對ConcurrentHashMap的remove操作解析

  ConcurrentHashMap的remove源碼：

/**
     * Removes the key (and its corresponding value) from this map.
     * This method does nothing if the key is not in the map.
     *
     * @param  key the key that needs to be removed
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>
     * @throws NullPointerException if the specified key is null
     */
    public V remove(Object key) {
	int hash = hash(key.hashCode());
        return segmentFor(hash).remove(key, hash, null);
    }

上面的刪除方法：獲取該key的hash值，找到所屬的桶（如果不知道桶可以去看下ConcurrentHashMap的實現原理）,並在桶中執行刪除操作

Segment<K,V>重寫了這個remove方法，源碼：

/**
         * Remove; match on key only if value null, else match both.
         */
        V remove(Object key, int hash, Object value) {
            lock();
            try {
                int c = count - 1;
                HashEntry<K,V>[] tab = table;
                int index = hash & (tab.length - 1);
                HashEntry<K,V> first = tab[index];
                HashEntry<K,V> e = first;
                while (e != null && (e.hash != hash || !key.equals(e.key)))
                    e = e.next;

                V oldValue = null;
                if (e != null) {
                    V v = e.value;
                    if (value == null || value.equals(v)) {
                        oldValue = v;
                        // All entries following removed node can stay
                        // in list, but all preceding ones need to be
                        // cloned.
                        ++modCount;
                        HashEntry<K,V> newFirst = e.next;
                        for (HashEntry<K,V> p = first; p != e; p = p.next)
                            newFirst = new HashEntry<K,V>(p.key, p.hash,
                                                          newFirst, p.value);
                        tab[index] = newFirst;
                        count = c; // write-volatile
                    }
                }
                return oldValue;
            } finally {
                unlock();
            }
        }

上面的解析：首先爲該桶加鎖，通過hash值找到該鏈在桶中的位置（如果不知道鏈可以去看下hashMap的實現原理）,while循環操作是爲了找到需要刪除的元素，類似於不斷循環，當不是當前hashEntry的時候就next,直接找到