問題描述

public V get(Object key) {Segment<K,V> s; // manually integrate access methods to reduce overheadHashEntry<K,V>[] tab;int h = hash(key);long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) { for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); e != null; e = e.next) {K k;if ((k = e.key) == key || (e.hash == h && key.equals(k))) return e.value; }}return null; }

為什么Vector的get就需要使用synchronized來加鎖而ConcurrentHashMap則不需要？而且CopyOnWriteArrayList也是使用了寫時復制還能實現讀寫并行，而顯然ConcrrentHashMap并沒有實現寫時復制，它是怎么保證讀寫并行時不會讀取到不一致中間狀態的呢？

問題解答

這篇文章解釋的挺好的