深入理解ConcurrentHashMap
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说明
HashMap是线程不安全的,在多线程的环境下,操作HashMap会导致线程安全的问题。若使用同步包装器下的HashMap,则会造成很大的性能问题。为此,JDK提供了ConcurrentHashMap来解决此问题。本文通过查看源码和其他优秀博文,对ConcurrentHashMap进行总结。
由于不同版本中的实现有所不同,本文基于JDK 7进行总结。
正文
ConcurrentHashMap采用了分段锁的设计,只有在同一个分段中内才存在竞争关系,所以在进行一般操作时,不需要对整个map加锁。分段锁极大提高了并发下的处理能力。
/* * The basic strategy is to subdivide the table among Segments, * each of which itself is a concurrently readable hash table..... * 由此看出,ConcurrentHashMap底层实现策略是多个段segment,每一个是具有并发可读性的HashTable * /
ConcurrentHashMap的数据结构
ConcurrentHashMap的成员变量
static final int DEFAULT_INITIAL_CAPACITY = 16;//初始容量为16static final float DEFAULT_LOAD_FACTOR = 0.75f;//负载因子为0.75static final int DEFAULT_CONCURRENCY_LEVEL = 16;/*并发度,即同时更新ConcurrentHashMap且不产生锁竞争的最大线程数,也就是分段锁的个数,默认为16.一经指定,便不可改变。如果元素增加导致扩容,也不会增加segment的数量,只会增加segment中数组链表的容量的大小。*/final Segment<K,V>[] segments;
创建Segement时,采用了延迟初始化机制,每次put操作前都要检查key对应的Segment是否为null,若是则调用ensureSegment()确保对应的Segement被创建。ensureSegment可能在并发环境下被调用,但是并未通过锁机制避免竞争,采用了Unsafe对象的getObjectVolatile()方法提供的原子读语义结合CAS来确保Segment创建的原子性。
/* all but one segments are constructed only when first needed (see ensureSegment).*/ if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { // recheck Segment<K,V> s = new Segment<K,V>(lf, threshold, tab); while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s)) break; } }
Segment是继承自ReentrantLock类,所以Segment对象可以充当锁的角色。每个segment中都由一个HashEntry类型的数组,HashEntry对象构成了数组链表,数组的大小为桶的个数。
static final class Segment<K,V> extends ReentrantLock implements Serializable { Segment(float lf, int threshold, HashEntry<K,V>[] tab) { this.loadFactor = lf; this.threshold = threshold; this.table = tab; } ... }
HashEntry的结构
static final class HashEntry<K,V> { final int hash; final K key; volatile V value; volatile HashEntry<K,V> next; HashEntry(int hash, K key, V value, HashEntry<K,V> next) { this.hash = hash; this.key = key; this.value = value; this.next = next; } /** * Sets next field with volatile write semantics. (See above * about use of putOrderedObject.) */ final void setNext(HashEntry<K,V> n) { UNSAFE.putOrderedObject(this, nextOffset, n); }
在HashEntry类中,hash,key声明为final,value,next声明为volatile。(不同于JDK 6版本中将next设置为final,因此无法在链中添加或删除结点,对于put操作,一律添加的链的头部,对于remove操作,需要将删除结点前的所有结点复制,再将复制完的最后一个结点的next设为当前删除结点的下一个结点。)final的不可变性,volatile的保证可见性是ConcurrentHashMap的get操作不需要同步加锁的重要原因,这里使用setNext方法保证了原子性。
ConcurrentHashMap的初始化
public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {//带参数的构造函数:初始容量,负载因子,并发度 if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) throw new IllegalArgumentException(); if (concurrencyLevel > MAX_SEGMENTS) concurrencyLevel = MAX_SEGMENTS; // Find power-of-two sizes best matching arguments int sshift = 0; int ssize = 1; while (ssize < concurrencyLevel) { ++sshift; ssize <<= 1; } /* 参数segmentShitft,segmentMask 在定位segment时使用,segmentShift=32-ssize向左移动的次数,segmentMask=ssize-1.ssize的最大长度为65536,对应的segmentShift最大值为16,segmentMask最大值为65536,对应的二进制16位全为1. */ this.segmentShift = 32 - sshift; this.segmentMask = ssize - 1; if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; int c = initialCapacity / ssize; if (c * ssize < initialCapacity) ++c; int cap = MIN_SEGMENT_TABLE_CAPACITY; while (cap < c) cap <<= 1; // create segments and segments[0] Segment<K,V> s0 = new Segment<K,V>(loadFactor, (int)(cap * loadFactor), (HashEntry<K,V>[])new HashEntry[cap]); Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize]; UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0] this.segments = ss; } public ConcurrentHashMap() {//无参构造函数,使用默认值调用三参数的构造函数 this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); }
Segment中的操作
put操作
在此版本中,put操作使用了自旋锁机制,提高了性能。(因为java线程是映射到系统的线程,阻塞唤醒都需要切换到内核态,频繁的操作会造成严重的性能问题,使用自旋旋可以减轻影响。详见周志明的《深入理解java虚拟机》)
put方法
进入此方法,尝试获取锁,能获取锁则进行下步操作,不能则调用scanAndLockForput()方法。
获取锁后,如果链中能找到与key相等的节点,并且当前执行的put()方法而不是putIfAbsent()方法,记录旧值,更新该结点的值,退出循环,完成put操作。
若没找到,此时需创建新的节点,但是在自旋等待时已经创建好,所以不需要创建,只更新它的next指针即可。这里调用了setNext()方法。
在执行插入操作时,将count值加1,首先会检查本次操作会不会导致segment中节点数量超过阈值threshold,若会,则先进行扩容和rehash操作。(这点与HashMap不同,HashMap的put操作,是先插入,后判断是否超过阈值,决定是否扩容)
final V put(K key, int hash, V value, boolean onlyIfAbsent) { HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value); V oldValue; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash;//定位段中的哪一个桶(segment中HashEntry数组中的某一位置) HashEntry<K,V> first = entryAt(tab, index); for (HashEntry<K,V> e = first;;) { if (e != null) { K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { oldValue = e.value; if (!onlyIfAbsent) { e.value = value; ++modCount; } break; } e = e.next; } else { if (node != null) node.setNext(first); else node = new HashEntry<K,V>(hash, key, value, first); int c = count + 1; if (c > threshold && tab.length < MAXIMUM_CAPACITY) rehash(node); else setEntryAt(tab, index, node); ++modCount; count = c; oldValue = null; break; } } } finally { unlock(); } return oldValue; }
scanAndLockForput方法
当put操作没有获取锁时,不是直接进入等待状态,而是调用此方法,在方法中循环查找链中是否有与key相等的节点,没有,则创建一个新的节点。尝试n次,直到尝试次数超过限制(retries > MAX_SCAN_RETRIES 最大尝试次数,单核为1,多核为64),才真正进入等待状态,即所谓的自旋锁等待。
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) {//若发现链头发生变化,则更新节点链的链头,重置retries值为-1,重新为尝试获取锁而自旋遍历 e = first = f; // re-traverse if entry changed retries = -1; } } return node; }
rehash操作
rehash操作是对某一个段进行操作,创建了一个比原来容量大两倍的数组,然后遍历数组及数组项中的每条链,对于每个节点都重新计算了index,然后创建一个新的节点插入到新的数组中,创建新节点是为了保证其它线程在rehash其间的get操作能够返回正确的值。为了减少新建节点的开销,做了两点优化:1.如果只有一个节点,就直接赋值给新的数组项,若是一个链,先遍历该链找到第一个后面所有节点的index相同的节点p,然后只重新创建节点p以前的节点即可。节点p为头节点的子链直接将p放到新桶中,后面的节点自然就连接上了。
在注释中,我们可以看到,由于扩容是按照2的幂次方进行的,所以新的索引值是原来的或者是原来的加上一个2的幂次方。
@SuppressWarnings("unchecked") 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; }
get操作
调用get方法,根据key值获得对应的value,进入方法后确定哪个段,哪个桶,再遍历链找出对应的value,不存在返回null。
与之前版本不同,在JDK 6中,若在链中找到对应的key值,而对应的value值为空,此时需要加锁重新读。此版本用UNSAFE.getObjectVolatile()避免了这个问题。
public V get(Object key) { Segment<K,V> s; // manually integrate access methods to reduce overhead HashEntry<K,V>[] tab; int h = hash(key.hashCode()); 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; }
remove操作
调用此方法,跟put方法一样,尝试获取锁,不能则进入自旋锁。
找到对应value值,直接更改next。在之前版本JDK 6中,需要创建新链以保证并发时不会出现脏读。next设为volatile和调用方法中使用UNSAFE.putOrderedObject()保证了可见性和原子性,避免了脏读的出现。
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; }
ConcurrentHashMap的操作
put()操作
调用put方法时,先判断value值是否为空。是,抛出异常,由此看出再ConcurrentHashMap中value值不允许为NULL。否,根据key的hashCode值再hash求出hash值,以此hash值再用segmentShift和segmentMask的值定位段(Segment),定位段必须确保段存在,否则调用ensureSegment().
segment数量是2的n次方,根据hash值的高n位就可以确定在哪一个segment。
确定segment后,再调用put方法
/*@throws NullPointerException if the specified key or value is null*/public V put(K key, V value) { Segment<K,V> s; if (value == null) throw new NullPointerException(); int hash = hash(key.hashCode()); int j = (hash >>> segmentShift) & segmentMask; if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck (segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment s = ensureSegment(j); return s.put(key, hash, value, false); } //由此方法可以看出ConcurrentHashMap中key或value值都不允许为null
ConcurrentHashMap中的isEmpty() ,size(),containsValue(),contains()操作都需要全局扫描Map,为减少锁对性能的影响,这里循环查找,并计算两次modCount值,若两次相等,说明两次遍历的过程中,整个的Map没有发生改变,查找的值是正确的,否则将segment逐个加锁计算。
public int size() { // Try a few times to get accurate count. On failure due to // continuous async changes in table, resort to locking. final Segment<K,V>[] segments = this.segments; int size; boolean overflow; // true if size overflows 32 bits long sum; // sum of modCounts long last = 0L; // previous sum int retries = -1; // first iteration isn't retry try { for (;;) { if (retries++ == RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) ensureSegment(j).lock(); // force creation } sum = 0L; size = 0; overflow = false; for (int j = 0; j < segments.length; ++j) { Segment<K,V> seg = segmentAt(segments, j); if (seg != null) { sum += seg.modCount; int c = seg.count; if (c < 0 || (size += c) < 0) overflow = true; } } if (sum == last) break; last = sum; } } finally { if (retries > RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) segmentAt(segments, j).unlock(); } } return overflow ? Integer.MAX_VALUE : size; }
putIfAbsent、replace、remove、clear操作与put方法类似,它们在Segment中都实现,只需要通过hash值找到Segment,然后调用相应方法即可。
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