ConcurrentHashMap源码解读

一、计算初始容量

 

1）源代码    

tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1)
//计算c最近的2的N次方的值，作为初始容量
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;
}

2） 对与右移的知识请到我的另一篇文章中查看

3）或 运算符，只要有一个为1，则结果为1，否则为0

4）假设传进来的是25，二进制为011001

  

我们最终需要获得最大的二进制为 100000，作为集合的初始值

如何计算得到这个最大的二进制呢？
答：终极思想用移位得出011111 然后 +1
如何得出  011111呢？思想：将最高位1以后的数字通过移位操作全都变成1，
将最高位1向右移动4次，每一次得到N位1（通过与原来数据或操作）保留下来
步骤：
1 取原始数据 011001 最高位的1 ，然后右移一位结果：001100
2 001100或操作 011001 = 011101
3 让数据011101 ，右移2位得到，然后重复1、2操作得到011111
4 最后的结果011111，加上1得到1000000

这个步骤基本上和上面的源代码相同，只不过这里只是执行了2个步骤，源代码中执行了4个步骤
为什么是4个步骤呢？ 因为正数二进制是32位，而我们想31位全都变成1，那必须执行4个步骤，然后最后+1

 二、数据放入结合

  

/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
    //参数校验
    if (key == null || value == null) throw new NullPointerException();
    // 计算key对应的hash值，这个值会决定放入的数据对应，数组的那一个位置
    int hash = spread(key.hashCode());
    
    int binCount = 0;
    //赋值，并且无条件循环，直到满足一定条件才会退出
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh;
        //判断数组是否已经初始化，如果没有则初始化数据结构
        if (tab == null || (n = tab.length) == 0)
            //初始化数据结构 数组见下面的 三
            tab = initTable();
        // (n - 1) & hash 这个相当于数组的hash下标， 获取key对应数组下标位置是否有数据
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            //没有数据，则使用创建一个Node对象，然后case将数据赋值到数组对应位置
            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 {
            V oldVal = null;
            //锁住 key数组对应下标的第一个对象
            synchronized (f) {
                //双重判断，以防f中途发生变化
                if (tabAt(tab, i) == f) {
                    //f 这个对象的hash值 大于0 ，当连表迁移的时候这个值会发生变化
                    if (fh >= 0) {
                        binCount = 1;
                        //遍历 查找到的这个连表
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            //如果连表中一个对象的 key和hash与传进来的key和hash一样
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                 (ek != null && key.equals(ek)))) {
                                oldVal = e.val;
                                if (!onlyIfAbsent)//如果 参数覆盖为false
                                    //用value覆盖原来对象的val
                                    e.val = value;
                                break;
                            }
                            Node<K,V> pred = e;
                            //如果查找到最后一个节点
                            if ((e = e.next) == null) {
                                //创建一个新节点放到连表中
                                pred.next = new Node<K,V>(hash, key,
                                                          value, null);
                                break;
                            }
                        }
                    }
                    //如果是红黑树
                    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) {
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    //新添加节点以后，将节点总数+1
    addCount(1L, binCount);
    return null;
}

 

三、初始化数据结构

//方法可能会有多个线程进入
private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
    //线程进来以后，如果数组没有初始化则一直循环
    while ((tab = table) == null || tab.length == 0) {
        //sizeCtl=-1<0  则集合正在初始化，当前线程让出cpu执行权限
        if ((sc = sizeCtl) < 0)
            Thread.yield(); // lost initialization race; just spin

        //cas操作，只有一个线程可以设置sizeCtl值为-1，进入的线程会对数组进行初始化
        else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
            try {
                //二次判断，防止初始化的线程初始化完成以后，改变 sizeCtl 数值以后，其他线程进入到这个方法再次进行初始化
                if ((tab = table) == null || tab.length == 0) {
                    //计算初始化数组的容量
                    int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                    @SuppressWarnings("unchecked")
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                    //初始化的数组赋值给对象的table变量，这个table使用volatile修饰的，一旦赋值以后其他线程不会进入到这个while循环里面了
                    table = tab = nt;
                    sc = n - (n >>> 2);
                }
            } finally {
                //恢复sizeCtl这个变量
                sizeCtl = sc;
            }
            break;
        }
    }
    return tab;
}

 四、总数+1

 //主线：计数需要baseCount+1，当有线程竞争的时候则计数放到counterCells里面
private final void addCount(long x, int check) {
    CounterCell[] as; long b, s;
    // 第一个线程和与第一个线程同时进到这个位置的线程 counterCells才能=null，后面的线程 (as = counterCells) != null一直为true
    // 即后面的线程才会执行 这段代码：!U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)
    if ((as = counterCells) != null ||
        //当counterCells已经被初始化的时候，每次过来的线程没有竞争的情况执行这个语句 将baseCount直接+1，如果+1执行失败，则执行内部代码块
        !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 || //判断counterCells 这个变量为空
            (a = as[ThreadLocalRandom.getProbe() & m]) == null || //判断 需要写入数值的counterCells数组对应的位置为空
            !(uncontended =   U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) { //将 counterCells数组对应的位置的CounterCell值+1
            // 初始化counterCells，初始化counterCells对应下面的counterCell对象，或者在counterCells 对应节点数值+x操作
            fullAddCount(x, uncontended);
            return;
        }
        if (check <= 1)
            return;
        //计算总数 counterCells和baseCount 数据相加 获取总数
        s = sumCount();
    }
    if (check >= 0) {
        Node<K,V>[] tab, nt; int n, sc;
        //计算得到的数据总数大于初始容量sizeCtl，并且table不为空，需要扩容了
        while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&
               (n = tab.length) < MAXIMUM_CAPACITY) {
            int rs = resizeStamp(n);
            if (sc < 0) { //如果正在扩容
                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);
            }
            //获取锁
            else if (U.compareAndSwapInt(this, SIZECTL, sc,
                                         (rs << RESIZE_STAMP_SHIFT) + 2))
                //扩容
                transfer(tab, null);
            s = sumCount();
        }
    }
}

五、初始化counterCells，初始化counterCells对应下面的counterCell对象，或者在counterCells 对应节点数值+x操作

private final void fullAddCount(long x, boolean wasUncontended) {
    int h;
    if ((h = ThreadLocalRandom.getProbe()) == 0) {
        ThreadLocalRandom.localInit();      // force initialization
        h = ThreadLocalRandom.getProbe();
        wasUncontended = true;
    }
    boolean collide = false;                // True if last slot nonempty
    //循环配合cas操作
    for (;;) {
        CounterCell[] as; CounterCell a; int n; long v;
        //判断counterCells 不为空，对数组进行数值+操作
        if ((as = counterCells) != null && (n = as.length) > 0) {
            // 需要操作的counterCells的下标对象为空
            if ((a = as[(n - 1) & h]) == null) {
                if (cellsBusy == 0) {            // Try to attach new Cell
                    //创建一个CounterCell
                    CounterCell r = new CounterCell(x); // Optimistic create
                    if (cellsBusy == 0 &&
                        U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) { //cellsBusy标识位设置为1，进行加锁
                        boolean created = false;
                        try {               // Recheck under lock
                            CounterCell[] rs; int m, j;
                            if ((rs = counterCells) != null &&
                                (m = rs.length) > 0 &&
                                rs[j = (m - 1) & h] == null) {
                                //创建好的 CounterCell放到数组对应的位置
                                rs[j] = r;
                                created = true;
                            }
                        } finally {
                            cellsBusy = 0;
                        }
                        if (created)
                            break;
                        continue;           // Slot is now non-empty
                    }
                }
                collide = false;
            }
            else if (!wasUncontended)       // CAS already known to fail
                wasUncontended = true;      // Continue after rehash
            //对应的 CounterCell对象数据+x，如果成功则跳出循环
            else if (U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))
                break;
            // 如果数组CounterCell[]的长度已经大于计算机cpu的核大小，则重新循环不会执行下面扩容
            else if (counterCells != as || n >= NCPU)
                collide = false;            // At max size or stale
            else if (!collide) //就是增加一次循环，增加数值+x的几率，如果失败则会执行下面的扩容
                collide = true;
            else if (cellsBusy == 0 &&
                     U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
                try {
                    //扩容CounterCell[]数组，以增加cpu的使用效率
                    if (counterCells == as) {// Expand table unless stale
                        CounterCell[] rs = new CounterCell[n << 1];
                        for (int i = 0; i < n; ++i)
                            rs[i] = as[i];
                        counterCells = rs;
                    }
                } finally {
                    cellsBusy = 0;
                }
                collide = false;
                continue;                   // Retry with expanded table
            }
            h = ThreadLocalRandom.advanceProbe(h);
        }
          //判断 锁标识位没有被占用，counterCells==null
        else if (cellsBusy == 0 && counterCells == as &&
                 U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {//加锁
            boolean init = false;
            try {                           // Initialize table
                if (counterCells == as) {
                    //初始化counterCells
                    CounterCell[] rs = new CounterCell[2];
                    //创建一个CounterCell放入数组
                    rs[h & 1] = new CounterCell(x);
                    counterCells = rs;
                    init = true;
                }
            } finally {
                cellsBusy = 0;
            }
            //防止其他线程进来直接break了
            if (init)
                break;
        }
        else if (U.compareAndSwapLong(this, BASECOUNT, v = baseCount, v + x))
            break;                          // Fall back on using base
    }
}

六、扩容

private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
    int n = tab.length, stride;
    //计算最小扩容大小
    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];//创建一个数组，容量为原来的2倍
            nextTab = nt;
        } catch (Throwable ex) {      // try to cope with OOME
            sizeCtl = Integer.MAX_VALUE;
            return;
        }
        //扩容后的数组
        nextTable = nextTab;
        //需要转移的索引数量
        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;
            //从最大索引位置计算，如果下一个迁移的索引已经为0，则退出循环
            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;
                    }
                }
            }
        }
    }
}