是否存在带有getAndWait（）方法的HashMap？例如。一个BlockingConcurrentHashMap实现？

问题描述

许多线程可能会填充 HashMap ，在某些情况下我需要等待（阻塞）直到HashMap中存在一个对象，例如：

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)

想知道这样的东西是否存在，我讨厌重新发明轮子。

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

推荐答案

据我所知，没有转移地图可用。虽然在理论上创建一个并不太困难。

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;
   }
  }

此课程中有明显的排除。没有提到锁粗化;不用说它不会表现出色，但你应该知道发生了什么

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

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