java ConcurrentHashMap分段加锁提高并发效率

// 表示整个hash表，初始化阶段是在第一次插入的时候，容量总是2的次幂
transient volatile Node<K,V>[] table;
// 下一个使用的表 只有在扩容的时候非空，其他情况都是null
private transient volatile Node<K,V>[] nextTable;
/**
 * Base counter value, used mainly when there is no contention,
 * but also as a fallback during table initialization
 * races. Updated via CAS.
 */
private transient volatile long baseCount;
// 用于初始化和扩容控制
// 0：默认值
// -1:正在初始化
// 大于0：为hash表的阈值
// 小于-1:有多个线程在进行扩容 该值为 -(1+正在扩容的线程数)
private transient volatile int sizeCtl;
/**
 * The next table index (plus one) to split while resizing.
 */
private transient volatile int transferIndex;
/**
 * Spinlock (locked via CAS) used when resizing and/or creating CounterCells.
 */
private transient volatile int cellsBusy;
/**
 * Table of counter cells. When non-null, size is a power of 2.
 */
private transient volatile CounterCell[] counterCells;
// views
private transient KeySetView<K,V> keySet;
private transient ValuesView<K,V> values;
private transient EntrySetView<K,V> entrySet;

/**
 * Creates a new, empty map with the default initial table size (16).
 */
public ConcurrentHashMap() {
}
/**
 * Creates a new, empty map with an initial table size
 * accommodating the specified number of elements without the need
 * to dynamically resize.
 *
 * @param initialCapacity The implementation performs internal
 * sizing to accommodate this many elements.
 * @throws IllegalArgumentException if the initial capacity of
 * elements is negative
 */
public ConcurrentHashMap(int initialCapacity) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException();
    int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
               MAXIMUM_CAPACITY :
               tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
    this.sizeCtl = cap;
}
/**
 * Creates a new map with the same mappings as the given map.
 *
 * @param m the map
 */
public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
    this.sizeCtl = DEFAULT_CAPACITY;
    putAll(m);
}
/**
 * Creates a new, empty map with an initial table size based on
 * the given number of elements ({@code initialCapacity}) and
 * initial table density ({@code loadFactor}).
 *
 * @param initialCapacity the initial capacity. The implementation
 * performs internal sizing to accommodate this many elements,
 * given the specified load factor.
 * @param loadFactor the load factor (table density) for
 * establishing the initial table size
 * @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, 1);
}
/**
 * Creates a new, empty map with an initial table size based on
 * the given number of elements ({@code initialCapacity}), table
 * density ({@code loadFactor}), and number of concurrently
 * updating threads ({@code concurrencyLevel}).
 *
 * @param initialCapacity the initial capacity. The implementation
 * performs internal sizing to accommodate this many elements,
 * given the specified load factor.
 * @param loadFactor the load factor (table density) for
 * establishing the initial table size
 * @param concurrencyLevel the estimated number of concurrently
 * updating threads. The implementation may use this value as
 * a sizing hint.
 * @throws IllegalArgumentException if the initial capacity is
 * negative or the load factor or concurrencyLevel are
 * nonpositive
 */
public ConcurrentHashMap(int initialCapacity,
                         float loadFactor, int concurrencyLevel) {
    if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
        throw new IllegalArgumentException();
    if (initialCapacity < concurrencyLevel)   // Use at least as many bins
        initialCapacity = concurrencyLevel;   // as estimated threads
    long size = (long)(1.0 + (long)initialCapacity / loadFactor);
    int cap = (size >= (long)MAXIMUM_CAPACITY) ?
        MAXIMUM_CAPACITY : tableSizeFor((int)size);
    this.sizeCtl = cap;
}

public V put(K key, V value) {
    return putVal(key, value, false);
}

final V putVal(K key, V value, boolean onlyIfAbsent) {
      // ConcurrentHashMap不允许key和value为null
    if (key == null || value == null) throw new NullPointerException();
      // 计算hash值
    int hash = spread(key.hashCode());
    int binCount = 0;
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh;
          // tab为null，哈希表还没有初始化，进行初始化哈希表
        if (tab == null || (n = tab.length) == 0)
            tab = initTable();
          // 该索引位置为null，表示还没有元素
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
              // 使用CAS的方式添加节点
            if (casTabAt(tab, i, null,
                         new Node<K,V>(hash, key, value, null)))
                break;                   // no lock when adding to empty bin
        }
          // 节点的hash值为-1，表示该哈希表正在扩容
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
              // 对头节点加锁
            synchronized (f) {
                  // 再次判断一下该节点是否为目标索引位置的头节点，防止期间被修改
                if (tabAt(tab, i) == f) {
                      // 表示是普通的链表
                    if (fh >= 0) {
                        binCount = 1;
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                 (ek != null && key.equals(ek)))) {
                                oldVal = e.val;
                                if (!onlyIfAbsent)
                                    e.val = value;
                                break;
                            }
                            Node<K,V> pred = e;
                            if ((e = e.next) == null) {
                                pred.next = new Node<K,V>(hash, key,
                                                          value, null);
                                break;
                            }
                        }
                    }
                      // 红黑树 TreeBin的hash值为TREEBIN，是-2
                    else if (f instanceof TreeBin) {
                        Node<K,V> p;
                        binCount = 2;
                        if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
                                                       value)) != null) {
                            oldVal = p.val;
                            if (!onlyIfAbsent)
                                p.val = value;
                        }
                    }
                }
            }
              // 可以看一下上述的赋值流程
              // 默认初始值是0
              // 链表时为1 在遍历时进行累加，直到找到所要添加的位置为止
              // 红黑树时为2
            if (binCount != 0) {
                  // 链表的长度是否达到8  达到8转为红黑树
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                  // oldVal不为null，表示只是对key的值进行的修改，没有添加元素，直接返回即可
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
      // 
    addCount(1L, binCount);
    return null;
}

static final int spread(int h) {
      // (h ^ (h >>> 16)与hashMap相同
      // HASH_BITS进行与运算
    return (h ^ (h >>> 16)) & HASH_BITS;
}

private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
      // hash表为null时才需要进行初始化
    while ((tab = table) == null || tab.length == 0) {
          // sizeCtl小于0表示有其他线程在进行初始化操作了
        if ((sc = sizeCtl) < 0)
            Thread.yield(); // lost initialization race; just spin
          // 将SIZECTL设为-1，表示该线程要开始初始化表了
        else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
            try {
                if ((tab = table) == null || tab.length == 0) {
                    int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                    @SuppressWarnings("unchecked")
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                    table = tab = nt;
                      // n右移两位  表示1/4n n-1/4n为3/4n  即为n*0.75
                    sc = n - (n >>> 2);
                }
            } finally {
                sizeCtl = sc;
            }
            break;
        }
    }
    return tab;
}

private final void addCount(long x, int check) {
    CounterCell[] as; long b, s;
    if ((as = counterCells) != null ||
        !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
        CounterCell a; long v; int m;
        boolean uncontended = true;
        if (as == null || (m = as.length - 1) < 0 ||
            (a = as[ThreadLocalRandom.getProbe() & m]) == null ||
            !(uncontended =
              U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) {
            fullAddCount(x, uncontended);
            return;
        }
        if (check <= 1)
            return;
        s = sumCount();
    }
    if (check >= 0) {
        Node<K,V>[] tab, nt; int n, sc;
        while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&
               (n = tab.length) < MAXIMUM_CAPACITY) {
            int rs = resizeStamp(n);
            if (sc < 0) {
                if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
                    sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
                    transferIndex <= 0)
                    break;
                if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
                    transfer(tab, nt);
            }
            else if (U.compareAndSwapInt(this, SIZECTL, sc,
                                         (rs << RESIZE_STAMP_SHIFT) + 2))
                transfer(tab, null);
            s = sumCount();
        }
    }
}