Add protected method to SpringELExpression

Issue: SWF-1476

spring-binding/src/main/java/org/springframework/binding/expression/spel/SpringELExpression.java

@@ -133,9 +133,17 @@ public class SpringELExpression implements Expression {
 		context.setVariables(getVariableValues(rootObject));
 		context.setTypeConverter(new StandardTypeConverter(conversionService));
 		context.getPropertyAccessors().addAll(propertyAccessors);
+		extendEvaluationContext(context);
 		return context;
 	}
 
+	/**
+	 * Invoked every time an evaluation context is created allowing further
+	 * initialization from sub-classes.
+	 */
+	protected void extendEvaluationContext(StandardEvaluationContext context) {
+	}
+
 	/**
 	 * Turn the map of variable-names-to-expressions into a map of variable-names-to-plain-objects
 	 * by evaluating each object against the input rootObject.

spring-binding/src/main/java/org/springframework/binding/expression/spel/SpringELExpressionParser.java

@@ -78,7 +78,18 @@ public class SpringELExpressionParser implements ExpressionParser {
 		Class<?> expectedResultType = context.getExpectedEvaluationResultType();
 		org.springframework.core.convert.ConversionService cs = conversionService.getDelegateConversionService();
 
-		return new SpringELExpression(spelExpression, expressionVars, expectedResultType, cs, propertyAccessors);
+		return createSpringELExpression(expressionVars, spelExpression, expectedResultType, cs);
+	}
+
+	/**
+	 * Create the {@link SpringELExpression}.
+	 */
+	protected SpringELExpression createSpringELExpression(Map<String, Expression> expressionVars,
+			org.springframework.expression.Expression spelExpression, Class<?> expectedResultType,
+			org.springframework.core.convert.ConversionService conversionService) {
+
+		return new SpringELExpression(spelExpression, expressionVars,
+				expectedResultType, conversionService, propertyAccessors);
 	}
 
 	private org.springframework.expression.Expression parseSpelExpression(String expression, ParserContext context) {