Understanding the Role of modCount in Java HashMap's Fail‑Fast Mechanism

Incorrect Conclusions

When searching online for the purpose of the modCount variable in java.util.HashMap , most explanations point to the fail‑fast mechanism.

The fail‑fast strategy means that because HashMap is not thread‑safe, any concurrent modification of the map while an iterator is in use will cause a ConcurrentModificationException . The implementation tracks modifications with the modCount field, copies its value to the iterator's expectedModCount at creation, and compares the two on each iteration; a mismatch triggers the exception. The modCount field is declared volatile to ensure visibility across threads.

This explanation appeared in JDK 5 and JDK 6, where modCount was indeed declared volatile . In JDK 7 and JDK 8 the declaration was removed.

Below are source screenshots for JDK 5, JDK 6, JDK 7 and JDK 8.

1. JDK5 source snapshot

2. JDK6 source snapshot

3. JDK7 source snapshot

4. JDK8 source snapshot

My Thoughts

Does the absence of a volatile declaration in JDK 7/8 mean we no longer need modCount to prevent concurrent modifications during iteration?

My reasoning: the Javadoc for ConcurrentModificationException states that the exception does not always indicate modification by a different thread; it can also be thrown when a single thread violates the collection’s contract, such as modifying a collection while iterating with a fail‑fast iterator.

Note that this exception does not always indicate that an object has
been concurrently modified by a
different
thread.  If a single
thread issues a sequence of method invocations that violates the
contract of an object, the object may throw this exception.  For
example, if a thread modifies a collection directly
while
it is
iterating over the collection with a fail-fast iterator, the iterator
will throw this exception.

Translation:

This exception does not only occur in multithreaded scenarios; it can also be thrown in a single‑threaded context when a fail‑fast iterator detects a structural change.

From this description I conclude:

The exception is not limited to multithreaded cases.

In single‑threaded code, using a fail‑fast iterator and modifying the collection during iteration will also trigger the exception.

The modCount field in HashMap exists to support the second scenario.

Thank you for reading, hope this helps :) Source: blog.csdn.net/dabusiGin/article/details/105483426