AbstractPlatformTransactionManager logs warning for custom isolation level in case of no actual transaction

Issue: SPR-12600

spring-tx/src/main/java/org/springframework/transaction/support/AbstractPlatformTransactionManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -385,6 +385,10 @@ public abstract class AbstractPlatformTransactionManager implements PlatformTran
 		}
 		else {
 			// Create "empty" transaction: no actual transaction, but potentially synchronization.
+			if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
+				logger.warn("Custom isolation level specified but no actual transaction initiated; " +
+						"isolation level will effectively be ignored: " + definition);
+			}
 			boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
 			return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
 		}