1. 简介

在之前写了HashMap源码解析介绍了HashMap这个数据结构，可惜它并不是线程安全的，在多线程情况下最好还是使用ConCurrentHashMap。在JDK8中它使用了CAS+synchronized实现来保证线程安全，而在JDK7中它使用了分段锁的实现方式来保证线程安全。下面我来剖析下它源码：

2. 实现

类继承结构

public class ConcurrentHashMap<K,V> extends AbstractMap<K,V>
    implements ConcurrentMap<K,V>, Serializable {}

属性

// Node数组最大大小
private static final int MAXIMUM_CAPACITY = 1 << 30;
// Node数组最大大小
private static final int DEFAULT_CAPACITY = 16;
// 最大数组大小，只在toArray方法中使用
static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
// 默认并发级别
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
// 负载因子
private static final float LOAD_FACTOR = 0.75f;
// 链表转换为红黑树链表长度的阈值
static final int TREEIFY_THRESHOLD = 8;
// 红黑树转换为链表的阈值
static final int UNTREEIFY_THRESHOLD = 6;
// 链表转换为红黑树数组大小的阈值
static final int MIN_TREEIFY_CAPACITY = 64;

private static final int MIN_TRANSFER_STRIDE = 16;

private static int RESIZE_STAMP_BITS = 16;

// 帮助扩容的最大线程数
private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;

private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;

static final int MOVED     = -1; // forwarding结点的Hash值
static final int TREEBIN   = -2; // 树根结点的Hash值
static final int RESERVED  = -3; // hash for transient reservations
static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash

// 获取可用处理器数量
static final int NCPU = Runtime.getRuntime().availableProcessors();

// Node结点的数组
transient volatile Node<K,V>[] table;

// 
private transient volatile Node<K,V>[] nextTable;

private transient volatile long baseCount;

// < -1表示有-(sizeCtl + 1)个线程在扩容
// == -1表示数组在初始化
// 大于0表示并且数组为初始化表示要初始化数组的大小，否则表示扩容的阈值
private transient volatile int sizeCtl;

private transient volatile int transferIndex;

private transient volatile int cellsBusy;

private transient volatile CounterCell[] counterCells;

private transient KeySetView<K,V> keySet;
private transient ValuesView<K,V> values;
private transient EntrySetView<K,V> entrySet;

结点类型

链表结点类型

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    // 使用volatile修饰来保证变量在多线程下的可见性
    volatile V val;
    volatile Node<K,V> next;

    Node(int hash, K key, V val, Node<K,V> next) {
        this.hash = hash;
        this.key = key;
        this.val = val;
        this.next = next;
    }

    public final K getKey()       { return key; }
    public final V getValue()     { return val; }
    public final int hashCode()   { return key.hashCode() ^ val.hashCode(); }
    public final String toString(){ return key + "=" + val; }
    public final V setValue(V value) {
        throw new UnsupportedOperationException();
    }

    public final boolean equals(Object o) {
        Object k, v, u; Map.Entry<?,?> e;
        return ((o instanceof Map.Entry) &&
                (k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
                (v = e.getValue()) != null &&
                (k == key || k.equals(key)) &&
                (v == (u = val) || v.equals(u)));
    }

    // 在以当前结点开始的链表中寻找含有k这个键的结点
    Node<K,V> find(int h, Object k) {
        Node<K,V> e = this;
        if (k != null) {
            do {
                K ek;
                if (e.hash == h &&
                    ((ek = e.key) == k || (ek != null && k.equals(ek))))
                    return e;
            } while ((e = e.next) != null);
        }
        return null;
    }
}

红黑树结点类型

static final class TreeNode<K,V> extends Node<K,V> {
    TreeNode<K,V> parent;  // red-black tree links
    TreeNode<K,V> left;
    TreeNode<K,V> right;
    TreeNode<K,V> prev;    // needed to unlink next upon deletion
    boolean red;

    TreeNode(int hash, K key, V val, Node<K,V> next,
                TreeNode<K,V> parent) {
        super(hash, key, val, next);
        this.parent = parent;
    }

    // 找出键为k的结点
    Node<K,V> find(int h, Object k) {
        return findTreeNode(h, k, null);
    }

    /**
        * Returns the TreeNode (or null if not found) for the given key
        * starting at given root.
        */
    final TreeNode<K,V> findTreeNode(int h, Object k, Class<?> kc) {
        // 省略代码
    }
}

// 用于存放红黑树的结构
static final class TreeBin<K,V> extends Node<K,V> {
    TreeNode<K,V> root;
    volatile TreeNode<K,V> first; // TreeNode根结点
    volatile Thread waiter;  // 等待线程
    volatile int lockState; // 锁状态
    // values for lockState
    static final int WRITER = 1; // set while holding write lock
    static final int WAITER = 2; // set when waiting for write lock
    static final int READER = 4; // increment value for setting read lock

    static int tieBreakOrder(Object a, Object b) {
        int d;
        if (a == null || b == null ||
            (d = a.getClass().getName().
                compareTo(b.getClass().getName())) == 0)
            d = (System.identityHashCode(a) <= System.identityHashCode(b) ?
                    -1 : 1);
        return d;
    }

    //省略后面大部分代码

构造函数

在构造函数中并不会创建Node数组，而是延迟到第一次put方法时才创建Node数组

public ConcurrentHashMap() {
}

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;
}

public ConcurrentHashMap(int initialCapacity, float loadFactor) {
    this(initialCapacity, loadFactor, 1);
}

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
    // 键值对数量 / 数组大小 <= 加载因子，根据initialCapacity与加载因子求出数组大小
    long size = (long)(1.0 + (long)initialCapacity / loadFactor);
    // 如果数组大小最大等于数组大小则设置数组初始大小为MAXIMUM_CAPACITY，
    // 否则调用tableSizeFor函数求出一个不小于size的第一个2的n次幂的正整数
    int cap = (size >= (long)MAXIMUM_CAPACITY) ?
        MAXIMUM_CAPACITY : tableSizeFor((int)size);
    // 设置要初始化的数组大小
    this.sizeCtl = cap;
}

// 求出一个不小于c的第一个2的n次幂的正整数
private static final int tableSizeFor(int c) {
    int n = c - 1;
    n |= n >>> 1;
    n |= n >>> 2;
    n |= n >>> 4;
    n |= n >>> 8;
    n |= n >>> 16;
    return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}

散列函数

通过高16位与低16位的异或运算得出新的散列值以减少碰撞几率

static final int spread(int h) {
    return (h ^ (h >>> 16)) & HASH_BITS;
}

put方法

//插入键值对，如果键已存在，默认更改其值
public V put(K key, V value) {
    return putVal(key, value, false);
}

/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
    // 键与值不得为空
    if (key == null || value == null) throw new NullPointerException();
    // 计算出键的散列值
    int hash = spread(key.hashCode());
    // 指定数组位置上结点的个数
    int binCount = 0;
    // 通过使用循环直到添加成功退出循环
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh;
        // 如果数组未初始化，则调用initTable方法初始化
        if (tab == null || (n = tab.length) == 0)
            tab = initTable();
            // 否则计算出键所在的桶，如果桶为空则调用casTabAt方法尝试在该位置添加结点
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            // 添加结点成功退出循环，否则表示有其他线程也尝试在此位置插入结点，出现冲突
            if (casTabAt(tab, i, null,
                            new Node<K,V>(hash, key, value, null)))
                break;                   // no lock when adding to empty bin
        }// 如果该位置不为空，并且该结点Hash值为MOVED表示当前正在扩容，调用helpTransfer方法帮助扩容
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
            // 使用synchronized关键字对该头结点进行加锁，只有获取头结点的锁的线程才有资格对该桶的链表或数组添加结点
            synchronized (f) {
                // 再一次判断加锁的结点是否为头结点
                if (tabAt(tab, i) == f) {
                    // 如果头结点Hash值大于等于0则表示该桶存放的是链表，遍历链表添加结点
                    if (fh >= 0) {
                        binCount = 1;
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            // 找到该结点
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                    (ek != null && key.equals(ek)))) {
                                oldVal = e.val;
                                // 判断是否替换旧值
                                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;
                            }
                        }
                    } // 否则判断头结点是否为为TreeBin的实例，如果是则该桶存放的是一棵红黑树
                    else if (f instanceof TreeBin) {
                        Node<K,V> p;
                        binCount = 2;
                        // 调用putTreeVal方法添加结点
                        if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
                                                        value)) != null) {
                            oldVal = p.val;
                            // 判断是否替换旧值
                            if (!onlyIfAbsent)
                                p.val = value;
                        }
                    }
                }
            }

            // 如果binCount不等于0则表示添加结点或替换新值成功
            if (binCount != 0) {
                // 如果结点数量大于等于8，将链表树形化
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                // 如果值不为空直接返回旧值
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    // 添加新结点成功，调用addCount方法判断是否需要扩容
    addCount(1L, binCount);
    return null;
}

initTable方法

这个方法是在调用put方法时数组未初始化时调用的，用于创建初始数组

private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
    // 使用循环来保证初始化成功
    while ((tab = table) == null || tab.length == 0) {
        // 如果sizeCtl小于0表示数组在初始化或者扩容
        if ((sc = sizeCtl) < 0)
            // 调用Thread.yield方法尝试让出CPU给其他相同优先级的线程
            Thread.yield(); // lost initialization race; just spin
            // 否则使用CAS更新SIZECTL为-1，表示要初始化数组
        else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
            try {
                // 再一次判断数组是否未初始化
                if ((tab = table) == null || tab.length == 0) {
                    // 若在构造对象时未给出初始化大小则使用默认大小16
                    int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                    @SuppressWarnings("unchecked")
                    // 创建新数组
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                    table = tab = nt;
                    // 设置下次扩容阈值为当前数组大小的0.75倍
                    sc = n - (n >>> 2);
                }
            } finally {
                // 无论是否初始化成功，更新sizeCtl的值
                sizeCtl = sc;
            }
            break;
        }
    }
    return tab;
}

helpTransfer方法

当发现数组正在扩容时调用helpTransfer方法帮助数组转移结点到新数组

final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
    Node<K,V>[] nextTab; int sc;
    if (tab != null && (f instanceof ForwardingNode) &&
        (nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) {
        int rs = resizeStamp(tab.length);
        while (nextTab == nextTable && table == tab &&
                (sc = sizeCtl) < 0) {
            if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
                sc == rs + MAX_RESIZERS || transferIndex <= 0)
                break;
            if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
                transfer(tab, nextTab);
                break;
            }
        }
        return nextTab;
    }
    return table;
}