1. Introduction

In multithreaded applications, data consistency and thread safety are critical concerns. The CopyOnWriteArrayList class in Java addresses these issues by offering a thread-safe variant of ArrayList.

As described in the official documentation:

“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.”

This article explores how CopyOnWriteArrayList works internally, its performance trade-offs, and common use cases.

2. What is CopyOnWriteArrayList?

CopyOnWriteArrayList is part of the java.util.concurrent package and implements the List interface. It is specifically designed for use in concurrent applications where reads vastly outnumber writes.

Key features:

• Thread-safe for both read and write operations
• Allows null elements
• Maintains insertion order
• Provides fast iteration without ConcurrentModificationException

If you’re unfamiliar with List implementations, check out our Introduction to Collections Framework in Java.

3. Internal Structure

Internally, CopyOnWriteArrayList is backed by an array, similar to ArrayList. However, its uniqueness lies in how it handles updates:

• When a write operation occurs (e.g., add(), remove()), the array is copied entirely.
• The write modifies the new array while readers continue to access the old one.
• Once the write is complete, the new array replaces the old reference.

This design ensures that readers always see a consistent snapshot of the data without needing locks.

CopyOnWriteArrayList<String> data = new CopyOnWriteArrayList<>();
data.add("Alpha");
data.add("Beta");

“Immutability during reads is the secret weapon of CopyOnWriteArrayList.”

4. Performance Characteristics

The class offers excellent performance in read-heavy environments, but can be costly during write operations:

Thus, it should be avoided in scenarios involving frequent modifications.

Compare this with ArrayList and LinkedList to understand trade-offs better.

5. Practical Use Cases

Use CopyOnWriteArrayList when:

• Reads significantly outnumber writes
• You need a thread-safe list without synchronization overhead
• The list size is relatively small

Typical applications include:

• Caching data in concurrent systems
• Observer lists in event-driven applications
• Read-mostly configuration settings

6. Code Examples

Adding Elements Concurrently

In a concurrent environment, CopyOnWriteArrayList allows multiple threads to add elements without locking issues:

CopyOnWriteArrayList<String> users = new CopyOnWriteArrayList<>();
users.add("UserA");
users.add("UserB");
System.out.println("Users: " + users);

Iterating Without ConcurrentModificationException

Even if other threads modify the list, iteration proceeds safely using a snapshot:

for (String user : users) {
    System.out.println(user);
}

Removing Elements Safely

Removing elements also triggers a new copy, ensuring thread isolation:

users.remove("UserA");
System.out.println("After removal: " + users);

7. Limitations and Alternatives

While CopyOnWriteArrayList is powerful for concurrent reads, it has some limitations:

• High memory usage due to frequent array copying
• Inefficient for write-heavy applications
• Not suitable for real-time systems requiring low latency

Alternatives include:

• Collections.synchronizedList() for lower write frequency
• ConcurrentLinkedQueue for FIFO access
• ArrayList for single-threaded use

8. Conclusion

CopyOnWriteArrayList provides a robust solution for thread-safe, read-heavy collections. Its internal copy-on-write mechanism ensures consistency and avoids synchronization overhead, making it ideal for specific concurrent use cases.

“When reads dominate, CopyOnWriteArrayList shines without contention.”

You can find the complete code of this article on GitHub.