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ConcurrentModificationException异常 - yanfei's tick tock
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<h1>ConcurrentModificationException异常</h1>
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<span class="date">2017/2/12 17:23 下午</span>
<span>posted in </span>
<span class="posted-in"><a href='%E5%B9%B6%E5%8F%91%E7%BC%96%E7%A8%8B.html'>并发编程</a></span>
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<h2 id="toc_0">原因</h2>
<p>AbstractList类的方法checkForComodification()如下：</p>
<pre><code class="language-java">final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
</code></pre>
<span id="more"></span><!-- more -->
<p>其中modCount为AbstractList的成员变量，表示对list的修改次数，每次调用ArrayList的add()、remove()方法时该值+1；而expectedModCount为AbstractList的内部类Itr的成员变量，表示对list修改次数的期望值，初始时，二者值相等。</p>
<p>发生该异常的原因是调用list.remove()导致modCount与expectedModCount不相等。</p>
<h2 id="toc_1">单线程环境下解决办法</h2>
<p>调用Itr中的remove()方法，该方法实际上也是调用的list.remove()，但多了expectedModCount = modCount的操作。</p>
<h2 id="toc_2">多线程环境下解决办法</h2>
<p>将ArrayList改为Vector并不能解决问题，因为Vector继承自AbstractList，所以通过Iterator来访问容器时，并不需要获取锁。因此会出现多个线程同时操作一个容器的情况，可能会导致某个线程中的expectedModCount与modCount不相等。</p>
<p>2种解决办法：<br/>
1. 通过Iterator迭代的时候使用synchronized或lock进行同步；<br/>
2. 使用并发容器CopyOnWriteArrayList代替ArrayList和Vector。</p>
<h2 id="toc_3">参考</h2>
<p><a href="http://www.cnblogs.com/dolphin0520/p/3933551.html">Java ConcurrentModificationException异常原因和解决方法</a></p>
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