Question

Many threads may populate a HashMap, in some cases I need to wait (block) until an object exists in the HashMap, such as:

BlockingConcurrentHashMap map = new BlockingConcurrentHashMap();
Object x = map.getAndWait(key, 1000);    //(object_to_get, max_delay_ms)

Wondering if such a thing exists already, I hate re-inventing wheels.

Answer 1

As far as I know, there is no ‘Transfer Map’ available. Though the creation of one in theory isn’t too difficult.

public class TransferMap<K,V> implements Map<K,V>{
  @GuardedBy("lock")
  private final HashMap<K,V> backingMap = new HashMap<K,V>();

  private final Object lock = new Object();
  public V getAndWait(Object key){
     synchronized(lock){
       V value = null;
         do{
            value = backingMap.get(key);

            if(value == null) lock.wait();

         }while(value == null); 
      }
      return value;
     }
   public V put(K key, V value){
      synchronized(lock){
         V value = backingMap.put(key,value);
         lock.notifyAll();
      }
     return value;
   }
  }

There are obvious exclusions in this class. Not to mentioned the lock coarsening; needless to say it won’t perform great, but you should get the idea of what is going on