如图，ConcurrentHashMap定位一个元素的过程需要进行两次Hash操作。
第一次Hash定位到Segment，第二次Hash定位到元素所在的链表的头部
坏处
这一种结构的带来的副作用是Hash的过程要比普通的HashMap要
好处
写操作的时候可以只对元素所在的Segment进行加锁即可，不会影响到其他的Segment，这样，在最理想的情况下，ConcurrentHashMap可以最高同时支持Segment数量大小的写操作（刚好这些写操作都非常平均地分布在所有的Segment上）。
注：
但是，在统计size的时候，可就是获取hashmap全局信息的时候，就可能需要获取所有的分段锁才能统计。
lock拿不到锁会一直等待。tryLock是去尝试，拿不到就返回false，拿到返回true。
tryLock是可以被打断的，被中断 的，lock是不可以。
在JDK1.7版本中，ConcurrentHashMap的数据结构是由一个Segment数组和多个HashEntry组成，如1.中图所示
Hashtable它把所有方法都加上synchronized关键字来实现线程安全。所有的方法都同步这样造成多个线程访问效率特别低。
HashTable的锁加在整个Hash表上，而ConcurrentHashMap将锁加在segment上（每个段上）
AbstractMap是 Map 接口的的实现类之一，也是 HashMap, TreeMap, ConcurrentHashMap 等类的父类。
ConcurrentMap它是一个接口，是一个能够支持并发访问的java.util.map集合
Serializable:一个对象序列化的接口，一个类只有实现了Serializable接口，它的对象才能被序列化
public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable { //serialVersionUID 用来表明实现序列化类的不同版本间的兼容性 private static final long serialVersionUID = 7249069246763182397L; /** * The default initial capacity for this table,该表的默认初始容量 * used when not otherwise specified in a constructor.在构造函数中未指定时使用 */ static final int DEFAULT_INITIAL_CAPACITY = 16; /** * The default load factor for this table, used when not otherwise specified in a constructor. * 该表的默认加载因子，在构造函数中未指定时使用。 */ static final float DEFAULT_LOAD_FACTOR = 0.75f; /** * The default concurrency level for this table, used when not otherwise specified in a constructor. * 此表的默认并发级别，在构造函数中未指定时使用。 */ static final int DEFAULT_CONCURRENCY_LEVEL = 16; /** * The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments. MUST * be a power of two <= 1<<30 to ensure that entries are indexable * using ints. * 最大容量 */ static final int MAXIMUM_CAPACITY = 1 << 30; /** * The minimum capacity for per-segment tables. Must be a power * of two, at least two to avoid immediate resizing on next use * after lazy construction. * 每个段表的最小容量。必须是2的幂，至少为2，以避免在延迟构造后再次使用时立即调整大小。 */ static final int MIN_SEGMENT_TABLE_CAPACITY = 2; /** * The maximum number of segments to allow; used to bound * constructor arguments. Must be power of two less than 1 << 24. * 允许的最大段数;用于绑定构造函数参数。 */ static final int MAX_SEGMENTS = 1 << 16; // slightly conservative /** * Number of unsynchronized retries in size and containsValue * methods before resorting to locking. This is used to avoid * unbounded retries if tables undergo continuous modification * which would make it impossible to obtain an accurate result. * 在使用锁定之前，在size和containsValue方法上的未同步重试次数。如果表经历了连续的修改，从而无法获得准确的结果，这可以用来避免无边界重试。 * 在size方法和containsValue方法，会优先采用乐观的方式不加锁，直到重试次数达到2，才会对所有Segment加锁 * 这个值的设定，是为了避免无限次的重试。后边size方法会详讲怎么实现乐观机制的。 */ static final int RETRIES_BEFORE_LOCK = 2;/** * Mask value for indexing into segments. The upper bits of a key's hash code are used to choose the segment. * 用于索引段的掩码值，用于根据元素的hash值定位所在的 Segment 下标 */final int segmentMask;/** * Shift value for indexing within segments. * 在段内索引的移位值 */final int segmentShift;/** * The segments, each of which is a specialized hash table. * Segment 组成的数组，每一个 Segment 都可以看做是一个特殊的 HashMap */final Segment<K,V>[] segments;/** * ConcurrentHashMap list entry. Note that this is never exported out as a user-visible Map.Entry. * ConcurrentHashMap列表条目。注意，这永远不会导出为用户可见的Map.Entry。 * HashEntry，存在于每个Segment中，它就类似于HashMap中的Node，用于存储键值对的具体数据和维护单向链表的关系 */static final class HashEntry<K,V> { final int hash; final K key; //value和next都用 volatile 修饰，用于保证内存可见性和禁止指令重排序 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; }static final class Segment<K,V> extends ReentrantLock implements Serializable { private static final long serialVersionUID = 2249069246763182397L; /** * The maximum number of times to tryLock in a prescan before possibly blocking on acquire in preparation for a locked * segment operation. On multiprocessors, using a bounded * number of retries maintains cache acquired while locating * nodes. * 在为锁定段操作做准备而可能阻塞之前，在预扫描中尝试lock的最大次数。在多处理器上，使用有限的重试次数来维护在定位节点时获取的缓存。 */ static final int MAX_SCAN_RETRIES = Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1; /** * The per-segment table. Elements are accessed via * entryAt/setEntryAt providing volatile semantics. * 每个segment中的键值对数组 */ transient volatile HashEntry<K,V>[] table; /** * The number of elements. Accessed only either within locks * or among other volatile reads that maintain visibility. * Segment中的元素个数 */ transient int count; /** * The total number of mutative operations in this segment. * Even though this may overflows 32 bits, it provides * sufficient accuracy for stability checks in CHM isEmpty() * and size() methods. Accessed only either within locks or * among other volatile reads that maintain visibility. * 每次 table 结构修改时，modCount增加1 */ transient int modCount; /** * The table is rehashed when its size exceeds this threshold. * 当表的大小超过这个阈值时，表将被重新散列。 * (The value of this field is always <tt>(int)(capacity * * loadFactor)</tt>.) * segment扩容的阈值 */ transient int threshold; /** * The load factor for the hash table. Even though this value * is same for all segments, it is replicated to avoid needing * links to outer object. * @serial * 加载因子 */ final float loadFactor; //构造函数 Segment(float lf, int threshold, HashEntry<K,V>[] tab) { this.loadFactor = lf; this.threshold = threshold; this.table = tab; }
/** * Creates a new, empty map with a default initial capacity (16), * load factor (0.75) and concurrencyLevel (16). * 创建一个新的空映射，具有默认的初始容量(16)，负载因子(0.75)和并发级别(16)。 */public ConcurrentHashMap() { this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);}/** * Creates a new, empty map with the specified initial capacity, * and with default load factor (0.75) and concurrencyLevel (16). * 使用指定的初始容量创建一个新的空映射，以及默认的负载因子(0.75)和并发级别(16)。 * @param initialCapacity the initial capacity. The implementation * performs internal sizing to accommodate this many elements. * @throws IllegalArgumentException if the initial capacity of * elements is negative. */public ConcurrentHashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);}/** * Creates a new, empty map with the specified initial capacity * and load factor and with the default concurrencyLevel (16). * 使用指定的初始容量，负载因子和默认的concurrencyLevel (16)创建一个新的空映射 * @param initialCapacity The implementation performs internal * sizing to accommodate this many elements. * @param loadFactor the load factor threshold, used to control resizing. * Resizing may be performed when the average number of elements per * bin exceeds this threshold. * @throws IllegalArgumentException if the initial capacity of * elements is negative or the load factor is nonpositive * * @since 1.6 */public ConcurrentHashMap(int initialCapacity, float loadFactor) { this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);}/** * Creates a new map with the same mappings as the given map. * 使用与给定映射相同的映射创建一个新映射。 * The map is created with a capacity of 1.5 times the number * of mappings in the given map or 16 (whichever is greater), * and a default load factor (0.75) and concurrencyLevel (16). * 容量为原map * 1.5倍 和 16 中大的那个，加载因子为0.75，concurrencyLevel为16 * @param m the map */public ConcurrentHashMap(Map<? extends K, ? extends V> m) { //构建新的table this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); //将原映射put进去 putAll(m);}/** * Creates a new, empty map with the specified initial capacity, load factor and concurrency level. * 使用指定的初始容量、负载因子和并发级别创建一个新的空映射。 * 所有的构造函数最终都会调用这个构造函数 * @param initialCapacity the initial capacity. The implementation * performs internal sizing to accommodate this many elements. * @param loadFactor the load factor threshold, used to control resizing. * Resizing may be performed when the average number of elements per * bin exceeds this threshold. * @param concurrencyLevel the estimated number of concurrently * updating threads. The implementation performs internal sizing * to try to accommodate this many threads. * @throws IllegalArgumentException if the initial capacity is * negative or the load factor or concurrencyLevel are * nonpositive. */@SuppressWarnings("unchecked")public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) { //如果加载因子<=0,初始容量为负，并发级别<=0，则抛出异常 if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) throw new IllegalArgumentException(); //并发级别不能大于16 if (concurrencyLevel > MAX_SEGMENTS) concurrencyLevel = MAX_SEGMENTS; // Find power-of-two sizes best matching arguments 找到2次幂大小的最佳匹配参数 //偏移量 //默认concurrencyLevel = 16, 所以ssize在默认情况下也是16,此时 sshift = 4 int sshift = 0; //segmen的大小 int ssize = 1; //找到>concurrencyLevel的最小2次幂 //sshift相当于ssize从1向左移的次数 while (ssize < concurrencyLevel) { ++sshift; ssize <<= 1; } //段偏移量，默认值28 this.segmentShift = 32 - sshift; //掩码 this.segmentMask = ssize - 1; //对初始容量再进行判断 if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; //计算一个segment中数组的数量 int c = initialCapacity / ssize; //向上取整 if (c * ssize < initialCapacity) ++c; //最小分段为2 int cap = MIN_SEGMENT_TABLE_CAPACITY; //同样地，将segment容量取到大于实际需要的最小2次幂 while (cap < c) cap <<= 1; // create segments and segments[0] //创建segment数组，并初始化segmen[0] Segment<K,V> s0 = new Segment<K,V>(loadFactor, (int)(cap * loadFactor), (HashEntry<K,V>[])new HashEntry[cap]); //创建ssize大小的数组 Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];//将obj对象的偏移量为offset的位置修改为value，因为Java中没有内存操作，而Unsafe的这个操作正好补充了内存操作的不足。也可以用于数组操作，比如ConcurrentHashMap中就大量用到了该操作 UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0] this.segments = ss;}/** * Maps the specified key to the specified value in this table. * 将指定的键映射到该表中的指定值。 * Neither the key nor the value can be null. * 键和值都不能为空。 * <p> The value can be retrieved by calling the <tt>get</tt> method * with a key that is equal to the original key. * 可以通过调用get方法检索该值，该方法具有与原始键相等的键 * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt> * @throws NullPointerException if the specified key or value is null *///告诉编译器忽略警告。不用在编译完成后出现警告@SuppressWarnings("unchecked")public V put(K key, V value) { Segment<K,V> s; //如果指定的值为空,抛出异常 if (value == null) throw new NullPointerException(); int hash = hash(key); //一个键值对在Segment数组中下标 int j = (hash >>> segmentShift) & segmentMask; //这里是用Unsafe类的原子操作找到Segment数组中j下标的 Segment 对象 if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck (segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment //返回segment类型，如果不存在则初始化 s = ensureSegment(j); //将键值对通过segment中put方法put，返回值为： return s.put(key, hash, value, false);}final V put(K key, int hash, V value, boolean onlyIfAbsent) { //这里通过tryLock尝试加锁，如果加锁成功，返回null，否则执行 scanAndLockForPut方法 HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value); //保存旧value V oldValue; try { HashEntry<K,V>[] tab = table; //二次哈希计算，求hashentry数组下标 int index = (tab.length - 1) & hash; //找到下标的头结点 HashEntry<K,V> first = entryAt(tab, index); //遍历操作 for (HashEntry<K,V> e = first;;) { //当首结点不为空的时候 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 { //当首结点为空，或者遍历晚时,以下 //node值不为空时，说明调用scanAndLockForPut()方法时，遍历没有找到该节点，创建了新结点给node，“预热” if (node != null) //直接头插法 node.setNext(first); else //新建结点，头插法 node = new HashEntry<K,V>(hash, key, value, first); count加1 int c = count + 1; //当c大于阈值且table长度没达到最大值的时候扩容 if (c > threshold && tab.length < MAXIMUM_CAPACITY) rehash(node); else //否则，将结点插到数组下标为index的位置 setEntryAt(tab, index, node); //增加修改次数 ++modCount; //count也++ count = c; //因为没有旧的value所以设置为null oldValue = null; break; } } } finally { unlock(); } //返回oldvalue return oldValue;}/** * Returns the value to which the specified key is mapped, * or {@code null} if this map contains no mapping for the key. * 返回指定键映射到的值，或是null * <p>More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code key.equals(k)}, * then this method returns {@code v}; otherwise it returns * {@code null}. (There can be at most one such mapping.) * * @throws NullPointerException if the specified key is null */public V get(Object key) { Segment<K,V> s; // manually integrate access methods to reduce overhead 手动集成访问方法以减少开销 HashEntry<K,V>[] tab; //计算hash值 int h = hash(key); //从主存中取出最新的结点 long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; //如果若Segment不为空，且链表也不为空，则遍历查找节点 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; //找到结点，返回value if ((k = e.key) == key || (e.hash == h && key.equals(k))) return e.value; } } //无，返回空 return null;}/** * Returns the segment for the given index, creating it and recording in segment table (via CAS) if not already present. * 返回给定索引的段，创建它并(通过CAS)在段表中记录(如果不存在)。 * @param k the index * @return the segment */@SuppressWarnings("unchecked")////k为 (hash >>> segmentShift) & segmentMask 计算出的segment下标private Segment<K,V> ensureSegment(int k) { final Segment<K,V>[] ss = this.segments; ////u代表 k 的偏移量，用于通过 UNSAFE 获取主内存最新的实际 K 值 long u = (k << SSHIFT) + SBASE; // raw offset Segment<K,V> seg; //从内存中取到最新的下标位置的 Segment 对象，判断是否为空 if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { //如果为空，则按照ss[0]为原型来建造segment Segment<K,V> proto = ss[0]; // use segment 0 as prototype //容量为ss[0]的长度 int cap = proto.table.length; //加载因子也为ss[0]的 float lf = proto.loadFactor; //算出阈值 int threshold = (int)(cap * lf); //再创建Segment 对应的 HashEntry 数组 HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap]; if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { // recheck 再次从内存中取到最新的下标位置的 Segment 对象，判断是否为空 //创建segment对象 Segment<K,V> s = new Segment<K,V>(lf, threshold, tab); //循环检查 u下标位置的 Segment 是否为空 while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { //不为空，说明有其他线程已经创建对象，则用seg保存 //若为空，则当前下标的Segment对象为空，就把它替换为最新创建出来的 s 对象 if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s)) break; } } } //返回segment return seg;}/** * Scans for a node containing given key while trying to * acquire lock, creating and returning one if not found. Upon * return, guarantees that lock is held. UNlike in most * methods, calls to method equals are not screened: Since * traversal speed doesn't matter, we might as well help warm * up the associated code and accesses as well. * put()方法第一步抢锁失败之后，就会执行此方法 * @return a new node if key not found, else null */private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) { //找到HashEntry数组的下标的首结点 HashEntry<K,V> first = entryForHash(this, hash); HashEntry<K,V> e = first; HashEntry<K,V> node = null; //初始化重试次数，为-1 int retries = -1; // negative while locating node //一直尝试抢锁 while (!tryLock()) { HashEntry<K,V> f; // to recheck first below // if (retries < 0) { //首结点为空，预先创建一个新的结点，在hashentry数组上，retries++ if (e == null) { if (node == null) // speculatively create node node = new HashEntry<K,V>(hash, key, value, null); retries = 0; } //如果first有值，并且相对应，则也把retries = 0 else if (key.equals(e.key)) retries = 0; else //不对应的话，就从判断语句开始 //同样的如果空，就新建结点，否则找到该结点，最后retries = 0 e = e.next; } else if (++retries > MAX_SCAN_RETRIES) { //lock拿不到锁会一直等待。tryLock是去尝试，拿不到就返回false，拿到返回true。 lock(); break; } //retries为偶数的时候&1为0，检查在这段时间内first结点是否有改变 else if ((retries & 1) == 0 && (f = entryForHash(this, hash)) != first) { e = first = f; // re-traverse if entry changed如果条目改变了，重新遍历 retries = -1; } } return node;}当 put 方法时，发现元素个数超过了阈值，则会扩容
但是segment互相之间并不影响
/** * Doubles size of table and repacks entries, also adding the given node to new table * 将表的大小增加一倍并重新打包条目，还将给定节点添加到新表中 */@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; //oldCapacity为原表的长度 int oldCapacity = oldTable.length; //新容量为原来的2倍 int newCapacity = oldCapacity << 1; //再计算新的阈值 threshold = (int)(newCapacity * loadFactor); //创建新容量的hashentry HashEntry<K,V>[] newTable = (HashEntry<K,V>[]) new HashEntry[newCapacity]; //哈希表大小掩码 用于计算索引值 int sizeMask = newCapacity - 1; //遍历原表 for (int i = 0; i < oldCapacity ; i++) { //// e 为链表的第一个结点 HashEntry<K,V> e = oldTable[i]; //如果首结点不为空 if (e != null) { //保存e的next结点 HashEntry<K,V> next = e.next; //重新计算e的index int idx = e.hash & sizeMask; //如果next为null，说明此位置没发生哈希冲突，直接将e插入 if (next == null) // Single node on list newTable[idx] = e; else { // Reuse consecutive sequence at same slot 重复使用同一槽位的连续序列 HashEntry<K,V> lastRun = e; int lastIdx = idx; //遍历列表 for (HashEntry<K,V> last = next; last != null; last = last.next) { //计算当前遍历到的节点的新下标 int k = last.hash & sizeMask; //若 k 不等于 lastIdx，则把last更新 if (k != lastIdx) { lastIdx = k; lastRun = last; } } //新表的lastidx位置放入和lastrun index相同的结点 newTable[lastIdx] = lastRun; // Clone remaining nodes 克隆剩余节点 for (HashEntry<K,V> p = e; p != lastRun; p = p.next) { //通过遍历建立新结点的方式 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); } } } } //添加新节点,put方法传入的结点 int nodeIndex = node.hash & sizeMask; // add the new node node.setNext(newTable[nodeIndex]); newTable[nodeIndex] = node; table = newTable;}
/** * Remove; match on key only if value null, else match both. */final V remove(Object key, int hash, Object value) { //抢锁 if (!tryLock()) scanAndLock(key, hash); V oldValue = null; try { HashEntry<K,V>[] tab = table; //找到哈希表对应下标的头结点 int index = (tab.length - 1) & hash; HashEntry<K,V> e = entryAt(tab, index); HashEntry<K,V> pred = null; //如果首结点不为null while (e != null) { K k; //记录next 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) /** * static final <K,V> void setEntryAt(HashEntry<K,V>[] tab, int i, * HashEntry<K,V> e) { * UNSAFE.putOrderedObject(tab, ((long)i << TSHIFT) + TBASE, e); * putOrderedObject: 将这个方法名拆成 put ordered Object */ setEntryAt(tab, index, next); else pred.setNext(next); ++modCount; --count; oldValue = v; } break; //用的Unsafe的方法直接替换数组对应的值（此时的数组对应的空，所以可以直接插入），然后就是解锁，返回旧的值了。 } pred = e; e = next; } } finally { unlock(); } return oldValue;}/** * Returns the number of key-value mappings in this map. * 返回此映射中的键-值映射的数量。 * If the map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns * <tt>Integer.MAX_VALUE</tt>. * * @return the number of key-value mappings in this map */public int size() { // Try a few times to get accurate count. On failure due to // continuous async changes in table, resort to locking. //试几次，得到准确的数字。如果由于表中的连续异步更改而导致失败，则使用锁定。 final Segment<K,V>[] segments = this.segments; int size; boolean overflow; // true if size overflows 32 bits long sum; // sum of modCounts 的和 long last = 0L; // previous sum int retries = -1; // first iteration isn't retry 重试次数 try { for (;;) { //如果超过重试次数，则不再重试，而是把所有Segment都加锁，再统计 size if (retries++ == RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) ensureSegment(j).lock(); // force creation } sum = 0L; size = 0; overflow = false; //遍历所有Segment //先不都锁上，每个段统计count，并记录modcount //最后如果modcount不相等，则重新循环，直到超出最大重试次数 //则强制锁上所有segment，然后统计次数返回 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;}1. ConcurrentHashMap中变量使用final和volatile修饰有什么用呢？
final ：HashEntry里面除了value值不是final修饰的，其他都被final修饰了，所以在HashEntry链表里面添加HashEntry的时候，只能添加到头节点，不能添加到尾节点，因为HashEntry里面的next值是被final修饰的，不能修改。
volatile:来保证某个变量内存的改变对其他线程即时可见，在配合CAS可以实现不加锁对并发操作的支持。
如：get操作可以无锁是由于Node的元素val和指针next是用volatile修饰的，在多线程环境下线程A修改结点的val或者新增节点的时候是对线程B可见的
2. 什么是哈希算法？
3. 为什么用两次hash？
4. hashmap在多线程下的隐患是什么？可以用用什么代替
jdk1.7版本存在put操作时存在丢失数据的情况
jdk1.8版本虽然解决了死循环问题，但是也有数据覆盖问题
可用ConcurrentHashMap代替HashMap
5. 并发问题分析
ConcurrentHashMap的get操作时候，新增，修改，删除都是要考虑并发问题的
。。。
6. segmentShift、segmentMask、sshift、ssize和SBASE关系
一个键值对在Segment数组中下标为：
(hash >>> segmentShift) & segmentMask
其中，