ConcurrentModificationException in Java
Upasana | November 21, 2020 | 1 min read | 1,287 views | Multithreading and Concurrency
ConcurrentModificationException is raised by fail-fast iterators when the underlying collection is modified structurally during iteration.
In the face of concurrent modification, the fail-fast iterator fails quickly and cleanly by throwing ConcurrentModificationException, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
There are two main scenarios when this exception can occur:
Collection is modified structurally while fail-fast iterator is iterating through it in the same thread.
One thread modifies the structure of collection, while other thread is iterating through it using fail-fast iterator.
There are two ways to handle this scenario:
Do not allow modification in underlying collection during the iteration.
Use synchronization mechanism to prevent any other thread from accessing the collection concurrently.
Do not use
Collection.remove()method instead always use
iterator.remove()method for any structural modification.
Do not use fail-fast iterators, instead use fail-safe iterators using concurrent collections. For example, ConcurrentHashMap instead of HashMap, CopyOnWriteArrayList instead of ArrayList, etc. Fail-safe iterators do not throw this exception.
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