| CopyOnWriteArrayList(Introduced in JDK 1.5) | SynchronizedList | |
| Creation | List list = new CopyOnWriteArrayList(); | List list = new ArrayList(); List syncList = Collections.synchronizedList(list); |
| Is Thread Safe? | YES.
CopyOnWriteArrayList is a thread-safe variant of ArrayList. It is designed for concurrent access from multiple threads. CopyOnWriteArrayList provides a thread-safe alternative for ArrayList same way ConcurrentHashMap provides a thread-safe alternative for HashMap and CopyOnWriteArraySet for HashSet. |
YES |
| How Thread Safety is Achieved? | Thread safety is achieved by making a fresh copy of the underlying array with every mutative operations (add, set, and so on). It is evident from the name also Copy on write whenever value is change create a copy. | Locks the SynchronizedList for all the operations on the underlying list.Basically it adds a synchronized block for all the operations.For example:Let us look into the add method of the SynchronizedList:
public void add(int index, E element) { |
| Iterator | fail-safe iterator | fail-fast iterator |
| Performance | CopyOnWriteArrayList implements all mutative operations (add, set, and so on) by making a fresh copy of the underlying array. So there is no additional overhead during a read operation but massive overhead during a write operation. | Very poor performance as the whole list is locked and only a single thread can access it at a given time. |
| Memory Overhead | YES.
Need to make a fresh copy of underlying list for mutative operations like add, set and so on. |
NO. |
| When to use | CopyOnWriteArrayList is a good choice when number of reads is significantly higher than number of writes. | Collections.synchronizedList() is a good choice when writes outnumber reads |
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