null el resolver

spring-binding/src/main/java/org/springframework/binding/expression/el/NullELResolver.java

@@ -0,0 +1,60 @@
+package org.springframework.binding.expression.el;
+
+import java.util.Iterator;
+
+import javax.el.ELContext;
+import javax.el.ELResolver;
+import javax.el.PropertyNotWritableException;
+
+/**
+ * Resolves the special 'null' variable, indicating a null value.
+ * 
+ * @author Keith Donald
+ */
+public class NullELResolver extends ELResolver {
+
+	private static final String NULL_VARIABLE_NAME = "null";
+
+	public Class getCommonPropertyType(ELContext context, Object base) {
+		return null;
+	}
+
+	public Iterator getFeatureDescriptors(ELContext context, Object base) {
+		return null;
+	}
+
+	public Class getType(ELContext context, Object base, Object property) {
+		if (base == null && NULL_VARIABLE_NAME.equals(property)) {
+			context.setPropertyResolved(true);
+			return null;
+		} else {
+			return null;
+		}
+	}
+
+	public Object getValue(ELContext context, Object base, Object property) {
+		if (base == null && NULL_VARIABLE_NAME.equals(property)) {
+			context.setPropertyResolved(true);
+			return null;
+		} else {
+			return null;
+		}
+	}
+
+	public boolean isReadOnly(ELContext context, Object base, Object property) {
+		if (base == null && NULL_VARIABLE_NAME.equals(property)) {
+			context.setPropertyResolved(true);
+			return true;
+		} else {
+			return false;
+		}
+	}
+
+	public void setValue(ELContext context, Object base, Object property, Object value) {
+		if (base == null && NULL_VARIABLE_NAME.equals(property)) {
+			context.setPropertyResolved(true);
+			throw new PropertyNotWritableException("The 'null' value cannot be set");
+		}
+	}
+
+}