final Segment<K,V>[] segments;

 transient volatile HashEntry<K,V>[] table;

 static final class HashEntry<K,V> {final int hash;final K key;volatile V value;volatile HashEntry<K,V> next;//其他省略
}    

Segment(float lf, int threshold, HashEntry<K,V>[] tab) {this.loadFactor = lf;//负载因子this.threshold = threshold;//阈值this.table = tab;//主干数组即HashEntry数组}

 1  public ConcurrentHashMap(int initialCapacity,
 2                                float loadFactor, int concurrencyLevel) {
 3           if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
 4               throw new IllegalArgumentException();
 5           //MAX_SEGMENTS 为1<<16=65536，也就是最大并发数为65536
 6           if (concurrencyLevel > MAX_SEGMENTS)
 7               concurrencyLevel = MAX_SEGMENTS;
 8           //2的sshif次方等于ssize，例:ssize=16,sshift=4;ssize=32,sshif=5
 9          int sshift = 0;
10          //ssize 为segments数组长度，根据concurrentLevel计算得出
11          int ssize = 1;
12          while (ssize < concurrencyLevel) {
13              ++sshift;
14              ssize <<= 1;
15          }
16          //segmentShift和segmentMask这两个变量在定位segment时会用到，后面会详细讲
17          this.segmentShift = 32 - sshift;
18          this.segmentMask = ssize - 1;
19          if (initialCapacity > MAXIMUM_CAPACITY)
20              initialCapacity = MAXIMUM_CAPACITY;
21          //计算cap的大小，即Segment中HashEntry的数组长度，cap也一定为2的n次方.
22          int c = initialCapacity / ssize;
23          if (c * ssize < initialCapacity)
24              ++c;
25          int cap = MIN_SEGMENT_TABLE_CAPACITY;
26          while (cap < c)
27              cap <<= 1;
28          //创建segments数组并初始化第一个Segment，其余的Segment延迟初始化
29          Segment<K,V> s0 =
30              new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
31                               (HashEntry<K,V>[])new HashEntry[cap]);
32          Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
33          UNSAFE.putOrderedObject(ss, SBASE, s0);
34          this.segments = ss;
35      }

 public V put(K key, V value) {Segment<K,V> s;//concurrentHashMap不允许key/value为空if (value == null)throw new NullPointerException();//hash函数对key的hashCode重新散列，避免差劲的不合理的hashcode，保证散列均匀int hash = hash(key);//返回的hash值无符号右移segmentShift位与段掩码进行位运算，定位segmentint j = (hash >>> segmentShift) & segmentMask;if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck(segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegments = ensureSegment(j);return s.put(key, hash, value, false);}

 public V get(Object key) {Segment<K,V> s; HashEntry<K,V>[] tab;int h = hash(key);long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;//先定位Segment，再定位HashEntryif ((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;}

final V put(K key, int hash, V value, boolean onlyIfAbsent) {HashEntry<K,V> node = tryLock() ? null :scanAndLockForPut(key, hash, value);//tryLock不成功时会遍历定位到的HashEnry位置的链表（遍历主要是为了使CPU缓存链表），若找不到，则创建HashEntry。tryLock一定次数后（MAX_SCAN_RETRIES变量决定），则lock。若遍历过程中，由于其他线程的操作导致链表头结点变化，则需要重新遍历。V oldValue;try {HashEntry<K,V>[] tab = table;int index = (tab.length - 1) & hash;//定位HashEntry，可以看到，这个hash值在定位Segment时和在Segment中定位HashEntry都会用到，只不过定位Segment时只用到高几位。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);elsenode = new HashEntry<K,V>(hash, key, value, first);int c = count + 1;　　　　　　　　　　　　　　//若c超出阈值threshold，需要扩容并rehash。扩容后的容量是当前容量的2倍。这样可以最大程度避免之前散列好的entry重新散列，具体在另一篇文章中有详细分析，不赘述。扩容并rehash的这个过程是比较消耗资源的。if (c > threshold && tab.length < MAXIMUM_CAPACITY)rehash(node);elsesetEntryAt(tab, index, node);++modCount;count = c;oldValue = null;break;}}} finally {unlock();}return oldValue;}