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ConcurrentHashMap.md

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ConcurrentHashMap

JDK1.6/JDK1.7 ConcurrentHashMap是HashMap的线程安全实现,相比于Hashtable每次锁住整张表的情况,concurrentHashMap使用了分段锁(降低锁粒度),每次只锁住一个结点。 JDK1.8中已经摒弃了分段所,采取CAS替代了分段锁实现了无锁操作。

Node

和HashMap的内部结点一致,定义了Node结点。

    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        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();
        }
		//比较两个节点是否相同
        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)));
        }

        /**
         * Virtualized support for map.get(); overridden in subclasses.
         */
		 //从链表中查找是否有哈希值和k均相同的结点
        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;
        }
    }

JDK 1.8 CAS实现无锁操作。

插入

  • put() 调用了putVal方法
    public V put(K key, V value) {
        return putVal(key, value, false);
    }
  • putVal()
    final V putVal(K key, V value, boolean onlyIfAbsent) {
        if (key == null || value == null) throw new NullPointerException();
        int hash = spread(key.hashCode());	//通过spread方法计算出hash值。
        int binCount = 0;
        for (Node<K,V>[] tab = table;;) {	//不断循环,和CAS配套使用
            Node<K,V> f; int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();	//第一次插入数据,创建新表。(参见initTable解析)
			//根据hash原子性的获取表中对应的Node结点
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
				//此时该结点为空,CAS,新建结点并存入表中
                if (casTabAt(tab, i, null,
                             new Node<K,V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)	//当前结点不为空,且rehash正在进行
				//如果要扩容的新表存在,则将新值存入新表,并返回新表。
                tab = helpTransfer(tab, f);
            else {
                V oldVal = null;
				//给要修改的结点上锁
                synchronized (f) {
					//当前数据还没有被修改过,未出现脏读。不然进入下一次循环,重新进行。
                    if (tabAt(tab, i) == f) {
						//此处插入的代码和HashMap一致
                        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)))) {//如果存在所需结点,则更新值
                                    oldVal = e.val;
                                    if (!onlyIfAbsent)
                                        e.val = value;
                                    break;
                                }
                                Node<K,V> pred = e;
                                if ((e = e.next) == null) {//如果遍历到链表结尾仍没有找到,则添加新的对象。
                                    pred.next = new Node<K,V>(hash, key,
                                                              value, null);
                                    break;
                                }
                            }
                        }
                        else if (f instanceof TreeBin) {//如果冲突大于8,则转成树形结构,此时插入或更新结点至红黑树。
                            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;
                            }
                        }
                    }
                }
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)	//如果在插入元素后造成了链表长度大于8,则树化链表
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);//check and rehash
        return null;
    }
  • initTable()
    /**
     * Initializes table, using the size recorded in sizeCtl.
     */
    private final Node<K,V>[] initTable() {
        Node<K,V>[] tab; int sc;
        while ((tab = table) == null || tab.length == 0) {
			//当sizeCtl小于0,说明表正在创建或rehash,放弃当前线程的控制权
            if ((sc = sizeCtl) < 0)
                Thread.yield(); // lost initialization race; just spin
			//其中第一个参数为需要改变的对象,第二个为偏移量(即之前求出来的valueOffset的值),第三个参数为期待的值,第四个为更新后的值。->将sizeCtl设置为-1,开始创建表。
            else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
                try {
                    if ((tab = table) == null || tab.length == 0) {
						//理论上sc的值应该为-1,但是由于多线程操作,可能当前值被别的线程修改,如果更新后的值是大于0,则选用sc的值作为哈希表的大小。
                        int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                        @SuppressWarnings("unchecked")
                        Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                        table = tab = nt;
                        sc = n - (n >>> 2);	//sc = 0.75 * sc
                    }
                } finally {
                    sizeCtl = sc;
                }
                break;
            }
        }
        return tab;
    }
  • transfer() 扩容
	//tab为当前的表地址,nextTab为扩容后的表
    private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
        int n = tab.length, stride;
		//NCPU可以使用的CPU的数量,通过哈希表中的Node总量和CPU的数量来确定步长,即每个线程所允许处理的bucket的数量。
        if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
            stride = MIN_TRANSFER_STRIDE; // subdivide range
        if (nextTab == null) {            // 新表为空,则创建一个新表
            try {
                @SuppressWarnings("unchecked")
                Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1];//扩容至两倍
                nextTab = nt;
            } catch (Throwable ex) {      // try to cope with OOME
                sizeCtl = Integer.MAX_VALUE;
                return;
            }
            nextTable = nextTab;
            transferIndex = n;//rehash过程中正在转移的结点的指针,转移是从尾部向前开始的。
        }
        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;
				// transferIndex = 0表示table中所有数组元素都已经有其他线程负责扩容
                else if ((nextIndex = transferIndex) <= 0) {
                    i = -1;
                    advance = false;
                }
				//更新transferIndex的值
				//更新成功,则当前线程负责完成索引为(nextBound,nextIndex)之间的桶首节点扩容
                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);	//设置load factor为0.75
                    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)	//如果当前结点为空,添加一个转移结点,被再次被遍历到的时候将会读到hash值为MOVED
                advance = casTabAt(tab, i, null, fwd);
            else if ((fh = f.hash) == MOVED)	//遍历到了一个已经被转移了的结点
                advance = true; // already processed
            else {	//添加分段锁机制,锁住bucket,开始扩容
                synchronized (f) {
                    if (tabAt(tab, i) == f) {//保证了在上锁之前值未被更改(宏观的原子性保证)
                        Node<K,V> ln, hn;
                        if (fh >= 0) {//拷贝链表的值
                            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) {//拷贝树
                            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;
                        }
                    }
                }
            }
        }
    }
  • get() 哈希表的查询是O(1)级别的(没有哈希冲突的情况下)
    public V get(Object key) {
        Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
        int h = spread(key.hashCode());
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (e = tabAt(tab, (n - 1) & h)) != null) {//通过CAS读取值,原子性
            if ((eh = e.hash) == h) {//当前bucket是单一结点,直接返回。
                if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                    return e.val;
            }
            else if (eh < 0)//说明当前元素是树形结构
                return (p = e.find(h, key)) != null ? p.val : null;
            while ((e = e.next) != null) {//当前bucket是链表元素,通过遍历得到结果
                if (e.hash == h &&
                    ((ek = e.key) == key || (ek != null && key.equals(ek))))
                    return e.val;
            }
        }
        return null;
    }

JDK1.6/1.7 分段锁实现

Segment

继承自ReentrantLock Hashtable之所以慢是因为在进行CRUD时通过synchronized锁住了整张表,我们要优化时应减小锁粒度,这样每次只对最小的范围进行上锁,允许了高并发的实现。

Segment<K,V> s0 = new Segment<K,V>(loadFactor, (int)(cap * loadFactor),(HashEntry<K,V>[])new HashEntry[cap]);

没有Java8中的Node结点,取代的是继承自ReentrantLock的分段锁。

 final Segment<K,V> segmentFor(int hash) { //定位锁
     return segments[(hash >>> segmentShift) & segmentMask];
 }
  • get() 它的get方法里将要使用的共享变量都定义成volatile.
  • put() get()和put()方法相较JAVA8简单很多,均是获得Segment后上锁进行操作。

Reference

1.ConcurrentHashMap源码解读(put/transfer/get)-jdk8 2.JDK7下ConcurrentHashMap源码分析