Lewis John McGibbney created GORA-228:
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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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