AccessorLValue reliably downcasts to CompilablePropertyAccessor in concurrent scenarios

Issue: SPR-14850
(cherry picked from commit e2b1dcb)

spring-expression/src/main/java/org/springframework/expression/spel/ast/PropertyOrFieldReference.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -371,10 +371,12 @@ public class PropertyOrFieldReference extends SpelNodeImpl {
 
 		@Override
 		public TypedValue getValue() {
-			TypedValue value = this.ref.getValueInternal(this.contextObject, this.evalContext, this.autoGrowNullReferences);
+			TypedValue value =
-			if (this.ref.cachedReadAccessor instanceof CompilablePropertyAccessor) {
+					this.ref.getValueInternal(this.contextObject, this.evalContext, this.autoGrowNullReferences);
-				CompilablePropertyAccessor accessor = (CompilablePropertyAccessor) this.ref.cachedReadAccessor;
+			PropertyAccessor accessorToUse = this.ref.cachedReadAccessor;
-				this.ref.exitTypeDescriptor = CodeFlow.toDescriptor(accessor.getPropertyType());
+			if (accessorToUse instanceof CompilablePropertyAccessor) {
+				this.ref.exitTypeDescriptor =
+						CodeFlow.toDescriptor(((CompilablePropertyAccessor) accessorToUse).getPropertyType());
 			}
 			return value;
 		}