What are fail-safe & fail-fast Iterators in Java

There are two types of iterators in Java: fail-safe and fail-fast.

What does this mean, and is the difference between them?

Answer

What is the difference between them …

“Fail-safe” (in engineering) means that something fails in a way that causes no or minimal damage. Strictly speaking, there is no such thing in Java as a fail-safe iterator. If an iterator fails (in the normal sense of “fail”), you can expect damage to occur.

I suspect that you actually mean “weakly consistent” iterators. The javadoc says:

“Most concurrent Collection implementations (including most Queues) also differ from the usual java.util conventions in that their Iterators and Spliterators provide weakly consistent rather than fast-fail traversal.”

Typically, weak consistency means that if a collection is modified concurrently with an iteration, the guarantees of what the iteration sees are weaker. (The details will be specified in each concurrent collection classes javadocs.)

“Fail-fast” (in systems design) means that the failure condition is checked aggressively so that the failure condition is (where possible¹) detected before too much damage can be done. In Java, a fail-fast iterator fails by throwing a ConcurrentModificationException.

The alternative to “fail-fast” and “weakly consistent” is semantic where the iteration fails unpredictably; e.g. to sometimes give the wrong answer or throw an unexpected exception. (This was the behavior of some standard implementations of the Enumeration API in early versions of Java.)

… and are they different from the iterator we use for collection.

No. These are properties of the iterators implemented by standard Collection types; i.e. they are either “fail fast” or “weakly consistent” … when used correctly with respect to synchronization and the Java memory model¹.

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Fail-fast iterators are typically implemented using a volatile counter on the collection object.

• When the collection is updated, the counter is incremented.
• When an Iterator is created, the current value of the counter is embedded in the Iterator object.
• When an Iterator operation is performed, the method compares the two counter values and throws a CME if they are different.

By contrast, weakly consistent iterators are typically light-weight and leverage properties of each concurrent collection’s internal data structures. There is no general pattern. If you are interested, read the source code for different collection classes.

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^{1 – The rider is that fail-fast iterator behavior assumes that the application is correctly implemented with respect to synchronization and the memory model. (In other words, the application is thread-safe.) For example, if you iterated an ArrayList without proper synchronization, the “fast fail” mechanism should detect the concurrent modification (though that isn’t guaranteed), but may not prevent the list from being corrupted due to the application’s unsafe behavior. To illustrate, the javadoc for Vector.iterator() says this:}

^{“The fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.”}