java.util.concurrent.ConcurrentHashMap
2012-02-28 11:48
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一、简介
ConcurrentHashMap是Map的一种并发实现,在该类中元素的read操作都是无锁了,而write操作需要被同步。这非常适合于读操作远大于写操作的情况。在实现过程中,ConcurrentHashMap将所有元素分成了若干个segment,每个segment是独立的,在一个segment上加锁并不影响其他segment的操作。segment本身是一个hashtable,对于一个加入ConcurrentHashMap的<key, value>对,key的hash值中的高位被用来索引segment,而低位用于segment中的索引。二、segment实现
segment是ConcurrentHashMap存储元素的基本段,它本身是一个hashtable的实现,read操作时无锁的,write需要同步,定义如下:public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable { /** * key, hash, next都是不可改的 * value值可被重写 */ static final class HashEntry<K,V> { final K key; final int hash; volatile V value; final HashEntry<K,V> next; ... } static final class Segment<K,V> extends ReentrantLock implements Serializable { transient volatile int count; transient volatile HashEntry[] table; // 当segment中元素个数达到threshold时,需要rehash transient int threshold; } ... }
segment的read操作:
static final class Segment<K,V> extends ReentrantLock implements Serializable { HashEntry<K,V> getFirst(int hash) { HashEntry[] tab = table; return (HashEntry<K,V>) tab[hash & (tab.length - 1)]; } V get(Object key, int hash) { // 该操作是无锁的 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; } ...
由于HashEntry当中的key和next都是final的,所以segment之上的操作不可能影响HashEntry列表之间相对的顺序,而value是可变的,当第一次读值失败时,尝试加锁读。
segment的replace操作:
static final class Segment<K,V> extends ReentrantLock implements Serializable { /** * replace操作是就地替换,HashEntry的value是非final的 */ boolean replace(K key, int hash, V oldValue, V newValue) { lock(); // replace操作是同步的 try { // 得到该hash值对应的entry列表 HashEntry<K,V> e = getFirst(hash); while (e != null && (e.hash != hash || !key.equals(e.key))) e = e.next; boolean replaced = false; if (e != null && oldValue.equals(e.value)) { // 替换 replaced = true; e.value = newValue; } return replaced; } finally { unlock(); } } ... }
segmet的put操作:
static final class Segment<K,V> extends ReentrantLock implements Serializable { V put(K key, int hash, V value, boolean onlyIfAbsent) { lock(); // put是同步的 try { int c = count; if (c++ > threshold) // ensure capacity rehash(); HashEntry[] tab = table; int index = hash & (tab.length - 1); HashEntry<K,V> first = (HashEntry<K,V>) tab[index]; HashEntry<K,V> e = first; while (e != null && (e.hash != hash || !key.equals(e.key))) e = e.next; V oldValue; if (e != null) { // 已存在则更新 oldValue = e.value; if (!onlyIfAbsent) e.value = value; } else { // 新添加则加入列表头部 oldValue = null; ++modCount; // HashEntry的next是只读的,新加入的entry只能放在头部 tab[index] = new HashEntry<K,V>(key, hash, first, value); count = c; // write-volatile } return oldValue; } finally { unlock(); } } ... }
segment的remove操作一种copy on write 的方法,保留被删元素之后的列表,copy被删元素之前的hashEntry:
static final class Segment<K,V> extends ReentrantLock implements Serializable { V remove(Object key, int hash, Object value) { lock(); try { int c = count - 1; HashEntry[] tab = table; int index = hash & (tab.length - 1); HashEntry<K,V> first = (HashEntry<K,V>)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)) { // copy on write oldValue = v; ++modCount; // e之后的列表可以保留,只需要重新创建e之前的HashEntry即可 HashEntry<K,V> newFirst = e.next; // copy on write e之前的HashEntry 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(); } } ... }
segment的rehash操作实现比较特别,为了保证rehash过程中copy的元素尽可能少,segment在rehash时Entry入口的个数是以2的倍数增长,这可以保证一个entry在rehash之后要么在原来的列表中,要么在下一个列表中:
static final class Segment<K,V> extends ReentrantLock implements Serializable { void rehash() { // 局部变量引用table HashEntry[] oldTable = table; int oldCapacity = oldTable.length; if (oldCapacity >= MAXIMUM_CAPACITY) return; // 右移1位相当于乘以2 HashEntry[] newTable = new HashEntry[oldCapacity << 1]; threshold = (int)(newTable.length * loadFactor); int sizeMask = newTable.length - 1; for (int i = 0; i < oldCapacity ; i++) { // 第i个entry列表 HashEntry<K,V> e = (HashEntry<K,V>)oldTable[i]; if (e != null) { HashEntry<K,V> next = e.next; // 在新table上的索引 int idx = e.hash & sizeMask; if (next == null) newTable[idx] = e; else { // 寻找该entry列表末端,rehash之后idx相同的元素 // 这些元素不需要被copy 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; } } // 将lastRun之后的整个列表挂到新位置上 newTable[lastIdx] = lastRun; // Clone all remaining nodes for (HashEntry<K,V> p = e; p != lastRun; p = p.next) { int k = p.hash & sizeMask; HashEntry<K,V> n = (HashEntry<K,V>)newTable[k]; newTable[k] = new HashEntry<K,V>(p.key, p.hash, n, p.value); } } } } table = newTable; } ... }
三、ConcurrentHashMap方法实现
ConcurrentHashMap在Segment的基础上,通过首先将<key, value>对hash到一个segment,再由segment实现对entry的管理。ConcurrentHashMap的get实现:
public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable { final Segment<K,V> segmentFor(int hash) { return (Segment<K,V>) segments[(hash >>> segmentShift) & segmentMask]; } public V get(Object key) { int hash = hash(key); // throws NullPointerException if key null return segmentFor(hash).get(key, hash); } ... }
ConcurrentHashMap的put和get方法:
public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable { public V put(K key, V value) { if (value == null) throw new NullPointerException(); int hash = hash(key); return segmentFor(hash).put(key, hash, value, false); } public V remove(Object key) { int hash = hash(key); return segmentFor(hash).remove(key, hash, null); } ... }
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