ConcurrentHashMap 源码

1.7 构造函数

    // 16 0.75 16public ConcurrentHashMap(int initialCapacity,float loadFactor, int concurrencyLevel) {if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)throw new IllegalArgumentException();if (concurrencyLevel > MAX_SEGMENTS) //1 << 16 65536concurrencyLevel = MAX_SEGMENTS;// Find power-of-two sizes best matching arguments//找到最接近的2的幂次方数int sshift = 0;int ssize = 1;//segment数组的长度while (ssize < concurrencyLevel) {++sshift;  //     1 2 3 [4]  5 取hash的高四位，put时候用高4(sshift)位求segment数组的index值,低4位用于求hashtable[]的index值ssize <<= 1;//左移 2 4 8 [16] 32}this.segmentShift = 32 - sshift; //28this.segmentMask = ssize - 1;// 16 -1(0000 1111) 用来取table index = hash & (ssize -1)if (initialCapacity > MAXIMUM_CAPACITY)//1 << 30 1073741824initialCapacity = MAXIMUM_CAPACITY;int c = initialCapacity / ssize; // 16/16 = 1 每个segment里面的hashEntry数组的数量if (c * ssize < initialCapacity) //initialCapacity=16 初始参数 保证总hashEntry数量大于初始化得数量++c;int cap = MIN_SEGMENT_TABLE_CAPACITY; // MIN_SEGMENT_TABLE_CAPACITY = 2while (cap < c)cap <<= 1;//左移 2的幂次数 hashEntry[]数组的长度也要是2的幂次方// 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]//Unsafe CAS技术this.segments = ss;}

1.7 ConcurrentHashmap.put()

    @SuppressWarnings("unchecked")public V put(K key, V value) {Segment<K,V> s;if (value == null)throw new NullPointerException();int hash = hash(key);//segmentShift默认值是32 -4 =28，int值一共32位 所以取hash值的高四位int j = (hash >>> segmentShift) & segmentMask;//使用hash值的高4位 &segmentMask是segment长度减1if ((s = (Segment<K,V>)UNSAFE.getObject          // Unsafe检查segment时候存在(segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegments = ensureSegment(j); //新建并使用CAS插入segmentreturn s.put(key, hash, value, false);//将键值对插入Segment}

Segment.put()

 Segment.putfinal 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;HashEntry<K,V> first = entryAt(tab, index);//获取第一个节点for (HashEntry<K,V> e = first;;) {//遍历链表if (e != null) {K k;if ((k = e.key) == key ||(e.hash == hash && key.equals(k))) {//找到key相等的HashEntry 并替换value值oldValue = e.value;if (!onlyIfAbsent) {e.value = value;++modCount;}break;}e = e.next;}//到达最后一个节点else {if (node != null)node.setNext(first);elsenode = new HashEntry<K,V>(hash, key, value, first);//创建新节点int c = count + 1;if (c > threshold && tab.length < MAXIMUM_CAPACITY)rehash(node);//扩容这个Segment的hashEntry[]数组elsesetEntryAt(tab, index, node);++modCount;count = c;oldValue = null;break;}}} finally {unlock();}return oldValue;}

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