• hashMap的线程不安全


    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

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  • 原文地址:https://www.cnblogs.com/chenglc/p/11008565.html
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