Java ConcurrentHashMap and for each loop

问题

Supposed I have the following ConcurrentHashMap:

ConcurrentHashMap<Integer,String> identificationDocuments = new ConcurrentHashMap<Integer,String>();
        
identificationDocuments.put(1, "Passport");
identificationDocuments.put(2, "Driver's Licence");

How would I safely iterate over the map with a for each loop and append the value of each entry to a string?

回答1:

Iterators produced by a ConcurrentHashMap are weakly consistent. That is:

they may proceed concurrently with other operations

they will never throw ConcurrentModificationException

they are guaranteed to traverse elements as they existed upon construction exactly once, and may (but are not guaranteed to) reflect any modifications subsequent to construction.

The last bullet-point is pretty important, an iterator returns a view of the map at some point since the creation of the iterator, to quote a different section of the javadocs for ConcurrentHashMap:

Similarly, Iterators, Spliterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration.

So when you loop through a keyset like the following, you need to double check if the item still exists in the collection:

for(Integer i: indentificationDocuments.keySet()){
    // Below line could be a problem, get(i) may not exist anymore but may still be in view of the iterator
    // someStringBuilder.append(indentificationDocuments.get(i));
    // Next line would work
    someStringBuilder.append(identificationDocuments.getOrDefault(i, ""));
}

The act of appending all the strings to the StringBuilder itself is safe, as long as you are doing it on one thread or have encapsulated the StringBuilder entirely in a thread-safe manner.

回答2:

I don't know If you are really asking this, but to iterate over any map you iterate over the keySet()

StringBuffer result = new StringBuffer("");

for(Integer i: indentificationDocuments.keySet()){
        result.append(indentificationDocuments.get(i));
}

return result.toString();

来源：https://stackoverflow.com/questions/31275265/java-concurrenthashmap-and-for-each-loop

标签

java

thread-safety