标签：
1.6的解释已经很多了,昨天本来做好的文章就不忍拿出来献丑了
这篇链接就不错 http://www.infoq.com/cn/articles/ConcurrentHashMap
本文就简单叙述1.7的更改部分.
1:最明显的，采用了尝试自旋锁的机制（多核情况下尝试自旋64次(ps:put的时候，自旋会预创建)，还是不行再锁）
典型代码
private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) { HashEntry<K,V> first = entryForHash(this, hash); HashEntry<K,V> e = first; HashEntry<K,V> node = null; int retries = -1; // negative while locating node while (!tryLock()) { HashEntry<K,V> f; // to recheck first below if (retries < 0) { if (e == null) { if (node == null) // speculatively create node node = new HashEntry<K,V>(hash, key, value, null); retries = 0; } else if (key.equals(e.key)) retries = 0; else e = e.next; } else if (++retries > MAX_SCAN_RETRIES) { lock(); break; } else if ((retries & 1) == 0 && (f = entryForHash(this, hash)) != first) { e = first = f; // re-traverse if entry changed retries = -1; } } return node; }
2:解决了弱一致性的问题,1.6使用volatile类型的数组，改变数组元素的值是直接操作数组,在并发中存在一致性问题，到1.7变为UNSAFE.getObjectVolatile和UNSAFE.putOrderedObject
典型代码
1 public V get(Object key) { 2 Segment<K,V> s; // manually integrate access methods to reduce overhead 3 HashEntry<K,V>[] tab; 4 int h = hash(key); 5 long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; 6 if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null && 7 (tab = s.table) != null) { 8 for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile 9 (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); 10 e != null; e = e.next) { 11 K k; 12 if ((k = e.key) == key || (e.hash == h && key.equals(k))) 13 return e.value; 14 } 15 } 16 return null; 17 }
3:remove和rehash操作优化，优化思路暂时没研究，但remove方法现在不需要拷贝remove前的数据了---待补充
remove代码
final V remove(Object key, int hash, Object value) { if (!tryLock()) scanAndLock(key, hash); V oldValue = null; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash; HashEntry<K,V> e = entryAt(tab, index); HashEntry<K,V> pred = null; while (e != null) { K k; HashEntry<K,V> next = e.next; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { V v = e.value; if (value == null || value == v || value.equals(v)) { if (pred == null) setEntryAt(tab, index, next); else pred.setNext(next); ++modCount; --count; oldValue = v; } break; } pred = e; e = next; } } finally { unlock(); } return oldValue; }
rehash代码
private void rehash(HashEntry<K,V> node) { /* * Reclassify nodes in each list to new table. Because we * are using power-of-two expansion, the elements from * each bin must either stay at same index, or move with a * power of two offset. We eliminate unnecessary node * creation by catching cases where old nodes can be * reused because their next fields won‘t change. * Statistically, at the default threshold, only about * one-sixth of them need cloning when a table * doubles. The nodes they replace will be garbage * collectable as soon as they are no longer referenced by * any reader thread that may be in the midst of * concurrently traversing table. Entry accesses use plain * array indexing because they are followed by volatile * table write. */ HashEntry<K,V>[] oldTable = table; int oldCapacity = oldTable.length; int newCapacity = oldCapacity << 1; threshold = (int)(newCapacity * loadFactor); HashEntry<K,V>[] newTable = (HashEntry<K,V>[]) new HashEntry[newCapacity]; int sizeMask = newCapacity - 1; for (int i = 0; i < oldCapacity ; i++) { HashEntry<K,V> e = oldTable[i]; if (e != null) { HashEntry<K,V> next = e.next; int idx = e.hash & sizeMask; if (next == null) // Single node on list newTable[idx] = e; else { // Reuse consecutive sequence at same slot HashEntry<K,V> lastRun = e; int lastIdx = idx; for (HashEntry<K,V> last = next; last != null; last = last.next) { int k = last.hash & sizeMask; if (k != lastIdx) { lastIdx = k; lastRun = last; } } newTable[lastIdx] = lastRun; // Clone remaining nodes for (HashEntry<K,V> p = e; p != lastRun; p = p.next) { V v = p.value; int h = p.hash; int k = h & sizeMask; HashEntry<K,V> n = newTable[k]; newTable[k] = new HashEntry<K,V>(h, p.key, v, n); } } } } int nodeIndex = node.hash & sizeMask; // add the new node node.setNext(newTable[nodeIndex]); newTable[nodeIndex] = node; table = newTable; }
4:增加了replace方法:如果key不存在，则直接返回false ，而非执行插入操作
final boolean replace(K key, int hash, V oldValue, V newValue) { if (!tryLock()) scanAndLock(key, hash); boolean replaced = false; try { HashEntry<K,V> e; for (e = entryForHash(this, hash); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { if (oldValue.equals(e.value)) { e.value = newValue; ++modCount; replaced = true; } break; } } } finally { unlock(); } return replaced; }
并发容器 concurrentHashMap--1.7 更改
标签：
原文地址：http://www.cnblogs.com/duchanggang/p/4638825.html
