1)put数据，覆盖导致丢失

假设2个线程:1)同时put数据 2)hash后都落在table[I] 3)按如下顺序执行createEntry代码段，最后map的数据如图状态1，thread2的put丢失。

timeline	thread1	thread2
1	Entry<K,V> e = table[bucketIndex];
2		Entry<K,V> e = table[bucketIndex];
3		table[bucketIndex] = new Entry<>(hash, key, value, e);
4	table[bucketIndex] = new Entry<>(hash, key, value, e);

put数据覆盖丢失

void createEntry(int hash, K key, V value, int bucketIndex) {
        Entry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new Entry<>(hash, key, value, e);
        size++;
    }

2)resize时，get返回null

假设2个线程:1)thread1 put数据触发resize 2)thead2 get数据,hash后都落在table[I] 3)且按如下顺序执行代码段，因为table[I]节点node.next==null,无法遍历链表，get返回null。

timeline	thread1	thread2
1	while(null != e) {……} 只执行了一个循环,table变为table'
2		for (Entry<K,V> e = table[indexFor(hash, table.length)];e != null;e = e.next) {……} table[I]节点e，e.next == null

get返回null

/**
     * 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);
                e.next = newTable[I];
                newTable[i] = e;
                e = next;
            }
        }
    }

/**
     * Returns the entry associated with the specified key in the
     * HashMap.  Returns null if the HashMap contains no mapping
     * for the key.
     */
    final Entry<K,V> getEntry(Object key) {
        if (size == 0) {
            return null;
        }

        int hash = (key == null) ? 0 : hash(key);
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k))))
                return e;
        }
        return null;
    }

3)resize时，put导致数据丢失

假设2个线程:1)thread1 put触发resize 2)thread resize未完成时，thead2 put又触发resize 3)且按如下顺序执行代码段，最终table状态为newtable‘导致部分之前put的数据丢失。

timeline	thread1	thread2
1	transfer(newTable, initHashSeedAsNeeded(newCapacity)); 如图newtable
2		transfer(newTable, initHashSeedAsNeeded(newCapacity)); 如图newtable’
3	table = newTable;
4		table = newTable;

同时resize

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];
        transfer(newTable, initHashSeedAsNeeded(newCapacity));
        table = newTable;
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }

    /**
     * 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);
                e.next = newTable[I];
                newTable[i] = e;
                e = next;
            }
        }
    }