HashMap ConcurrentModificationException同时初始化另一个HashMap

Question

In a multi-threaded code, I am seeing ConcurrentModificationException on line 3

line 1:   Map<String, String> attributMap = new HashMap<>();
line 2:   if(attributeMap.size() > 0)
line 3:       tagMyEvent(new HashMap<>(attributeMap));

java.util.ConcurrentModificationException
    1   at java.util.HashMap$HashIterator.nextEntry(HashMap.java:851)
    2   at java.util.HashMap$EntryIterator.next(HashMap.java:891)
    3   at java.util.HashMap$EntryIterator.next(HashMap.java:890)
    4   at java.util.HashMap.putAllForCreate(HashMap.java:485)
    5   at java.util.HashMap.<init>(HashMap.java:257)
!   6   at tagMyEvent (test.java:line 3)  

Only one reason for this crash I could guess is:

• attributMap is being modified when the new HashMap<>(attributMap) is being created.

Will changing of the above code to this, solve the problem:

line 1:   Map<String, String> attributMap = new HashMap<>();
line 2:   if(attributeMap.size() > 0)
line 3:       tagMyEvent(new ConcurrentHashMap<>(attributeMap));

OR

line 1:   Map<String, String> attributMap = new ConcurrentHashMap<>();
line 2:   if(attributeMap.size() > 0)
line 3:       tagMyEvent(new ConcurrentHashMap<>(attributeMap));

If not, can someone suggest a solution or throw some light on what exactly is causing this problem.

Thanks in advance.

Answer 1

If anything, "attributeMap" is the one that needs to be concurrent - because if it's just a regular HashMap, it would still refuse to be iterated upon inside "new ConcurrentHashMap", and we've achieved nothing...

That said, I'd also kindly suggest

1. Consider whether ConcurrentHashMap is really good enough for your needs, that depends what other operations you run on it, and what's the level of cooperation between threads, or how immediately they should see each other's changes...
2. I assume we only saw a partial code snippet (obviously you're not initializing "attributeMap" locally in a method, because then there would be no concurrency problem). So consider testing for other problems as well - eg if the complete code does some additional manipulation that was omitted from this post for simplicity. Remember that some manipulations could yield a ConcurrentModificationException even if you only have a single thread...