在并发哈希图中读写

Question

I have doubt understanding concurrenthashmap . If a section of conc hashmap is locked up by a writer thread then is it possible for a reader thread to read the same section of hashmap map simultaneously ? or does it need the lock to be released from the writer thread ?

Answer 1

If you look at the source code of ConcurrentHashMap.get() then you can easily understand it.

No lock is required while reading from concurrent map but if the value is null it checks again under readValueUnderLock

    get(Object key, int hash) {
         if (count != 0) { // read-volatile
             HashEntry<K,V> e = getFirst(hash);
             while (e != null) {
                 if (e.hash == hash && key.equals(e.key)) {
                     V v = e.value;
                     if (v != null)
                         return v;
                     return readValueUnderLock(e); // recheck
                 }
                 e = e.next;
             }
         }
         return null;
     }

    readValueUnderLock(HashEntry<K,V> e) {
         lock();
         try {
             return e.value;
         } finally {
             unlock();
         }
     }

Look at the ConcurrentHashMap.put() method as well that first calls lock() and finally calls unlock()

    put(K key, int hash, V value, boolean onlyIfAbsent) {
         lock();
         try {
            ...
         } finally {
             unlock();
         }
     }

Answer 2

读取不需要锁。更新地图需要锁。