concurrent.CopyOnWriteArrayList;

CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<String>();
list.add("Java");
list.add("Python");