Improve @SessionAttributes support during redirect

Before this change attributes listed with @SessionAttributes would not
be saved in the session when there was a redirect and the controller
method declared a parameter of type RedirectAttributes.

This change ensures it's the "default" model that is always the one
checked for @SessionAttributes under all circumstances since
RedirectAttributes is really only meant to provide String values to
insert into or append to the the redirect URL.

Issue: SPR-12542

spring-web/src/main/java/org/springframework/web/method/annotation/ModelFactory.java

@@ -224,14 +224,15 @@ public final class ModelFactory {
 	 * @throws Exception if creating BindingResult attributes fails
 	 */
 	public void updateModel(NativeWebRequest request, ModelAndViewContainer mavContainer) throws Exception {
+		ModelMap defaultModel = mavContainer.getDefaultModel();
 		if (mavContainer.getSessionStatus().isComplete()){
 			this.sessionAttributesHandler.cleanupAttributes(request);
 		}
 		else {
-			this.sessionAttributesHandler.storeAttributes(request, mavContainer.getModel());
+			this.sessionAttributesHandler.storeAttributes(request, defaultModel);
 		}
-		if (!mavContainer.isRequestHandled()) {
-			updateBindingResult(request, mavContainer.getModel());
+		if (!mavContainer.isRequestHandled() && mavContainer.getModel() == defaultModel) {
+			updateBindingResult(request, defaultModel);
 		}
 	}
 

spring-web/src/main/java/org/springframework/web/method/support/ModelAndViewContainer.java

@@ -137,6 +137,19 @@ public class ModelAndViewContainer {
 		return (!this.redirectModelScenario || (this.redirectModel == null && !this.ignoreDefaultModelOnRedirect));
 	}
 
+	/**
+	 * Return the "default" model created at instantiation.
+	 * <p>In general it is recommended to use {@link #getModel()} instead which
+	 * returns either the "default" model (template rendering) or the "redirect"
+	 * model (redirect URL preparation). Use of this method may be needed for
+	 * advanced cases when access to the "default" model is needed regardless,
+	 * e.g. to save model attributes specified via {@code @SessionAttributes}.
+	 * @return the default model, never {@code null}
+	 */
+	public ModelMap getDefaultModel() {
+		return this.defaultModel;
+	}
+
 	/**
 	 * Provide a separate model instance to use in a redirect scenario.
 	 * The provided additional model however is not used used unless

spring-web/src/test/java/org/springframework/web/method/annotation/ModelFactoryTests.java

@@ -215,6 +215,32 @@ public class ModelFactoryTests {
 		assertNull(this.sessionAttributeStore.retrieveAttribute(this.webRequest, attributeName));
 	}
 
+	// SPR-12542
+
+	@Test
+	public void updateModelWhenRedirecting() throws Exception {
+		String attributeName = "sessionAttr";
+		String attribute = "value";
+		ModelAndViewContainer container = new ModelAndViewContainer();
+		container.addAttribute(attributeName, attribute);
+
+		String queryParam = "123";
+		String queryParamName = "q";
+		container.setRedirectModel(new ModelMap(queryParamName, queryParam));
+		container.setRedirectModelScenario(true);
+
+		WebDataBinder dataBinder = new WebDataBinder(attribute, attributeName);
+		WebDataBinderFactory binderFactory = mock(WebDataBinderFactory.class);
+		given(binderFactory.createBinder(this.webRequest, attribute, attributeName)).willReturn(dataBinder);
+
+		ModelFactory modelFactory = new ModelFactory(null, binderFactory, this.sessionAttrsHandler);
+		modelFactory.updateModel(this.webRequest, container);
+
+		assertEquals(queryParam, container.getModel().get(queryParamName));
+		assertEquals(1, container.getModel().size());
+		assertEquals(attribute, this.sessionAttributeStore.retrieveAttribute(this.webRequest, attributeName));
+	}
+
 
 	private String bindingResultKey(String key) {
 		return BindingResult.MODEL_KEY_PREFIX + key;