CannotCreateTransactionException non-forked integration testing "Session is closed!"

[javapda]

We are converting to using spring-3.1.0.RC2 and hibernate-4.0.0.CR7

We have several hundred integration tests. When they are run in a forked mode (<forkMode>always</forkMode>, which takes much longer) all tests pass. When they are run in non-forked mode (much quicker) they begin to fail after the 28th integration test class is run.

Each of the failing integration tests passes independently.

Here is a stack trace (test result) from the first of the failures:

-------------------------------------------------------------------------------
Test set: com.gs.core.services.impl.ServiceImplIntegrationTest
-------------------------------------------------------------------------------
Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.036 sec <<< FAILURE!
testGetDiseaseByClinicalCode(com.gs.core.services.impl.ServiceImplIntegrationTest)  Time elapsed: 0.018 sec  <<< ERROR!
org.springframework.transaction.CannotCreateTransactionException: Could not open Hibernate Session for transaction; nested exception is org.hibernate.SessionException: Session is closed!
	at org.springframework.orm.hibernate4.HibernateTransactionManager.doBegin(HibernateTransactionManager.java:428)
	at org.springframework.transaction.support.AbstractPlatformTransactionManager.getTransaction(AbstractPlatformTransactionManager.java:371)
	at org.springframework.test.context.transaction.TransactionalTestExecutionListener$TransactionContext.startTransaction(TransactionalTestExecutionListener.java:513)
	at org.springframework.test.context.transaction.TransactionalTestExecutionListener.startNewTransaction(TransactionalTestExecutionListener.java:271)
	at org.springframework.test.context.transaction.TransactionalTestExecutionListener.beforeTestMethod(TransactionalTestExecutionListener.java:164)
	at org.springframework.test.context.TestContextManager.beforeTestMethod(TestContextManager.java:358)
	at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:73)
	at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:83)
	at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:72)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:231)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:49)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
	at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
	at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:71)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:174)
	at org.apache.maven.surefire.junit4.JUnit4TestSet.execute(JUnit4TestSet.java:59)
	at org.apache.maven.surefire.suite.AbstractDirectoryTestSuite.executeTestSet(AbstractDirectoryTestSuite.java:120)
	at org.apache.maven.surefire.suite.AbstractDirectoryTestSuite.execute(AbstractDirectoryTestSuite.java:103)
	at org.apache.maven.surefire.Surefire.run(Surefire.java:169)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.apache.maven.surefire.booter.SurefireBooter.runSuitesInProcess(SurefireBooter.java:350)
	at org.apache.maven.surefire.booter.SurefireBooter.main(SurefireBooter.java:1021)
Caused by: org.hibernate.SessionException: Session is closed!
	at org.hibernate.internal.AbstractSessionImpl.errorIfClosed(AbstractSessionImpl.java:126)
	at org.hibernate.internal.SessionImpl.connection(SessionImpl.java:449)
	at org.springframework.orm.hibernate4.HibernateTransactionManager.doBegin(HibernateTransactionManager.java:342)
	... 30 more

Q: Why would forking of the integration tests pass (a new jvm for each test) versus non-forked failing?

Q: Any suggestions on how best to track down the issue?

Thanks in advance. :-)