Java ConcurrentModificationException：是否可以在迭代时向哈希表添加元素？

Question

I am iterating through a Hashtable and at one point, I add something in to the Hashtable which is clearly giving me a ConcurrentModificationException . I understand why I am getting the error, but is there a way around this such that I could still iterate through the Hashtable and add values simultaneously?

Answer 1

From the docs

The iterators returned by the iterator method of the collections returned by all of this class's "collection view methods" are fail-fast: if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Hashtable's keys and elements methods are not fail-fast.

Note that 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.

If you need this kind of behavior you can safely copy the set of keys and iterate through the copy. Another option if the hashtable is large and copying the keyset is likely to be expensive is to add to a separate collection during the iteration and add the elements of the separate collection post iteration.

Answer 2

You may also want to know about CopyOnWriteSet, which is specifically designed for safe iteration while set is modified. Note that iterator sees only the original set. Any additions will not be visible until next iteration.

http://download.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/CopyOnWriteArraySet.html

This is most useful in many readers / few writers scenario. It is unlikely to be the most efficient solution if reading and writing happens in the same code path.

Answer 3

Make a new Hashtable that you add new entries to; then when you are done iterating, add in the entries from the first table.

Optionally, if you need to, you can skip keys that exist in the original table.

Answer 4

Another alternative would be to use a ConcurrentHashMap instead of a HashMap. However:

• The iterators for a ConcurrentHashMap are defined to return objects reflecting a state some time at or after the creation of the iterator. A more precise statement of the behaviour is in the javadocs for the relevant methods.

• A ConcurrentHashMap is probably slower than a regular HashMap.