ConcurrentHashMap 源码解析

字数统计: 2.1k字   |   阅读时长: 11分

put操作

  • put()
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final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();
    int hash = spread(key.hashCode());  //hash (h ^ (h >>> 16)) & HASH_BITS;减少hash冲突，保证均匀分布
    int binCount = 0; 	//统计节点长度
    for (Node<K,V>[] tab = table;;) {   //CAS自旋
        Node<K,V> f; int n, i, fh;
        if (tab == null || (n = tab.length) == 0)  //数组为空，则初始化
            tab = initTable();
        //(n - 1) & hash 取模运算得到元素的位置，tabAt利用Unsafe类得到当前元素是否有数据
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            //如果没有，利用CAS新增数据
            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)
            tab = helpTransfer(tab, f);
        else { // f != null 即数组当前节点有值
            V oldVal = null;
            //synchronized 加锁 使锁的力度细化到数组当前节点
            synchronized (f) {
                if (tabAt(tab, i) == f) {  //已经加锁，为什么还要判断？
                    if (fh >= 0) {   //fh = f.hash
                        binCount = 1;
                        //处理链表
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            //put数组与当前节点数据相同 则根据onlyIfAbsent判断覆盖
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                 (ek != null && key.equals(ek)))) {
                                oldVal = e.val;
                                if (!onlyIfAbsent) //覆盖
                                    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;
                            }
                        }
                    }
                    //f是红黑树节点
                    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;
                        }
                    }
                }
            }
            if (binCount != 0) {
                //节点长度 >= 8， 转红黑树
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    addCount(1L, binCount);
    return null;
}
  • initTable()
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private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
    while ((tab = table) == null || tab.length == 0) {  //数组为空
        if ((sc = sizeCtl) < 0)        //sizeCtl是扩容长度
            Thread.yield(); // lost initialization race; just spin
        else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) { //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，这句话的意思是如果根据SIZECTL偏移量得到的值（sizeCtl）跟sc相等，则将sizeCtl赋值为-1，此时如果又进来一个初始化操作，则线程在if ((sc = sizeCtl) < 0)处让出cpu，确保只有一个线程在做初始化操作，保证线程安全
            try {
                if ((tab = table) == null || tab.length == 0) {
                    int n = (sc > 0) ? sc : DEFAULT_CAPACITY;  //DEFAULT_CAPACITY = 16
                    @SuppressWarnings("unchecked")
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                    table = tab = nt;
                    sc = n - (n >>> 2);    //利用位移运算得到扩容长度，相当于 n * 0.75(HashMap中的扩容因子)
                }
            } finally {
                sizeCtl = sc;
            }
            break;
        }
    }
    return tab;
}
  • tabAt
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static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
    //ABASE 为数组本身的偏移量，ASHIFT是元素大小后面0的个数
    return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
}
  • treeifyBin
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private final void treeifyBin(Node<K,V>[] tab, int index) {
    Node<K,V> b; int n, sc;
    if (tab != null) {
        //数组的长度<64则进行
        if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
            tryPresize(n << 1);
        else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
            synchronized (b) {
                if (tabAt(tab, index) == b) {
                    TreeNode<K,V> hd = null, tl = null;
                    for (Node<K,V> e = b; e != null; e = e.next) {
                        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));
                }
            }
        }
    }
}
  • tryPresize
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private final void tryPresize(int size) {
    int c = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY :
        tableSizeFor(size + (size >>> 1) + 1); //计算出大于size的最小2^n的值
    int sc;
    while ((sc = sizeCtl) >= 0) {
        Node<K,V>[] tab = table; int n;
        if (tab == null || (n = tab.length) == 0) {
            n = (sc > c) ? sc : c;
            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;
                }
            }
        }
        else if (c <= sc || n >= MAXIMUM_CAPACITY)
            break;
        else if (tab == table) {
            // Integer.numberOfLeadingZeros(n) | (1 << (RESIZE_STAMP_BITS - 1))
            //resizeStamp的作用是把n前导0的个数记在了rs的低16位上, rs的最高位(第16位)是1
            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;
                //记录帮助线程的个数，并扩容
                if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
                    transfer(tab, nt);
            }
            //利用CAS将sc赋值，把rs赋值到sc的高16位，低16位+2 此时rs为一个非常大的负数
            else if (U.compareAndSwapInt(this, SIZECTL, sc,
                                         (rs << RESIZE_STAMP_SHIFT) + 2))
                transfer(tab, null);
        }
    }
}
  • transfer
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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
    //初始化扩容的数组 长度为n*2
    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;
    //hash值是MOVED(-1)的Node节点
    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;;) {  //自旋
        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)
            //将旧tab的i节点hash值赋值成-1
            advance = casTabAt(tab, i, null, fwd);
        else if ((fh = f.hash) == MOVED) //-1表示已处理完
            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;
                        //扩容后当前节点的值分为两种
                        //1、hash后还在当前i节点
                        //2、hash后在i+n节点
                        //这个for循环的依据是链表后面的节点扩容后都在1或者2上面，目的是为了找到最后一个连续的节点
                        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;
                        }
                        //遍历链表重新分配节点
                        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); //将旧节点hash赋值-1
                        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;
                    }
                }
            }
        }
    }
}
  • addCount()
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private final void addCount(long x, int check) {
    CounterCell[] as; long b, s;
    if ((as = counterCells) != null ||
        //累加baseCount的值，计算size的时候会用
        !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();
        }
    }
}
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    缺失模块。
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      excerpt: false
      categories: false
      tags: true

很惭愧<br><br>只做了一点微小的工作<br>谢谢大家