Why is a ConcurrentModificationException thrown and how to debug it

java

concurrent-modification-exception

thread-safety

java-collections

byNikita Barsukov·Sep 8, 2024

A ConcurrentModificationException is thrown when elements in a Collection are modified concurrently by multiple threads or when the structure of the collection is altered during an iteration. Avoid the exception by using the Iterator's remove() or utilize thread-safe variants when in a concurrent environment.

List<String> safeList = new ArrayList<>(Arrays.asList("a", "b", "c"));
for (Iterator<String> it = safeList.iterator(); it.hasNext();)
    // Better to axe 'b' out! (Joke: you don't 'b'elong here 'b'uddy)
    if ("b".equals(it.next())) it.remove();

// In a thread jungle, be Tarzan
List<String> tsList = new CopyOnWriteArrayList<>(safeList);
tsList.removeIf("b"::equals); // Thread-safe eviction of 'b'

Defining ConcurrentModificationException

The ConcurrentModificationException stems from the fail-fast behavior of Java collections' iterators, signifying an unexpected structural adjustment not done through the iterator's own methods. Multi-threading adds complexity, thus synchronization is imperative.

Countermeasures to the exception

Harness thread-safe Java Collections, for instance, ConcurrentHashMap.
Enforce synchronization when handling collections in a multi-threaded environment.
For concurrent bonanzas, employ CopyOnWriteArrayList or similar classes.

Best practices and advanced techniques

Even concurrent collections like ConcurrentHashMap can sling a ConcurrentModificationException if improperly used. Here are some tactics to stay clear of it.

Safe modifications during iteration

Put Java 8's removeIf and Predicate to use for safe element eviction during iteration.
Introduce accurate locking and synchronization in multi-threaded access scenarios.
In the debugging stage, scan the stack trace to pinpoint the birthplace of the ConcurrentModificationException.
Confine the scope of the collection references to curb surprise modifications.

With Hibernate, use versioning for an exclusive row-wise DB access.
Tweak the isolation level to shun dirty reads which might be the root cause.
Carefully manage concurrent DB updates and deletions to prevent data integrity issues leading to ConcurrentModificationExceptions.

Iterator types and thread safety

Employ fail-safe iterators, as provided by ConcurrentHashMap.
Note that fail-fast iterators are built for bug detection. An undetected modification is not a licence to assume absence of a ConcurrentModificationException.

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