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POConcurrent Modification Exception
 

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    <p>I'm currently working on a multi-threaded application, and I occasionally receive a concurrently modification exception (approximately once or twice an hour on average, but occurring at seemingly random intervals).</p> <p>The faulty class is essentially a wrapper for a map -- which extends <code>LinkedHashMap</code> (with accessOrder set to true). The class has a few methods:</p> <pre><code>synchronized set(SomeKey key, SomeValue val) </code></pre> <p>The set method adds a key/value pair to the internal map, and is protected by the synchronized keyword.</p> <pre><code>synchronized get(SomeKey key) </code></pre> <p>The get method returns the value based on the input key.</p> <pre><code>rebuild() </code></pre> <p>The internal map is rebuilt once in a while (~every 2 minutes, intervals do not match up with the exceptions). The rebuild method essentially rebuilds the values based on their keys. Since rebuild() is fairly expensive, I did not put a synchronized keyword on the method. Instead, I am doing:</p> <pre><code>public void rebuild(){ /* initialization stuff */ List&lt;SomeKey&gt; keysCopy = new ArrayList&lt;SomeKey&gt;(); synchronized (this) { keysCopy.addAll(internalMap.keySet()); } /* do stuff with keysCopy, update a temporary map */ synchronized (this) { internalMap.putAll(tempMap); } } </code></pre> <p>The exception occurs at </p> <pre><code>keysCopy.addAll(internalMap.keySet()); </code></pre> <p>Inside the synchronized block.</p> <p>Suggestions are greatly appreciated. Feel free to point me to specific pages/chapters in <em>Effective Java</em> and/or <em>Concurrency in Practice</em>.</p> <p>Update 1:</p> <p>Sanitized stacktrace:</p> <pre><code>java.util.ConcurrentModificationException at java.util.LinkedHashMap$LinkedHashIterator.nextEntry(LinkedHashMap.java:365) at java.util.LinkedHashMap$KeyIterator.next(LinkedHashMap.java:376) at java.util.AbstractCollection.toArray(AbstractCollection.java:126) at java.util.ArrayList.addAll(ArrayList.java:473) at a.b.c.etc.SomeWrapper.rebuild(SomeWraper.java:109) at a.b.c.etc.SomeCaller.updateCache(SomeCaller.java:421) ... </code></pre> <p>Update 2:</p> <p>Thanks everyone for the answers so far. I think the problem lies within the LinkedHashMap and its accessOrder attribute, although I am not entirely certain atm (investigating).</p> <p><strong>If accessOrder on a LinkedHashMap is set to true, and I access its keySet then proceed to add the keySet to a linkedList via addAll, do either of these actions mutate the order (i.e. count towards an "access")?</strong></p>
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    1. COOT, but arn't you in danger of missing any changes to the class in the commented region between the two synchronized methods in rebuild()?
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    2. COYes, you are absolutely correct; but according to the design docs, it's acceptable to have somewhat stale data :)
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    3. CODamn, I wish I had design docs like that ;)
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