SPR-6801 @ModelAttribute instantiation refinement.

Instantiate the model attribute from a URI var or a request param only
if the name matches and there is a registered Converter<String, ?>.

org.springframework.web.servlet/src/main/java/org/springframework/web/servlet/mvc/method/annotation/support/ServletModelAttributeMethodProcessor.java

@@ -22,6 +22,10 @@ import java.util.Map;
 import javax.servlet.ServletRequest;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.core.convert.ConversionService;
+import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.core.convert.converter.Converter;
+import org.springframework.util.StringUtils;
 import org.springframework.validation.DataBinder;
 import org.springframework.web.bind.ServletRequestDataBinder;
 import org.springframework.web.bind.WebDataBinder;
@@ -36,9 +40,9 @@ import org.springframework.web.servlet.mvc.method.annotation.ServletRequestDataB
  * A Servlet-specific {@link ModelAttributeMethodProcessor} that applies data
  * binding through a WebDataBinder of type {@link ServletRequestDataBinder}. 
  * 
- * <p>Adds a fall-back strategy to instantiate a model attribute from a 
- * URI template variable combined with type conversion, if the model attribute 
- * name matches to a URI template variable name.
+ * <p>Also adds a fall-back strategy to instantiate the model attribute from a
+ * URI template variable or from a request parameter if the name matches the
+ * model attribute name and there is an appropriate type conversion strategy.
  *
  * @author Rossen Stoyanchev
  * @since 3.1
@@ -56,36 +60,89 @@ public class ServletModelAttributeMethodProcessor extends ModelAttributeMethodPr
 	}
 
 	/**
-	 * Add a fall-back strategy to instantiate the model attribute from a URI 
-	 * template variable with type conversion, if the model attribute name 
-	 * matches to a URI variable name.
+	 * Instantiate the model attribute from a URI template variable or from a
+	 * request parameter if the name matches to the model attribute name and
+	 * if there is an appropriate type conversion strategy. If none of these
+	 * are true delegate back to the base class.
+	 * @see #createAttributeFromUriValue
 	 */
 	@Override
-	protected Object createAttribute(String attributeName, 
-									 MethodParameter parameter, 
-									 WebDataBinderFactory binderFactory, 
-									 NativeWebRequest request) throws Exception {
-		
-		Map<String, String> uriVariables = getUriTemplateVariables(request);
-		
-		if (uriVariables.containsKey(attributeName)) {
-			DataBinder binder = binderFactory.createBinder(request, null, attributeName);
-			return binder.convertIfNecessary(uriVariables.get(attributeName), parameter.getParameterType());
+	protected final Object createAttribute(String attributeName, 
+										   MethodParameter parameter, 
+										   WebDataBinderFactory binderFactory, 
+										   NativeWebRequest request) throws Exception {
+
+		String value = getRequestValueForAttribute(attributeName, request);
+		if (value != null) {
+			Object attribute = createAttributeFromRequestValue(value, attributeName, parameter, binderFactory, request);
+			if (attribute != null) {
+				return attribute;
+			}
 		}
-		
+
 		return super.createAttribute(attributeName, parameter, binderFactory, request);
 	}
+	
+	/**
+	 * Obtain a value from the request that may be used to instantiate the
+	 * model attribute through type conversion from String to the target type. 
+	 * <p>The default implementation looks for the attribute name to match
+	 * a URI variable first and then a request parameter.
+	 * @param attributeName the model attribute name
+	 * @param request the current request
+	 * @return the request value to try to convert or {@code null}
+	 */
+	protected String getRequestValueForAttribute(String attributeName, NativeWebRequest request) {
+		Map<String, String> variables = getUriTemplateVariables(request);
+		if (StringUtils.hasText(variables.get(attributeName))) {
+			return variables.get(attributeName);
+		}
+		else if (StringUtils.hasText(request.getParameter(attributeName))) {
+			return request.getParameter(attributeName);
+		}
+		else {
+			return null;
+		}
+	}
 
 	@SuppressWarnings("unchecked")
-	private Map<String, String> getUriTemplateVariables(NativeWebRequest request) {
-		
-		Map<String, String> uriTemplateVars = 
+	protected final Map<String, String> getUriTemplateVariables(NativeWebRequest request) {
+		Map<String, String> variables = 
 			(Map<String, String>) request.getAttribute(
 					HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
-		
-		return (uriTemplateVars != null) ? uriTemplateVars : Collections.<String, String>emptyMap();
+		return (variables != null) ? variables : Collections.<String, String>emptyMap();
 	}
 
+	/**
+	 * Create a model attribute from a String request value (e.g. URI template
+	 * variable, request parameter) using type conversion.
+	 * <p>The default implementation converts only if there a registered 
+	 * {@link Converter} that can perform the conversion.
+	 * @param sourceValue the source value to create the model attribute from
+	 * @param attributeName the name of the attribute, never {@code null}
+	 * @param parameter the method parameter
+	 * @param binderFactory for creating WebDataBinder instance
+	 * @param request the current request
+	 * @return the created model attribute, or {@code null}
+	 * @throws Exception
+	 */
+	protected Object createAttributeFromRequestValue(String sourceValue,
+												 String attributeName, 
+												 MethodParameter parameter, 
+												 WebDataBinderFactory binderFactory, 
+												 NativeWebRequest request) throws Exception {
+		DataBinder binder = binderFactory.createBinder(request, null, attributeName);
+		ConversionService conversionService = binder.getConversionService();
+		if (conversionService != null) {
+			TypeDescriptor source = TypeDescriptor.valueOf(String.class);
+			TypeDescriptor target = new TypeDescriptor(parameter);
+			if (conversionService.canConvert(source, target)) {
+				return binder.convertIfNecessary(sourceValue, parameter.getParameterType(), parameter);
+			}
+		}
+		return null;
+	}
+	
 	/**
 	 * {@inheritDoc}
 	 * <p>Downcast {@link WebDataBinder} to {@link ServletRequestDataBinder} before binding.