SPR-7608 Add fallback mechanism for instantiating a model attribute from a path variable

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

@@ -64,9 +64,9 @@ public class PathVariableMethodArgumentResolver extends AbstractNamedValueMethod
 	@Override
 	@SuppressWarnings("unchecked")
 	protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
-		String key = HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE;
-		int scope = RequestAttributes.SCOPE_REQUEST;
-		Map<String, String> uriTemplateVars = (Map<String, String>) request.getAttribute(key, scope);
+		Map<String, String> uriTemplateVars = 
+			(Map<String, String>) request.getAttribute(
+					HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
 		return (uriTemplateVars != null) ? uriTemplateVars.get(name) : null;
 	}
 

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

@@ -16,17 +16,31 @@
 
 package org.springframework.web.servlet.mvc.method.annotation.support;
 
+import java.beans.PropertyEditor;
+import java.util.Map;
+
 import javax.servlet.ServletRequest;
 
 import org.springframework.beans.BeanUtils;
+import org.springframework.core.MethodParameter;
+import org.springframework.core.convert.converter.Converter;
+import org.springframework.validation.DataBinder;
 import org.springframework.web.bind.ServletRequestDataBinder;
 import org.springframework.web.bind.WebDataBinder;
 import org.springframework.web.bind.annotation.ModelAttribute;
+import org.springframework.web.bind.support.WebDataBinderFactory;
 import org.springframework.web.context.request.NativeWebRequest;
+import org.springframework.web.context.request.RequestAttributes;
 import org.springframework.web.method.annotation.support.ModelAttributeMethodProcessor;
+import org.springframework.web.servlet.HandlerMapping;
 
 /**
- * A Servlet-specific {@link ModelAttributeMethodProcessor} variant that casts the {@link WebDataBinder}
+ * A Servlet-specific {@link ModelAttributeMethodProcessor} variant with the following further benefits:
+ * <ul>
+ * 	<li>Casts the data binder down to {@link ServletRequestDataBinder} prior to invoking bind on it
+ * 	<li>Attempts to instantiate the model attribute using a path variable and type conversion
+ * </ul>
+ *  that casts
  * instance to {@link ServletRequestDataBinder} prior to invoking data binding.
  *
  * @author Rossen Stoyanchev
@@ -44,12 +58,43 @@ public class ServletModelAttributeMethodProcessor extends ModelAttributeMethodPr
 	}
 
 	/**
-	 * {@inheritDoc}
-	 * <p>This method downcasts the binder instance to {@link ServletRequestDataBinder} and invokes
-	 * its bind method passing a {@link ServletRequest} to it.
+	 * Instantiates the model attribute by trying to match the model attribute name to a path variable.
+	 * If a match is found an attempt is made to convert the String path variable to the expected 
+	 * method parameter type through a registered {@link Converter} or {@link PropertyEditor}.
+	 * If this fails the call is delegated back to the parent for default constructor instantiation.   
 	 */
 	@Override
-	protected void doBind(WebDataBinder binder, NativeWebRequest request) {
+	@SuppressWarnings("unchecked")
+	protected Object createAttribute(String attributeName, 
+									 MethodParameter parameter, 
+									 WebDataBinderFactory binderFactory, 
+									 NativeWebRequest request) throws Exception {
+		Map<String, String> uriTemplateVars = 
+			(Map<String, String>) request.getAttribute(
+					HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
+
+		if (uriTemplateVars != null && uriTemplateVars.containsKey(attributeName)) {
+			try {
+				String var = uriTemplateVars.get(attributeName);
+				DataBinder binder = binderFactory.createBinder(request, null, attributeName);
+				return binder.convertIfNecessary(var, parameter.getParameterType());
+
+			} catch (Exception exception) {
+				logger.info("Model attribute '" + attributeName + "' matches to a URI template variable name. "
+						+ "The URI template variable however couldn't converted to a model attribute instance: "
+						+ exception.getMessage());
+			}
+		}
+		
+		return super.createAttribute(attributeName, parameter, binderFactory, request);
+	}
+
+	/**
+	 * {@inheritDoc}
+	 * <p>This implementation downcasts to {@link ServletRequestDataBinder} before invoking the bind operation.
+	 */
+	@Override
+	protected void bindRequestParameters(WebDataBinder binder, NativeWebRequest request) {
 		ServletRequest servletRequest = request.getNativeRequest(ServletRequest.class);
 		ServletRequestDataBinder servletBinder = (ServletRequestDataBinder) binder;
 		servletBinder.bind(servletRequest);