不要在并发场景下使用hashmap的原因

    /**
     * Transfers all entries from current table to newTable.
     */
    void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        for (Entry<K,V> e : table) {
            while(null != e) {
                Entry<K,V> next = e.next;
                if (rehash) {
                    e.hash = null == e.key ? 0 : hash(e.key);
                }
                int i = indexFor(e.hash, newCapacity);  //所在散列桶序号
                e.next = newTable[i];   //这边做了逆序处理，在多线程使用时会导致死循环
                newTable[i] = e;
                e = next;
            }
        }
    }

    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        //oldCap代表有没有数据插入，table创建采用lazy模式，只有使用时才创建
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {   //已经有数据，证明不是初始化
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                    oldCap >= DEFAULT_INITIAL_CAPACITY)//老的threshold大于16时，新的threshold才进行double处理
                newThr = oldThr << 1; // double threshold
        }//oldCap == 0，初始化，创建hashmap时没有设置capacity
        else if (oldThr > 0) // initial capacity was placed in threshold
            //oldThr > 0 && oldCap == 0
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            //oldThr == 0 && oldCap == 0，初始化
            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) {//插入数据时扩容
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {//该散列桶已有数据，需要将该散列桶中数据重新映射
                    oldTab[j] = null;
                    if (e.next == null)//只有一个数据
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)//该散列桶中数据红黑树存储
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order 保证原来的顺序，否则在多线程误用时会导致死循环
                        //有多个数据，而且是链表形式存储
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            next = e.next;
                            /**
                             * 该hash在老哈希桶数组位置为(oldCap-1) & hash，在扩容后哈希桶下标为(2*oldCap-1) & hash
                             * 2*oldCap-1相当于在oldCap-1的基础上左侧高位再加一个1，所以插入数据hash值在oldCap-1高一位是0还是1将决定
                             * 数据是否需要迁移，如果是1，则迁移到 j+oldCap位置，如果是0，则保持原位置
                             */
                            //与JDK7不同的是JDK8在扩容时原数据没有进行链表倒置操作，因为这个在并发编程时会导致
                            if ((e.hash & oldCap) == 0) {//在原来的散列桶
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            else {//不在原来的散列桶
                                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;
    }

    /**
     * JDK8的hashmap虽然没有jdk7及以前版本存在的死循环问题，但仍然存在丢失数据问题
     */
    @Test
    public void test(){
        Map<String,String> map = new HashMap<>();
        Thread[] ts = new Thread[50];
        for(int i=0;i<50;i++){
            int k = i;
            Thread t = new Thread(()->{
                for(int j=0;j<500;j++){
                    map.put(String.format("1001%2d%3d",k,j),"h");
                }
            });
            t.start();
            ts[i] = t;
        }
        for(int i=0;i<50;i++){
            try {
                ts[i].join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(map.keySet().size());

        Map<String,String> map1 = new HashMap<>();
        for(int i=0;i<50;i++){
            int k = i;
            Thread t = new Thread(()->{
                for(int j=0;j<500;j++){
                    map1.put(String.format("1001%2d%3d",k,j),"h");
                }
            });
            t.start();
            try {
                t.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        for(int i=0;i<50;i++){
            try {
                ts[i].join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(map1.keySet().size());
    }