In concurrent Java applications, managing collections safely across multiple threads is crucial. When the read operations significantly outnumber write operations, CopyOnWriteArrayList and CopyOnWriteArraySet provide a powerful solution. These collections are designed to eliminate the need for explicit synchronization, offering a snapshot-based, thread-safe mechanism ideal for read-heavy workloads.
This guide explores the internals, performance, use cases, Java version enhancements, and common pitfalls related to CopyOnWriteArrayList and CopyOnWriteArraySet.
Definition and Purpose
Both CopyOnWriteArrayList and CopyOnWriteArraySet are part of the java.util.concurrent package. Their key design principle is copy-on-write, which means any mutation (add, remove, etc.) results in a new copy of the underlying array. This makes them lock-free for reads, which can continue uninterrupted.
They are especially useful for:
- Event listeners
- Read-mostly configurations
- Observer patterns
Java Syntax and Class Structure
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.CopyOnWriteArraySet;
List<String> list = new CopyOnWriteArrayList<>();
list.add("A");
Set<String> set = new CopyOnWriteArraySet<>();
set.add("X");
Internal Working and Memory Model
- Backed by an internal array (
Object[]) - Write operations like
add()orremove()create a fresh array copy - Iterators are snapshot-based and do not reflect concurrent modifications
- No locking is required for reads
Performance and Big-O Complexity
| Operation | Time Complexity | Notes |
|---|---|---|
| add/remove | O(n) | Full array copied on each write |
| get | O(1) | Array index access |
| contains | O(n) | Linear scan |
| iteration | O(n) | Snapshot-based, lock-free |
Real-World Use Cases
- Managing GUI listeners in Swing/JavaFX
- Analytics event tracking (e.g., Google Tag Manager)
- Shared configuration options or access control lists
Functional Programming Support
CopyOnWriteArrayList<String> logEvents = new CopyOnWriteArrayList<>();
logEvents.add("login");
logEvents.add("logout");
logEvents.stream()
.filter(e -> e.startsWith("log"))
.forEach(System.out::println);
Supports Java 8+ features like:
removeIf(Predicate)forEach(Consumer)stream()andparallelStream()
Pros and Cons
✅ Pros
- Thread-safe without explicit locking
- Safe and predictable iteration
- No
ConcurrentModificationException
❌ Cons
- Memory intensive on write-heavy loads
- Slower writes due to full copying
- Not suitable for large datasets with frequent mutations
Anti-Patterns and Misuse
- ❌ Using for high-frequency writes like logging or data ingestion
- ❌ Wrapping mutable elements inside a copy-on-write structure
- ❌ Assuming it behaves like a synchronized collection
Refactoring Legacy Code
Before:
List<String> listeners = Collections.synchronizedList(new ArrayList<>());
After:
List<String> listeners = new CopyOnWriteArrayList<>();
This removes the need for manual synchronization blocks.
Best Practices
- Use only for read-heavy scenarios
- Minimize mutation operations in performance-critical code
- Don’t use
size()orcontains()in tight loops unnecessarily - Avoid nesting copy-on-write collections
📌 What's New in Java Versions
Java 8
- Stream support (
stream(),forEach()) removeIf(Predicate)
Java 9
- Compact memory footprint enhancements
Java 10–17
- Type inference with
var - Better performance with compact strings
Java 21
- Structured concurrency simplifies parallel logic using snapshot collections
- Better efficiency with virtual threads for parallel reads
Conclusion and Key Takeaways
- Copy-on-write collections are a niche but powerful tool
- Favor them for collections with infrequent writes and frequent reads
- Java 8+ methods unlock expressive and safe usage patterns
FAQ
1. Are CopyOnWriteArrayList and ArrayList the same?
No. CopyOnWriteArrayList is thread-safe and copies the array on writes.
2. When should I avoid CopyOnWriteArrayList?
Avoid when you have frequent updates or large datasets.
3. Is CopyOnWriteArraySet a wrapper?
Yes, it’s a wrapper over CopyOnWriteArrayList that enforces uniqueness.
4. Does iteration reflect updates?
No, it’s a snapshot of the array taken at the time of iterator creation.
5. Can I use streams safely?
Yes. Streams operate on a consistent snapshot.
6. How does removeIf work?
Internally copies the array, removes elements, and resets the array reference.
7. Does CopyOnWriteArrayList throw ConcurrentModificationException?
No, its iterators are immune to concurrent modifications.
8. Are they part of the core Java API?
Yes, available since Java 1.5 in java.util.concurrent.
9. Is memory overhead significant?
Yes, every write creates a full new copy of the underlying array.
10. Can I serialize CopyOnWriteArrayList?
Yes, both collections implement Serializable.