Java8 中的 ConcurrentHashMap 的put()方法简单解析

太过爱你忘了你带给我的痛 2022-01-31 03:35 276阅读 0赞

**1.put()方法：**大方向上可以分为三块：1.初始化，2扩容，3数据迁移

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) {
            //key和value都不允许null
            if (key == null || value == null) throw new NullPointerException();
            int hash = spread(key.hashCode());
            // 用于记录相应链表的长度
            int binCount = 0;
            //遍历数组
            for (Node<K,V>[] tab = table;;) {
                Node<K,V> f; int n, i, fh;
                //数组不存在或者没数据的情况，初始化
                if (tab == null || (n = tab.length) == 0)
                    tab = initTable();
                //如果数组当前位置为null,用一次 CAS 操作将这个新值放入其中
                else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
                    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 {
                    //如果f是头结点
                    V oldVal = null;
                    //加锁
                    synchronized (f) {
                        //
                        if (tabAt(tab, i) == f) {
                            //头结点的 hash 值大于 0，说明是链表
                            if (fh >= 0) {
                                //链表长度+1
                                binCount = 1;
                                //遍历链表
                                for (Node<K,V> e = f;; ++binCount) {
                                    K ek;
                                    if (e.hash == hash &&
                                        ((ek = e.key) == key ||
                                         (ek != null && key.equals(ek)))) {
                                        // 如果发现了"相等"的 key，判断是否要进行值覆盖
                                        oldVal = e.val;
                                        if (!onlyIfAbsent)
                                            e.val = value;
                                        break;
                                    }
                                    // 到了链表的最末端，将新值放到链表的最末端
                                    Node<K,V> pred = e;
                                    if ((e = e.next) == null) {
                                        pred.next = new Node<K,V>(hash, key,
                                                                  value, null);
                                        break;
                                    }
                                }
                            }
                            //如果是一个红黑树的结构，将新值放到红黑树中
                            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;
                                }
                            }
                        }
                    }
                    //链表的长度大于8；转换为红黑树
                    if (binCount != 0) {
                        if (binCount >= TREEIFY_THRESHOLD)
                            treeifyBin(tab, i);
                        if (oldVal != null)
                            return oldVal;
                        break;
                    }
                }
            }
            addCount(1L, binCount);
            return null;
        }

**1.1 private final void treeifyBin(Node<K,V>\[\] tab, int index) ：**以链表存储还是按红黑树结构存储

private final void treeifyBin(Node<K,V>[] tab, int index) {
            Node<K,V> b; int n, sc;
            if (tab != null) {
                //判断当前表容量
                if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
                    tryPresize(n << 1);
                else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
                    //加锁
                    synchronized (b) {
                        //找到对应的数组
                        if (tabAt(tab, index) == b) {
                            TreeNode<K,V> hd = null, tl = null;
                            //遍历链表
                            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;
                            }
                            //建立一个红黑树
                            setTabAt(tab, index, new TreeBin<K,V>(hd));
                        }
                    }
                }
            }
        }

**1.1.1 private final Node<K,V>\[\] initTable()：** cas 加锁，并设置主内存变量sizeCtl

private final Node<K,V>[] initTable() {
            Node<K,V>[] tab; int sc;
            while ((tab = table) == null || tab.length == 0) {
                if ((sc = sizeCtl) < 0)
                    Thread.yield(); // lost initialization race; just spin
                // CAS，将 sizeCtl 设置为 -1，代表抢到了锁
                else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
                    try {
                        //表为空或者没有数据
                        if ((tab = table) == null || tab.length == 0) {
                            int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                            @SuppressWarnings("unchecked")
                            //初始化一个表（初始化一个数组）
                            Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                            table = tab = nt;
                            //如果 n 为 16 的话，那么这里 sc = 12  也就是 0.75 * n
                            sc = n - (n >>> 2);
                        }
                    } finally {
                        //赋值到主内存中的sizeCtl 
                        sizeCtl = sc;
                    }
                    break;
                }
            }
            return tab;
        }

**1.2 private final void tryPresize(int size)：**按数组合链表结构创建map

private final void tryPresize(int size) {
            //size+0.5size+1,再往上取最近的 2 的 n 次方
            int c = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY :
                tableSizeFor(size + (size >>> 1) + 1);
            int sc;
            //sizeCtl 是主内存中的一个值
            while ((sc = sizeCtl) >= 0) {
                //获取当前表
                Node<K,V>[] tab = table; int n;
                if (tab == null || (n = tab.length) == 0) {
                    //可以理解为当前的表空间
                    n = (sc > c) ? sc : c;
                    //CAS 线程安全限制，提高效率，其实是加锁-1
                    if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
                        try {
                            if (table == tab) {
                                @SuppressWarnings("unchecked")
                                //创建数组空间
                                Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                                table = nt;
                                sc = n - (n >>> 2);
                            }
                        } finally {
                            //赋值到主内存中的sizeCtl 
                            sizeCtl = sc;
                        }
                    }
                }
                //
                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;
                        //用 CAS 将 sizeCtl 加 1，然后执行 transfer 方法
                        if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
                            transfer(tab, nt);
                    }
                    //将 sizeCtl 设置为 (rs << RESIZE_STAMP_SHIFT) + 2)
                    else if (U.compareAndSwapInt(this, SIZECTL, sc,
                                                 (rs << RESIZE_STAMP_SHIFT) + 2))
                        transfer(tab, null);
                }
            }
        }

**1.3  private final void transfer(Node<K,V>\[\] tab, Node<K,V>\[\] nextTab) :**数据迁移,判断以何种结构存储

private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
            int n = tab.length, stride;
            // stride 在单核下直接等于 n，多核模式下为 (n>>>3)/NCPU，最小值是 16
            if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
                //设置16
                stride = MIN_TRANSFER_STRIDE; // subdivide range
            //入参为null
            if (nextTab == null) {            // initiating
                try {
                    @SuppressWarnings("unchecked")
                    //位移一位，表空间扩大一倍
                    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;
                }
                //给到当前新表
                nextTable = nextTab;
                //控制迁移的位置
                transferIndex = n;
            }
            //数组长度
            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
            //索引和边界从零开始
            for (int i = 0, bound = 0;;) {
                Node<K,V> f; int fh;
                while (advance) {
                    int nextIndex, nextBound;
                    //如果索引值比边界值大或者相同或者结束了，迁移就结束了
                    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;
                    // 所有的迁移操作已经完成
                    if (finishing) {
                        nextTable = null;
                        // 将新的 nextTab 赋值给 table 属性，完成迁移
                        table = nextTab;
                        // 重新计算 sizeCtl：n 是原数组长度，所以 sizeCtl 是新数组长度
                        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
                    }
                }
                // 如果位置 i 处是空的，cas节点出来放入刚刚初始化的 ForwardingNode 空节点
                else if ((f = tabAt(tab, i)) == null)
                    advance = casTabAt(tab, i, null, fwd);
                // 该位置处是一个 ForwardingNode，代表该位置已经迁移过了
                else if ((fh = f.hash) == MOVED)
                    advance = true; // already processed
                else {
                    // 对数组该位置处的结点加锁，开始处理数组该位置处的迁移工作
                    synchronized (f) {
                        if (tabAt(tab, i) == f) {
                            Node<K,V> ln, hn;
                            // 头结点的 hash 大于 0，说明是链表的 Node 节点
                            if (fh >= 0) {
                                //和jdk1.7类似，分两个链表处理
                                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;
                            }
                            //判断是不是红黑树，
                            else if (f instanceof TreeBin) {
                                //按红黑树规则插入红黑树的节点
                                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;
                                    }
                                }
                                // 如果一分为二后，节点数少于 8，那么将红黑树转换回链表
                                ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) :
                                    (hc != 0) ? new TreeBin<K,V>(lo) : t;
                                // // 如果一分为二后，节点数少于 8，那么将红黑树转换回链表
                                hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :
                                    (lc != 0) ? new TreeBin<K,V>(hi) : t;
                                // 将 ln 放置在新数组的位置 i
                                setTabAt(nextTab, i, ln);
                                // 将 hn 放置在新数组的位置 i+n
                                setTabAt(nextTab, i + n, hn);
                                //将原数组该位置处设置为 fwd，代表该位置已经处理完毕
                                setTabAt(tab, i, fwd);
                                //设置处理完成的标志
                                advance = true;
                            }
                        }
                    }
                }
            }
        }