ConcurrentHashMap与同步

Question

I am maintaining some legacy code and found some implementation with synchronized key-word on ConcurrentHashMap . It seem unnecessary to me:

public class MyClass{

    private final Map<MyObj, Map<String, List<String>>> conMap = new ConcurrentHashMap<>();

    //...

    //adding new record into conMap:
    private void addToMap(MyObj id, String name, String value){
        conMap.putIfAbsent(id, new ConcurrentHashMap<>());
        Map<String, List<String>> subMap = conMap.get(id);
        synchronized(subMap){                            // <-- is it necessary?
            subMap.putIfAbsent(name, new ArrayList<>());
            subMap.get(name).add(value);
        }
    }

    //...

    public void doSomthing((MyObj id){
        List<Map<String, List<String>>> mapsList = new LinkedList<>();
        for(MyObj objId: conMap.keySet()){              
            if(objId.key1.equals(id.key1)){
                mapsList.add(conMap.get(objId));
            }
        }

        for(Map<String, List<String>> map: mapsList){
            synchronized(map){                       // <-- is it necessary?
                if(timeout <= 0){
                    log(map.size());
                    for(List<String> value: map.values(){
                        log(id, value);
                    }
                }
                else{
                    int sum = 0;
                    for(map.Entry<String, List<String>> val: map.entrySet()){
                        sum += val.getValue().size();
                    }
                    log(sum);
                    map.wait(timeout);
            }
    }

    //...

}

So, is it reasonable to use synchronized key on object that already concurrent? Or those are two different things?

Answer 1

ConcurrentHashMap synchronizes each individual method call itself, so that no other thread can access the map (and possibly break the internal data structure of the map).

Synchronized block synchronizes two or more consecutive method calls, so that no other thread can modify the data structure between the calls (and possibly break the consistency of the data, with regards to the application logic).

Note that the synchornized block only works if all access to the HashMap is performed from synchronized blocks using the same monitor object.

Answer 2

It is sort of necessary, as multiple threads may try to append to the same ArrayList at the same time. The synchonized is protecting against that happening as ArrayList is obviously not synchronized.

Since Java 8 we have computeIfAbsent which means the puts followed by gets they are doing can be simplified. I would write it like this, no synchronization required:

conMap.computeIfAbsent(id, k -> new ConcurrentHashMap<>())
    .computeIfAbsent(name, k -> new CopyOnWriteArrayList<>()) // or other thread-safe list
    .add(value);

Answer 3

In this case:

    synchronized(subMap){                            // <-- is it necessary?
        subMap.putIfAbsent(name, new ArrayList<>());
        subMap.get(name).add(value);
    }

the synchronized is necessary. Without it, you could have two threads simultaneously updating the same ArrayList instance. Since ArrayList is not thread-safe, the addToMap method would not be thread-safe either.

In this case:

        synchronized(map){                       // <-- is it necessary?
            if(/*condition*/){
                log(map.size());
                for(List<String> value: map.values(){
                    log(id, value);
                }
            }
            else{
                int sum = 0;
                for(map.Entry<String, List<String>> val: map.entrySet()){
                    sum += val.getValue().size();
                }
                log(sum);
                map.wait(timeout);
        }

the synchronized is necessary.

• In the if branch, the log method (or something called from it) will probably call ArrayList::toString which will iterate each ArrayList . Without the synchronizing at the submap level, there could be a simultaneous add by another thread (eg an addToMap call). That means that there are memory hazards, and a ConcurrentModificationException may be possible in the toString() method.

• In the else branch, the size() call is accessing a size field in each ArrayList in the submap. Without the synchronizing at the submap level, there could be a simultaneous add on one of those list. That could cause the size() method to return a stale value. In addition, you are not guaranteed to see map entries added to a submap while your are iterating it. If either of those events happen, the sum could be inaccurate. (Whether that is really an issue depends on the requirements for this method: inaccurate counts could be acceptable.)

Answer 4

Other answers don't adequately this bit...

   for(Map<String, List<String>> map: mapsList){
        synchronized(map){                       // <-- is it necessary?
            if(/*condition*/){
                ...iterate over map...
            }
            else {
                ...iterate over map...
            }
        }
   }

Is it necessary? Hard to tell.

What is /*condition*/ ? Does synchronizing on map prevent some other thread A from changing the value of /*condition*/ after thread B has tested it, but before or while thread B is performing either of the two branches? If so, then the synchronized block could be very important.

How about those iterations? Does synchronizing on map prevent some other thread A from changing the contents of the map while thread B is iterating? If so, then the synchronized block could be very important.