CopyOnWriteArrayList

CopyOnWriteArrayList is a thread-safe List implementation in the java.util.concurrent package that uses copy-on-write semantics. It stores its elements in an internal array and replaces that array on each mutating operation. It implements List, RandomAccess, Cloneable, and Serializable.

The concurrency model relies on a volatile reference to the internal array and a private write lock. Reads access the array without synchronization and observe a consistent snapshot. Mutating operations acquire the lock, copy the current array, apply the change to the copy, and then update the reference to the new array.

Iterators are snapshot-style: they reflect the state of the list at the moment the iterator was created and do not reflect subsequent modifications. They do not throw ConcurrentModificationException, making iteration safe even while other threads modify the list.

Performance and use: copy-on-write means reads are fast and do not incur synchronization overhead, but writes are expensive and memory-intensive because the entire array is copied for each modification. This makes the class suitable for read-heavy use with relatively small lists.

Limitations and alternatives: due to repeated copying, it is not well-suited for large lists or workloads with frequent modifications. For other concurrency needs, alternatives include synchronized wrappers around ArrayList or more scalable concurrent collections such as ConcurrentLinkedQueue or CopyOnWriteArraySet, depending on the use case.