JAVA高级面试进阶训练营视频教程

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HashMap是一个线程不安全的集合，如果在遍历的过程中同时对该集合进行修改操作，例如put，add，remove等，
会抛出java.util.ConcurrentModificationException异常，那么究竟这个异常为何抛出，下面从源码层面来分析一下。

跟踪代码：

　查看HashMap源码，具体抛该异常的地方为：

final Node<K,V> nextNode() {
            Node<K,V>[] t;
            Node<K,V> e = next;
            if (modCount != expectedModCount) throw new ConcurrentModificationException(); if (e == null)
                throw new NoSuchElementException();
            if ((next = (current = e).next) == null && (t = table) != null) {
                do {} while (index < t.length && (next = t[index++]) == null);
            }
            return e;
        }

　　如果HashMap中modCount和expectedModCount不相等，则会抛出异常

查看modCount：

　　具体用途是记录该HashMap修改次数，比如在对一个HashMap put操作时，会对modCount进行++modCount操作（红色标注的地方）

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)
            tab = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    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;
    }

　　而在remove操作的时候，也会对modCount进行同样的操作：

final Node<K,V> removeNode(int hash, Object key, Object value,
                               boolean matchValue, boolean movable) {
        Node<K,V>[] tab; Node<K,V> p; int n, index;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (p = tab[index = (n - 1) & hash]) != null) {
            Node<K,V> node = null, e; K k; V v;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            else if ((e = p.next) != null) {
                if (p instanceof TreeNode)
                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
                else {
                    do {
                        if (e.hash == hash &&
                            ((k = e.key) == key ||
                             (key != null && key.equals(k)))) {
                            node = e;
                            break;
                        }
                        p = e;
                    } while ((e = e.next) != null);
                }
            }
            if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount; --size;
                afterNodeRemoval(node);
                return node;
            }
        }
        return null;
    }

查看expectedModCount：

　　它是HashIterator中的一个变量，在对HashMap迭代的时候，将modCount赋给expectedModCount，具体代码：

HashIterator() {
            expectedModCount = modCount;
            Node<K,V>[] t = table;
            current = next = null;
            index = 0;
            if (t != null && size > 0) { // advance to first entry
                do {} while (index < t.length && (next = t[index++]) == null);
            }
        }

何时调用HashIterator():

　　查看HashMap entrySet()源码： 

public Set<Map.Entry<K,V>> entrySet() {
        Set<Map.Entry<K,V>> es;
        return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
    }

　　此处新建一个EntrySet对象，而在对EntrySet进行迭代的时候，会调用：

public final Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator();
        }

　　新建一个EntryIterator对象，查看该类描述：

final class EntryIterator extends HashIterator
        implements Iterator<Map.Entry<K,V>> {
        public final Map.Entry<K,V> next() { return nextNode(); }
    }

　　它继承HashIterator，因此在new EntryIterator()的时候会默认调用它父类HashIterator的无参构造方法。

总结：

　　HashMap迭代遍历的时候，会初始化expectedModCount=modCount，这时候对HashMap进行修改操作，modCount会+1，继续遍历的时候expectedModCount!=modCount，继而抛出java.util.ConcurrentModificationException异常。