What is the Purpose of CopyOnWriteArrayList ?

The CopyOnWriteArrayList is a thread-safe implementation of the List interface in Java that creates a copy of the array when modifications are made, ensuring safe concurrent access.

Introduction

Java provides various thread-safe collections for handling concurrency in multi-threaded environments. CopyOnWriteArrayList is one such class in java.util.concurrent that allows multiple threads to read data efficiently without requiring explicit synchronization.

Why Use CopyOnWriteArrayList?

In traditional ArrayList, concurrent modifications can lead to ConcurrentModificationException. However, CopyOnWriteArrayList handles this issue by making a copy of the underlying array whenever a modification occurs.

Key Features of CopyOnWriteArrayList

• Thread-safe without external synchronization.
• Optimized for scenarios where reads vastly outnumber writes.
• Modifications (add/remove/set) create a new copy of the array.
• Iterators do not throw ConcurrentModificationException.

Code Example: Basic Usage

import java.util.concurrent.CopyOnWriteArrayList; public class CopyOnWriteExample { public static void main(String[] args) { CopyOnWriteArrayList list = new CopyOnWriteArrayList<>(); // Adding elements list.add("Java"); list.add("Concurrency"); list.add("Thread-safe List"); // Iterating for (String item : list) { System.out.println(item); } } }

Code Example: Handling Concurrency

import java.util.concurrent.CopyOnWriteArrayList; public class ConcurrentAccessDemo { public static void main(String[] args) { CopyOnWriteArrayList list = new CopyOnWriteArrayList<>(); list.add(10); list.add(20); list.add(30); // Creating a thread that modifies the list new Thread(() -> { list.add(40); System.out.println("Element added from Thread 1"); }).start(); // Creating another thread that iterates the list new Thread(() -> { for (Integer num : list) { System.out.println("Reading: " + num); } }).start(); } }

Advantages and Disadvantages

Advantages

• Safe for concurrent read operations.
• Does not require explicit synchronization.
• Efficient when modifications are infrequent.

Disadvantages

• High memory overhead due to copying on modification.
• Not suitable for scenarios with frequent modifications.

When to Use CopyOnWriteArrayList?

Use CopyOnWriteArrayList when:

• Multiple threads need to read data simultaneously.
• Modifications are rare compared to read operations.
• Iterators need to be fail-safe and should not throw exceptions.

Conclusion

The CopyOnWriteArrayList is an excellent choice for read-heavy, multi-threaded environments. It ensures thread safety without requiring explicit synchronization, making it a reliable option for concurrent programming.