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What it looks like

java.util.ConcurrentModificationException

Extends RuntimeException in java.util.

What it means

Most collections return fail-fast iterators. If the collection changes (an add or remove, not a mere set) after the iterator was created, and not through that iterator, the next call to next() throws ConcurrentModificationException. Despite the name it happens in single-threaded code too, the classic case being a for-each loop that removes from the list it is looping over.

Common causes

• Removing or adding to a list inside a for-each loop over that same list.
• One thread modifying a non-thread-safe collection while another iterates it.
• A callback or listener that changes the collection during iteration.
• Modifying a map while iterating over its keySet() or entrySet().

How to fix it

1. Use list.removeIf(predicate) or map.entrySet().removeIf(...).
2. Remove through the Iterator: it.remove().
3. Iterate over a copy when the loop body must change the original: new ArrayList<>(list).
4. For real concurrency use ConcurrentHashMap or CopyOnWriteArrayList, or synchronise access.

Broken and fixed

for (String s : names) {
    if (s.isEmpty()) names.remove(s); // throws on the next iteration
}

names.removeIf(String::isEmpty);

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In Spring and enterprise code

The fail-fast check is best effort and is not a thread-safety guarantee; do not rely on catching it. Shared state in singleton Spring beans is a typical source of the multi-threaded variant.

Debug it faster

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