ConcurrentHashMap源码解析

面试常见问题

ConcurrentHashMap实现原理

ConcurrentHashMap如何保证线程安全

本文基于JDK1.8

一、构造方法和基本属性

JDK8中ConcurrentHashMap参考了JDK8 HashMap的实现，构造方法和基本属性与HashMap大致相同，可参考HashMap源码解,以下主要列举不同的地方。

/**
 * Encodings for Node hash fields. See above for explanation.
 */
static final int MOVED     = -1; // hash for forwarding nodes
static final int TREEBIN   = -2; // hash for roots of trees
static final int RESERVED  = -3; // hash for transient reservations
// Hash节点正常可用位
static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash

二、主要方法解析

2.1 spread(int h)

/**
 * 这里的spread方法作用与HashMap中的hash方法相同，//先对低16位进行扰动处理，然后屏蔽符号位，结果为32      * 位int型非负数
 */
static final int spread(int h) {
    return (h ^ (h >>> 16)) & HASH_BITS;
}

2.2 tabAt(Node<K,V>[] tab, int i)

/**
 * 该方法通过Unsafe类直接进行内存寻址定位,用来返回节点数组的指定位置的节点的原子操作
 */
static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
    return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
}

2.3 casTabAt(Node<K,V>[] tab, int i, Node<K,V> c, Node<K,V> v)

/**
 * 比较交换（CAS）
 */
static final <K,V> boolean casTabAt(Node<K,V>[] tab, int i, Node<K,V> c, Node<K,V> v) {
    return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
}

2.4 setTabAt(Node<K,V>[] tab, int i, Node<K,V> v)

/**
 * 
 */   
static final <K,V> void setTabAt(Node<K,V>[] tab, int i, Node<K,V> v) {
    U.putObjectVolatile(tab, ((long)i << ASHIFT) + ABASE, v);
}

2.5 put(K key, V value)

/**
 * Maps the specified key to the specified value in this table.
 * Neither the key nor the value can be null.(键值都不能为空)
 *
 * <p>The value can be retrieved by calling the {@code get} method
 * with a key that is equal to the original key.
 *
 * @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 {@code key}, or
 *         {@code null} if there was no mapping for {@code key}
 * @throws NullPointerException if the specified key or value is null
 */
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) {
    // 键值都不能为null,否则抛出NPE异常
    if (key == null || value == null) throw new NullPointerException();
    int hash = spread(key.hashCode());
    int binCount = 0;
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; 
        int n, i, fh;
        if (tab == null || (n = tab.length) == 0)
            tab = initTable();
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            // 如果i位置没有数据，则比较并交换
            if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value, null)))
                break; // no lock when adding to empty bin
        }
        /*
         * 如果检测到某个节点的hash值是MOVED，则表示正在进行数组扩张的数据复制阶段，
         * 则当前线程也会参与去复制，通过允许多线程复制的功能，以此来减少数组的复制所带来的性能损失
         */
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
            // 如果在这个位置有元素的话，就采用synchronized的方式加锁
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    if (fh >= 0) {// 标识该节点为列表节点
                        binCount = 1;
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            // 如果key相同，则覆盖旧value值
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                 (ek != null && key.equals(ek)))) {
                                oldVal = e.val;
                                if (!onlyIfAbsent)
                                    e.val = value;
                                break;
                            }
                            // 插入列表尾部
                            Node<K,V> pred = e;
                            if ((e = e.next) == null) {
                                pred.next = new Node<K,V>(hash, key, value, null);
                                break;
                            }
                        }
                    }
                    else if (f instanceof TreeBin) {
                        Node<K,V> p;
                        binCount = 2;
                        // 红黑树结构旋转插入
                        if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key, value)) != null) {
                            oldVal = p.val;
                            if (!onlyIfAbsent)
                                p.val = value;
                        }
                    }
                }
            }
            if (binCount != 0) {
                // 如果列表长度大于等于8则转换为红黑树
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    // 统计size，并检测是否需要扩容
    addCount(1L, binCount);
    return null;
}

2.6 initTable()

/**
 * Initializes table, using the size recorded in sizeCtl.
 */
private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; 
    int sc;
    // 如果table为null则进行初始化
    while ((tab = table) == null || tab.length == 0) {
        //sizeCtl<0表示其他线程已经在初始化了或者扩容了，挂起当前线程
        if ((sc = sizeCtl) < 0)
            Thread.yield(); // lost initialization race; just spin
        //CAS操作SIZECTL为-1，表示初始化状态
        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;
                    // 初始化后，sizeCtl长度为数组长度的3/4
                    sc = n - (n >>> 2);
                }
            } finally {
                sizeCtl = sc;
            }
            break;
        }
    }
    return tab;
}

2.7 treeifyBin(Node<K,V>[] tab, int index)

/**
 * Replaces all linked nodes in bin at given index unless table is
 * too small, in which case resizes instead.
 * 当数组长度小于64的时候，扩张数组长度一倍，否则的话把链表转为树
 */
private final void treeifyBin(Node<K,V>[] tab, int index) {
    Node<K,V> b; int n, sc;
    if (tab != null) {
        if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
            // 数组长度小于64，则扩容1倍
            tryPresize(n << 1);
        else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
            synchronized (b) {//使用synchronized同步器，将该节点出的链表转为红黑树
                if (tabAt(tab, index) == b) {
                    TreeNode<K,V> hd = null, tl = null;
                    for (Node<K,V> e = b; e != null; e = e.next) {
                        // 将列表节点转换为红黑树节点
                        TreeNode<K,V> p = new TreeNode<K,V>(e.hash, e.key, e.val, null, null);
                        if ((p.prev = tl) == null)
                            hd = p;
                        else
                            tl.next = p;
                        tl = p;
                    }
                    setTabAt(tab, index, new TreeBin<K,V>(hd));
                }
            }
        }
    }
}

2.8 tryPresize(int size)

/**
 * Tries to presize table to accommodate the given number of elements.
 *
 * @param size number of elements (doesn't need to be perfectly accurate)
 */
private final void tryPresize(int size) {
    // 如果指定容量大小大于等于最大容量的一半，则设置为最大容量，否则扩大为size的两倍
    int c = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY :
        tableSizeFor(size + (size >>> 1) + 1);
    int sc;
    while ((sc = sizeCtl) >= 0) {
        Node<K,V>[] tab = table; int n;
        // 如果还未初始化，则进行初始化
        if (tab == null || (n = tab.length) == 0) {
            n = (sc > c) ? sc : c;
            // 初始化tab的时候，把sizeCtl设为-1
            if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
                try {
                    if (table == tab) {
                        @SuppressWarnings("unchecked")
                        Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                        table = nt;
                        sc = n - (n >>> 2);
                    }
                } finally {
                    sizeCtl = sc;
                }
            }
        }
         /*
         * 一直扩容到的c小于等于sizeCtl或者数组长度大于最大长度的时候，则退出
         * 所以在一次扩容之后，不是原来长度的两倍，而是2的n次方倍
         */
        else if (c <= sc || n >= MAXIMUM_CAPACITY)
            break;
        else if (tab == table) {
            int rs = resizeStamp(n);
            if (sc < 0) {
                Node<K,V>[] nt;
                if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
                    sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
                    transferIndex <= 0)
                    break;
                /*
                 * transfer的线程数加一,该线程将进行transfer的帮忙
                 * 在transfer的时候，sc表示在transfer工作的线程数
                 */
                if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
                    transfer(tab, nt);
            }
            /*
             * 没有在初始化或扩容，则开始扩容
             */
            else if (U.compareAndSwapInt(this, SIZECTL, sc,(rs << RESIZE_STAMP_SHIFT) + 2))
                transfer(tab, null);
        }
    }
}

2.9 transfer(Node<K,V>[] tab, Node<K,V>[] nextTab)

/**
 * Moves and/or copies the nodes in each bin to new table. See
 * above for explanation.
 */
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
    int n = tab.length, stride;
    if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
        stride = MIN_TRANSFER_STRIDE; // subdivide range
    if (nextTab == null) {            // initiating
        try {
            @SuppressWarnings("unchecked")
            Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1];
            nextTab = nt;
        } catch (Throwable ex) {      // try to cope with OOME
            sizeCtl = Integer.MAX_VALUE;
            return;
        }
        nextTable = nextTab;
        transferIndex = n;
    }
    int nextn = nextTab.length;
    ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab);
    boolean advance = true;
    boolean finishing = false; // to ensure sweep before committing nextTab
    for (int i = 0, bound = 0;;) {
        Node<K,V> f; int fh;
        while (advance) {
            int nextIndex, nextBound;
            if (--i >= bound || finishing)
                advance = false;
            else if ((nextIndex = transferIndex) <= 0) {
                i = -1;
                advance = false;
            }
            else if (U.compareAndSwapInt
                     (this, TRANSFERINDEX, nextIndex,
                      nextBound = (nextIndex > stride ?
                                   nextIndex - stride : 0))) {
                bound = nextBound;
                i = nextIndex - 1;
                advance = false;
            }
        }
        if (i < 0 || i >= n || i + n >= nextn) {
            int sc;
            if (finishing) {
                nextTable = null;
                table = nextTab;
                sizeCtl = (n << 1) - (n >>> 1);
                return;
            }
            if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {
                if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)
                    return;
                finishing = advance = true;
                i = n; // recheck before commit
            }
        }
        else if ((f = tabAt(tab, i)) == null)
            advance = casTabAt(tab, i, null, fwd);
        else if ((fh = f.hash) == MOVED)
            advance = true; // already processed
        else {
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    Node<K,V> ln, hn;
                    if (fh >= 0) {
                        int runBit = fh & n;
                        Node<K,V> lastRun = f;
                        for (Node<K,V> p = f.next; p != null; p = p.next) {
                            int b = p.hash & n;
                            if (b != runBit) {
                                runBit = b;
                                lastRun = p;
                            }
                        }
                        if (runBit == 0) {
                            ln = lastRun;
                            hn = null;
                        }
                        else {
                            hn = lastRun;
                            ln = null;
                        }
                        for (Node<K,V> p = f; p != lastRun; p = p.next) {
                            int ph = p.hash; K pk = p.key; V pv = p.val;
                            if ((ph & n) == 0)
                                ln = new Node<K,V>(ph, pk, pv, ln);
                            else
                                hn = new Node<K,V>(ph, pk, pv, hn);
                        }
                        setTabAt(nextTab, i, ln);
                        setTabAt(nextTab, i + n, hn);
                        setTabAt(tab, i, fwd);
                        advance = true;
                    }
                    else if (f instanceof TreeBin) {
                        TreeBin<K,V> t = (TreeBin<K,V>)f;
                        TreeNode<K,V> lo = null, loTail = null;
                        TreeNode<K,V> hi = null, hiTail = null;
                        int lc = 0, hc = 0;
                        for (Node<K,V> e = t.first; e != null; e = e.next) {
                            int h = e.hash;
                            TreeNode<K,V> p = new TreeNode<K,V>
                                (h, e.key, e.val, null, null);
                            if ((h & n) == 0) {
                                if ((p.prev = loTail) == null)
                                    lo = p;
                                else
                                    loTail.next = p;
                                loTail = p;
                                ++lc;
                            }
                            else {
                                if ((p.prev = hiTail) == null)
                                    hi = p;
                                else
                                    hiTail.next = p;
                                hiTail = p;
                                ++hc;
                            }
                        }
                        ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) :
                            (hc != 0) ? new TreeBin<K,V>(lo) : t;
                        hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :
                            (lc != 0) ? new TreeBin<K,V>(hi) : t;
                        setTabAt(nextTab, i, ln);
                        setTabAt(nextTab, i + n, hn);
                        setTabAt(tab, i, fwd);
                        advance = true;
                    }
                }
            }
        }
    }
}