Which Java Concurrency List is Best for a Fixed Thread Pool?

What is the best monitor-free List implementation in the java.util.concurrent package for use with a fixed thread pool?

When working with a fixed thread pool that requires frequent read and write operations to a shared List in Java, choosing the right data structure is critical for performance and thread safety. The Java `java.util.concurrent` package offers several options, but it’s essential to select a monitor-free implementation that supports concurrent access without introducing synchronizations overhead.

import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;

public class ThreadSafeListExample {
    public static void main(String[] args) {
        List<String> list = new CopyOnWriteArrayList<>();

        // Sample threads modifying the list
        Runnable writer = () -> {
            for (int i = 0; i < 10; i++) {
                list.add("Item " + i);
            }
        };

        Runnable reader = () -> {
            for (String item : list) {
                System.out.println(item);
            }
        };

        Thread writerThread = new Thread(writer);
        Thread readerThread = new Thread(reader);

        writerThread.start();
        readerThread.start();
    }
}

Causes

• Threads need to read and write simultaneously to the list, which can lead to ConcurrentModificationExceptions if not handled properly.
• Traditional collections like ArrayList and LinkedList are not designed for concurrent access and can become a bottleneck.

Solutions

• Use `CopyOnWriteArrayList`, which is designed to handle concurrent writes by creating a new copy of the underlying array whenever a write operation occurs, thus ensuring that read operations do not block and always have access to up-to-date data.
• Alternatively, consider using `ConcurrentLinkedQueue` if your design allows for a queue structure instead of a list. This can provide more efficient concurrent access by utilizing a non-blocking algorithm.

Mistake: Using ArrayList or LinkedList in a concurrent environment.

Solution: Always utilize thread-safe alternatives like CopyOnWriteArrayList for safe concurrent access.

Mistake: Not understanding the performance implications of CopyOnWriteArrayList.

Solution: Avoid excessive write operations, as CopyOnWriteArrayList creates a new array for every write. Limit write frequency for better performance.