[jira] [Created] (GORA-228) java.util.ConcurrentModificationException when using MemStore for concurrent tests

["Lewis John McGibbney (JIRA)" <ji...@apache.org>]

Lewis John McGibbney created GORA-228:
-----------------------------------------

             Summary: java.util.ConcurrentModificationException when using MemStore for concurrent tests
                 Key: GORA-228
                 URL: https://issues.apache.org/jira/browse/GORA-228
             Project: Apache Gora
          Issue Type: Sub-task
          Components: gora-core
    Affects Versions: 0.3
            Reporter: Lewis John McGibbney
             Fix For: 0.4


Finally, a multithreaded test in [3] fails with the following

{code}
java.util.ConcurrentModificationException
	at java.util.TreeMap$NavigableSubMap$SubMapIterator.nextEntry(TreeMap.java:1594)
	at java.util.TreeMap$NavigableSubMap$SubMapKeyIterator.next(TreeMap.java:1655)
	at org.apache.gora.memory.store.MemStore$MemResult.nextInner(MemStore.java:81)
	at org.apache.gora.query.impl.ResultBase.next(ResultBase.java:112)
	at org.apache.nutch.storage.TestGoraStorage.readWrite(TestGoraStorage.java:74)
	at org.apache.nutch.storage.TestGoraStorage.access$100(TestGoraStorage.java:41)
	at org.apache.nutch.storage.TestGoraStorage$1.call(TestGoraStorage.java:107)
	at org.apache.nutch.storage.TestGoraStorage$1.call(TestGoraStorage.java:102)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
	at java.util.concurrent.FutureTask.run(FutureTask.java:166)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
	at java.lang.Thread.run(Thread.java:722)
{code}

I believe that the final failure is due to to the use of TreeMap [5] as a private object in MemStore. TreeMap implementations are not synchronized. If multiple threads access a map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with an existing key is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be "wrapped" using the Collections.synchronizedSortedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map e.g.

   SortedMap m = Collections.synchronizedSortedMap(new TreeMap(...));

N.B. The NOTE on TreeMap's come right from the Oracle JavaDoc.
[3] http://svn.apache.org/viewvc/nutch/branches/2.x/src/test/org/apache/nutch/storage/TestGoraStorage.java?view=markup
[4] http://svn.apache.org/viewvc/nutch/branches/2.x/src/test/org/apache/nutch/util/AbstractNutchTest.java?view=markup
[5] http://docs.oracle.com/javase/6/docs/api/java/util/TreeMap.html

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