Do not retain cache transaction managers

Previously, cache transaction managers may be  retained outside the
boundaries of an application context with AspectJ since an aspect is
basically a singleton for the current class loader.

This commit adds a "clearTransactionManagerCache" that is similar to the
"clearMetadataCache" introduced in CacheAspectSupport: whenever the
context is disposed, the cache is cleared to remove any reference to a
transaction manager defined by that context.

Issue: SPR-12518

spring-aspects/src/main/java/org/springframework/transaction/aspectj/AbstractTransactionAspect.aj

@@ -19,6 +19,7 @@ package org.springframework.transaction.aspectj;
 import org.aspectj.lang.annotation.SuppressAjWarnings;
 import org.aspectj.lang.reflect.MethodSignature;
 
+import org.springframework.beans.factory.DisposableBean;
 import org.springframework.transaction.interceptor.TransactionAspectSupport;
 import org.springframework.transaction.interceptor.TransactionAttributeSource;
 
@@ -44,7 +45,7 @@ import org.springframework.transaction.interceptor.TransactionAttributeSource;
  * @author Juergen Hoeller
  * @since 2.0
  */
-public abstract aspect AbstractTransactionAspect extends TransactionAspectSupport {
+public abstract aspect AbstractTransactionAspect extends TransactionAspectSupport implements DisposableBean {
 
 	/**
 	 * Construct the aspect using the given transaction metadata retrieval strategy.
@@ -56,6 +57,11 @@ public abstract aspect AbstractTransactionAspect extends TransactionAspectSuppor
 		setTransactionAttributeSource(tas);
 	}
 
+	@Override
+	public void destroy() {
+		clearTransactionManagerCache(); // An aspect is basically a singleton
+	}
+
 	@SuppressAjWarnings("adviceDidNotMatch")
 	Object around(final Object txObject): transactionalMethodExecution(txObject) {
 		MethodSignature methodSignature = (MethodSignature) thisJoinPoint.getSignature();

spring-tx/src/main/java/org/springframework/transaction/interceptor/TransactionAspectSupport.java

@@ -338,6 +338,13 @@ public Object doInTransaction(TransactionStatus status) {
 		}
 	}
 
+	/**
+	 * Clear the cached transaction managers.
+	 */
+	protected void clearTransactionManagerCache() {
+		this.transactionManagerCache.clear();
+	}
+
 	/**
 	 * Determine the specific transaction manager to use for the given transaction.
 	 */