JSF Entityconverter ConcurrentModificationException

Question

I'm using a generic entity converter in my Project. I've noticed that sometimes the converter throws a java.util.ConcurrentModificationException There is also a 4 years old Post which has no answer.

I tried to use Iterator instead of a plain for-loop but it didn't helped.

Is there a way to create a thread-safe converter?

@FacesConverter(value = "entityConverter")
public class EntityConverter implements Converter {

    private static Map<Object, String> entities = new WeakHashMap<Object, String>();

    @Override
    public String getAsString(FacesContext context, UIComponent component, Object entity) {
        synchronized (entities) {
            if (!entities.containsKey(entity)) {
                String uuid = UUID.randomUUID().toString();
                entities.put(entity, uuid);
                return uuid;
            } else {
                return entities.get(entity);
            }
        }
    }

    @Override
    public Object getAsObject(FacesContext context, UIComponent component, String uuid) {

        Iterator<Map.Entry<Object, String>> tempIt = entities.entrySet().iterator();
        while (tempIt.hasNext()) {
            Map.Entry<Object, String> nextObj = tempIt.next();
            if (nextObj.getValue().equals(uuid)) {
                return nextObj.getKey();
            }
        }

        return null;
    }

}

score 2 · Accepted Answer · answered May 11 '18 at 07:30

2

Wrap the body of getAsObject in a block synchronized on entities, as in the other method.

answered May 11 '18 at 07:30

Andy Turner

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Thanks alot! I don't know why I didn't tried that first! – Mihawk May 11 '18 at 07:52

Aritz · Answer 2 · 2018-05-15T06:30:42.283

Another way to achieve it is to use Java's ConcurrentMap in order to guarantee thread-safety. In a one-liner:

private static ConcurrentMap<Object, String> entities = new ConcurrentHashMap<>();

@Override
public String getAsString(FacesContext context, UIComponent component, Object entity) {
    return entities.putIfAbsent(entity, UUID.randomUUID().toString());
}

Starting from Java 8, you could also take advantage from computeIfAbsent, which will only calculate the UUID if the key is absent:

return entities.putIfAbsent(entity, 
    entities.computeIfAbsent(k -> UUID.randomUUID().toString()));

Or even enhance it with Guava's caching utilities, if you want to keep weak references.

It would be better to use `computeIfAbsent`, so you only compute a new uuid when necessary. — Andy Turner, May 12 '18 at 10:29

JSF Entityconverter ConcurrentModificationException

2 Answers2