线程安全的并发容器1.7版本的ConcurrentHashMap原理分析（二十二）

private HashEntry scanAndLockForPut(K key, int hash, V value) {HashEntry first &＃61; entryForHash(this, hash);HashEntry e &＃61; first;HashEntry node &＃61; null;int retries &＃61; -1; // negative while locating nodewhile (!tryLock()) {HashEntry f; // to recheck first belowif (retries <0) {if (e &＃61;&＃61; null) {if (node &＃61;&＃61; null) // speculatively create nodenode &＃61; new HashEntry(hash, key, value, null);retries &＃61; 0;}else if (key.equals(e.key))retries &＃61; 0;elsee &＃61; e.next;}else if (&＃43;&＃43;retries > MAX_SCAN_RETRIES) {lock();break;}else if ((retries & 1) &＃61;&＃61; 0 &&(f &＃61; entryForHash(this, hash)) !&＃61; first) {e &＃61; first &＃61; f; // re-traverse if entry changedretries &＃61; -1;}}return node;}

put方法里 rehash(node);

private void rehash(HashEntry node) {HashEntry[] oldTable &＃61; table;int oldCapacity &＃61; oldTable.length;int newCapacity &＃61; oldCapacity <<1;/* oldCapacity*2 */threshold &＃61; (int)(newCapacity * loadFactor);HashEntry[] newTable &＃61;(HashEntry[]) new HashEntry[newCapacity];int sizeMask &＃61; newCapacity - 1;for (int i &＃61; 0; i e &＃61; oldTable[i];if (e !&＃61; null) {HashEntry next &＃61; e.next;int idx &＃61; e.hash & sizeMask;if (next &＃61;&＃61; null) // Single node on listnewTable[idx] &＃61; e;else { // Reuse consecutive sequence at same slotHashEntry lastRun &＃61; e;int lastIdx &＃61; idx;for (HashEntry last &＃61; next;last !&＃61; null;last &＃61; last.next) {int k &＃61; last.hash & sizeMask;if (k !&＃61; lastIdx) {lastIdx &＃61; k;lastRun &＃61; last;}}newTable[lastIdx] &＃61; lastRun;// Clone remaining nodesfor (HashEntry p &＃61; e; p !&＃61; lastRun; p &＃61; p.next) {V v &＃61; p.value;int h &＃61; p.hash;int k &＃61; h & sizeMask;HashEntry n &＃61; newTable[k];newTable[k] &＃61; new HashEntry(h, p.key, v, n);}}}}int nodeIndex &＃61; node.hash & sizeMask; // add the new nodenode.setNext(newTable[nodeIndex]);newTable[nodeIndex] &＃61; node;table &＃61; newTable;}

扩容是新创建了数组&＃xff0c;然后进行迁移数据&＃xff0c;最后再将 newTable 设置给属性table。为了避免让所有的节点都进行复制操作&＃xff1a;由于扩容是基于 2 的幂指来操作&＃xff0c; 假设扩容前某 HashEntry 对应到 Segment 中数组的 index 为 i&＃xff0c;数组的容量为 capacity&＃xff0c;那么扩容后该 HashEntry 对应到新数组中的 index 只可能为 i 或者 i&＃43;capacity&＃xff0c;因此很多HashEntry 节点在扩容前后 index 可以保持不变。

public V remove(Object key) {int hash &＃61; hash(key);Segment s &＃61; segmentForHash(hash);return s &＃61;&＃61; null ? null : s.remove(key, hash, null);}

或&＃xff1a;用key和value做参数

public boolean remove(Object key, Object value) {int hash &＃61; hash(key);Segment s;return value !&＃61; null && (s &＃61; segmentForHash(hash)) !&＃61; null &&s.remove(key, hash, value) !&＃61; null;}

与 put 方法类似&＃xff0c;都是在操作前需要拿到锁&＃xff0c;以保证操作的线程安全性。