jdk8中是如何解决jdk7中的HashMap死循环的？

 

jdk8中是如何解决jdk7中的HashMap死循环的？

HashMap死循环问题，作为考核一个袁同学的知识功底和经验水平，大家津津乐道！对于没有真实遇到过这种问题的同学，可能也没有去研究过这种问题的同学，真是害苦了！

具体是啥问题呢？
我简单描述下: 某同学将HashMap()作为一个共享变量，供所有线程使用，然后还乐呵呵地进行put/get/remove..., 然后后来他的cpu被打满了，业务响应不了了，请排查问题发生的原因？

jdk1.7 中才有哦！

如下例子，反正我是没有模拟出死循环出来！

 

public class HashMapDeadLoopTest extends Thread {

    private static Map<Integer, Integer> sharedMap = new HashMap<>(2);

    private static AtomicInteger keyCounter = new AtomicInteger(1);

    private static LoopPointerObjectDump table[] = new LoopPointerObjectDump[3];

    private static int size = 0;

    public static void main(String[] args) throws InterruptedException, IOException {
        // 开400个线程同时跑（过小的线程数不易模拟出结果），死循环命中率很高，但是机器可能被卡死，小心运行
        int i, runNum = 400;
        HashMapDeadLoopTest[] tests = new HashMapDeadLoopTest[runNum];
        for( i = 0; i < runNum; i++) {
            tests[i] = new HashMapDeadLoopTest();
            tests[i].start();
        }

        // 自己模拟单个线程出现死循环情况，说明
//        HashMapDeadLoopTest test = new HashMapDeadLoopTest();
//        LoopPointerObjectDump next1 = new LoopPointerObjectDump();
//        LoopPointerObjectDump next2 = new LoopPointerObjectDump(next1);
//
//        test.put(next2);
//        LoopPointerObjectDump[] newTable = new LoopPointerObjectDump[3];
//        newTable[0] = next2;
//        test.transfer(newTable, true);
        System.in.read();

    }
    void put(LoopPointerObjectDump e) {
        table[size++] = e;
    }
    void transfer(LoopPointerObjectDump[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        int loopId = 0;
        for (LoopPointerObjectDump e : table) {
            while(null != e) {
                LoopPointerObjectDump next = e.next;
                // 假设线程一看不到线程最后的这个变更，next已经被赋旧值，打开注释慢慢查看死循环效果
                // if(++loopId == 1) {
                //    next = e;
                // }
                // if(loopId > 100000) {
                //    break;
                // }

                int i = 0;
                e.next = newTable[i];
                newTable[i] = e;
                e = next;
            }
            System.out.println("loopId:" + loopId);
        }
    }
    @Override
    public void run() {
        while (keyCounter.get() < 1000000)
        {
            sharedMap.put(keyCounter.get(), keyCounter.get());
            keyCounter.addAndGet(2);
            System.out.println(Thread.currentThread().getName() + " : put a-> " + keyCounter.get());
        }
    }

    private static class LoopPointerObjectDump {
        private LoopPointerObjectDump next;

        public LoopPointerObjectDump() {
            next = null;
        }

        public LoopPointerObjectDump(LoopPointerObjectDump next) {
            this.next = next;
        }

        public void setNext(LoopPointerObjectDump next) {
            this.next = next;
        }
    }
}

不管怎么样，原理咱们还是来理解下的！

// java.utils.HashMap.put()
    public V put(K key, V value) {
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);
        }
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key);
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        // 新元素，插入
        addEntry(hash, key, value, i);
        return null;
    }
    void addEntry(int hash, K key, V value, int bucketIndex) {
        if ((size >= threshold) && (null != table[bucketIndex])) {
            // 容量达到阀值，扩容
            resize(2 * table.length);
            hash = (null != key) ? hash(key) : 0;
            bucketIndex = indexFor(hash, table.length);
        }

        createEntry(hash, key, value, bucketIndex);
    }
    void resize(int newCapacity) {
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
        // 转移老容器内的元素到新容器内，重新平衡分布元素
        transfer(newTable, initHashSeedAsNeeded(newCapacity));
        table = newTable;
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }
    void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        for (Entry<K,V> e : table) {
            // 注意，只要达到 e != null 条件，死循环就生成了
            while(null != e) {
                // 获取末尾元素
                Entry<K,V> next = e.next;
                if (rehash) {
                    e.hash = null == e.key ? 0 : hash(e.key);
                }
                int i = indexFor(e.hash, newCapacity);
                // 此处可能形成循环链 select * from com.test.HashMapDeadLoopTest, 查看 循环链
                e.next = newTable[i];
                newTable[i] = e;
                e = next;
            }
        }
    }
    
    
    jdk8 中的 HashMap, 是这样的！
    
    
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        // 容量为空时重新赋值
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        // 元素不存在，则直接插入数组即可，线程安全性？
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            // 原值已存在，直接替换
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            // 如果是 LinkedHashMap 实现的话，会使用红黑树作为数据结构，调用其 putTreeVal()
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    // 找到最后一个 next 不会 null 的位置，插入元素
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        // 如果树的深度大于阀值-1, 则重新调整，平衡二叉树
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    // 找到元素存在，直接进入后续更新
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            // 当元素存在时，更新，并返回旧值
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                // 存在才添加判定
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                // LinkedHashMap 预留
                afterNodeAccess(e);
                return oldValue;
            }
        }
        // 修改+1
        ++modCount;
        // 容量超过阀值，扩容
        if (++size > threshold)
            resize();
        // LinkedHashMap 预留
        afterNodeInsertion(evict);
        return null;
    }
    
    
    /**
     * Replaces all linked nodes in bin at index for given hash unless
     * table is too small, in which case resizes instead.
     */
    final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        // 对于小于容量 64 的情况，直接扩容解决
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            TreeNode<K,V> hd = null, tl = null;
            do {
                // 否则将数据结构升级为 TreeNode 
                TreeNode<K,V> p = replacementTreeNode(e, null);
                if (tl == null)
                    hd = p;
                else {
                    // 二叉树建立 tl -> p ->
                    p.prev = tl;
                    tl.next = p;
                }
                tl = p;
            } while ((e = e.next) != null);
            if ((tab[index] = hd) != null)
                // 如果结构发生变更，则做一次平衡操作
                hd.treeify(tab);
        }
    }
    
    
        /**
         * Forms tree of the nodes linked from this node.
         * @return root of tree
         */
        final void treeify(Node<K,V>[] tab) {
            TreeNode<K,V> root = null;
            for (TreeNode<K,V> x = this, next; x != null; x = next) {
                next = (TreeNode<K,V>)x.next;
                x.left = x.right = null;
                if (root == null) {
                    x.parent = null;
                    x.red = false;
                    root = x;
                }
                else {
                    K k = x.key;
                    int h = x.hash;
                    Class<?> kc = null;
                    for (TreeNode<K,V> p = root;;) {
                        int dir, ph;
                        K pk = p.key;
                        // 根据hash值决定 node 放在左子树还有右子树
                        if ((ph = p.hash) > h)
                            dir = -1;
                        else if (ph < h)
                            dir = 1;
                        else if ((kc == null &&
                                  (kc = comparableClassFor(k)) == null) ||
                                 (dir = compareComparables(kc, k, pk)) == 0)
                            dir = tieBreakOrder(k, pk);

                        TreeNode<K,V> xp = p;
                        // 找到叶子节点为空的位置，插入 x , 并做平衡操作
                        if ((p = (dir <= 0) ? p.left : p.right) == null) {
                            x.parent = xp;
                            if (dir <= 0)
                                xp.left = x;
                            else
                                xp.right = x;
                            root = balanceInsertion(root, x);
                            break;
                        }
                    }
                }
            }
            moveRootToFront(tab, root);
        }
        
        // 平衡二叉树的节点插入
        static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root,
                                                    TreeNode<K,V> x) {
            x.red = true;
            for (TreeNode<K,V> xp, xpp, xppl, xppr;;) {
                if ((xp = x.parent) == null) {
                    x.red = false;
                    return x;
                }
                else if (!xp.red || (xpp = xp.parent) == null)
                    return root;
                if (xp == (xppl = xpp.left)) {
                    if ((xppr = xpp.right) != null && xppr.red) {
                        xppr.red = false;
                        xp.red = false;
                        xpp.red = true;
                        x = xpp;
                    }
                    else {
                        if (x == xp.right) {
                            root = rotateLeft(root, x = xp);
                            xpp = (xp = x.parent) == null ? null : xp.parent;
                        }
                        if (xp != null) {
                            xp.red = false;
                            if (xpp != null) {
                                xpp.red = true;
                                root = rotateRight(root, xpp);
                            }
                        }
                    }
                }
                else {
                    if (xppl != null && xppl.red) {
                        xppl.red = false;
                        xp.red = false;
                        xpp.red = true;
                        x = xpp;
                    }
                    else {
                        if (x == xp.left) {
                            root = rotateRight(root, x = xp);
                            xpp = (xp = x.parent) == null ? null : xp.parent;
                        }
                        if (xp != null) {
                            xp.red = false;
                            if (xpp != null) {
                                xpp.red = true;
                                root = rotateLeft(root, xpp);
                            }
                        }
                    }
                }
            }
        }
        
        /**
         * Ensures that the given root is the first node of its bin.
         */
        static <K,V> void moveRootToFront(Node<K,V>[] tab, TreeNode<K,V> root) {
            int n;
            if (root != null && tab != null && (n = tab.length) > 0) {
                int index = (n - 1) & root.hash;
                TreeNode<K,V> first = (TreeNode<K,V>)tab[index];
                if (root != first) {
                    Node<K,V> rn;
                    tab[index] = root;
                    TreeNode<K,V> rp = root.prev;
                    if ((rn = root.next) != null)
                        ((TreeNode<K,V>)rn).prev = rp;
                    if (rp != null)
                        rp.next = rn;
                    if (first != null)
                        first.prev = root;
                    root.next = first;
                    root.prev = null;
                }
                assert checkInvariants(root);
            }
        }
        
    // 扩容
    /**
     * Initializes or doubles table size.  If null, allocates in
     * accord with initial capacity target held in field threshold.
     * Otherwise, because we are using power-of-two expansion, the
     * elements from each bin must either stay at same index, or move
     * with a power of two offset in the new table.
     *
     * @return the table
     */    
    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            // 超过最大容量限制后，不再扩容，而是直接返回原值
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            // 判定扩容(*2)后不超过最大限制，且原有容量需大于最小容量，后更新扩容阀值，否则走后续逻辑扩容
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            next = e.next;
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }