added a convenience utility

spring-binding/src/main/java/org/springframework/binding/expression/support/AbstractGetValueExpression.java

@@ -0,0 +1,22 @@
+package org.springframework.binding.expression.support;
+
+import org.springframework.binding.expression.EvaluationException;
+import org.springframework.binding.expression.Expression;
+
+/**
+ * Trivial helper for concrete expression types that do not support setting their values. Simply throws an unsupported
+ * operation exception if {@link #setValue(Object, Object)} is called.
+ * 
+ * Subclasses must implement {@link #getValue(Object)}.
+ * 
+ * @author Keith Donald
+ */
+public abstract class AbstractGetValueExpression implements Expression {
+
+	public abstract Object getValue(Object context) throws EvaluationException;
+
+	public void setValue(Object context, Object value) throws EvaluationException {
+		throw new UnsupportedOperationException("Setting this expression's value is not supported");
+	}
+
+}