hashMap是非线程安全的，表现在两种情况下：
1 扩容：
t1线程对map进行扩容，此时t2线程来读取数据，原本要读取位置为2的元素，扩容后此元素位置未必是2，则出现读取错误数据。
2 hash碰撞
两个线程添加元素发生hash碰撞，都要将此元素添加到链表的头部，则会发生数据被覆盖。
详情：
HashMap底层是一个Node数组，一旦发生Hash冲突的的时候，HashMap采用拉链法解决碰撞冲突，Node结构：
/** * Basic hash bin node, used for most entries. (See below for * TreeNode subclass, and in LinkedHashMap for its Entry subclass.) */ static class Node<K,V> implements Map.Entry<K,V> { final int hash; final K key; V value; Node<K,V> next; Node(int hash, K key, V value, Node<K,V> next) { this.hash = hash; this.key = key; this.value = value; this.next = next; } public final K getKey() { return key; } public final V getValue() { return value; } public final String toString() { return key + "=" + value; } public final int hashCode() { return Objects.hashCode(key) ^ Objects.hashCode(value); } public final V setValue(V newValue) { V oldValue = value; value = newValue; return oldValue; } public final boolean equals(Object o) { if (o == this) return true; if (o instanceof Map.Entry) { Map.Entry<?,?> e = (Map.Entry<?,?>)o; if (Objects.equals(key, e.getKey()) && Objects.equals(value, e.getValue())) return true; } return false; } }
Node的变量：
final int hash; final K key; V value; Node<K,V> next;
执行put方法添加元素后调用此方法：
/** * Implements Map.put and related methods * * @param hash hash for key * @param key the key * @param value the value to put * @param onlyIfAbsent if true, don't change existing value * @param evict if false, the table is in creation mode. * @return previous value, or null if none */ final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<K,V>[] tab; Node<K,V> p; int n, i; if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; if ((p = tab[i = (n - 1) & hash]) == null) //1位置 此位置为空，则直接创建newNode tab[i] = newNode(hash, key, value, null); else { //如果此处有元素 Node<K,V> e; K k; //如果此处有元素，且key值相同，则覆盖元素 if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) e = p; //如果当前位置的元素的结构是树节点，则将待添加的数据作为树节点，添加到红黑树结构中 else if (p instanceof TreeNode) e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //剩余情况，将新元素追加到当前链表元素的next节点 else { for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) { p.next = newNode(hash, key, value, null); //如果链表长度达到8，要将链表结构转化为红黑树 if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st treeifyBin(tab, hash); break; } //如果发现相同的key，同样覆盖 if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; p = e; } } if (e != null) { // existing mapping for key V oldValue = e.value; if (!onlyIfAbsent || oldValue == null) e.value = value; afterNodeAccess(e); return oldValue; } } ++modCount; if (++size > threshold) resize(); afterNodeInsertion(evict); return null; }
在并发情况下，添加元素，在 1位置 会出现线程安全问题
扩容问题：
/** * 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() { //旧数据 Node<K,V>[] oldTab = table; //旧map长度 int oldCap = (oldTab == null) ? 0 : oldTab.length; //要调整大小的下一个值 int oldThr = threshold; int newCap, newThr = 0; if (oldCap > 0) { //如果当前数组长度大于等于最大值（1 << 30），就把threshold调到最大，无法再创建更大的数组了，直接返回原有数据 if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; } //如果当前长度*2 小于最大值，并且当前长度大于等于默认长度16，阈值*2 else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; // double threshold } //如果阈值大于0，则将新数组的长度设置为阈值=16 else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr; // 零初始阈值表示使用默认值 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"}) //创建新的数组 Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; table = newTab; //将旧数据迁移到新数组 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) //为每个不为空的key重新计算hash值 newTab[e.hash & (newCap - 1)] = e; //处理红黑树中数据 else if (e instanceof TreeNode) ((TreeNode<K,V>)e).split(this, newTab, j, oldCap); //处理链表数据 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; }
线程1在取数据时，map被其它线程扩容，则造成取到错误数据