Issue #1643: InternalException: null

Hi!

when I run my application where 10 threads are concurrently accessing ObjectDB (only reading) via using ExecutorService, then these 2 mistakes are coming rapidly:

1.

Caused by: com.objectdb.o._JdoObjectNotFoundException: Entity is not found: eu.extech.data.versioning.ProductVersionHistory#3669
at com.objectdb.o.JDE.g(JDE.java:93)
at com.objectdb.o.ERR.f(ERR.java:56)
at com.objectdb.o.JDE.f(JDE.java:52)
at com.objectdb.o.OBC.onObjectDBError(OBC.java:1503)
at com.objectdb.jdo.PMImpl.getObjectById(PMImpl.java:465)
at eu.extech.serverImpl.jdo.JDOConnection.objectById(JDOConnection.java:330)

2.

[ObjectDB 2.6.1_02] Unexpected exception (Error 990)
  Generated by Java HotSpot(TM) 64-Bit Server VM 1.8.0_05 (on Mac OS X 10.9.5).
Please report this error on http://www.objectdb.com/database/issue/new
com.objectdb.o.InternalException: null
com.objectdb.o.InternalException
at com.objectdb.o.InternalException.f(InternalException.java:236)
at com.objectdb.o.LDR.F(LDR.java:508)
at com.objectdb.o.LDR.E(LDR.java:470)
at com.objectdb.o.OBC.UO(OBC.java:1080)
at com.objectdb.o.ENT.d(ENT.java:1139)
at com.objectdb.o.ENT.extractMap(ENT.java:1522)
at objectdb.java.util.HashMap.__odbBeforeAccess(Unknown Source)
at objectdb.java.util.HashMap.keySet(Unknown Source)
at eu.extech.quai.ProductStateAttributeStorage.restore(ProductStateAttributeStorage.java:77)
at eu.extech.quai.ProductStateAttributeStorage.createBasicDatabaseObjectCopy(ProductStateAttributeStorage.java:87)
at eu.extech.serverImpl.QuantServerImpl.retrieveProductStateAttributeStorageForProductID(QuantServerImpl.java:4856)
at eu.extech.serverImpl.QuantServerImpl.access$3(QuantServerImpl.java:4851)
at eu.extech.serverImpl.QuantServerImpl$6.run(QuantServerImpl.java:2184)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

Every worker thread has its own connection to the DB. 

When running the app without the multithreading, no errors are coming.

Build 2.6.1_05 includes an addition of a synchronized wrapper, which might have been missing in the context of your second reported stack trace. If it does't solve the issue, we may need a test case that demonstrates the exception.