Remove use of ServletException in ModelFactory

This commit changes the use of HttpSessionRequiredException in
ModelFactory::initModel to an IllegalStateException, because the former
extends ServletException and cannot be used in WebFlux.

Closes gh-33043

Author: poutsma

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

@@ -37,7 +37,6 @@
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 import org.springframework.validation.BindingResult;
-import org.springframework.web.HttpSessionRequiredException;
 import org.springframework.web.bind.WebDataBinder;
 import org.springframework.web.bind.annotation.ModelAttribute;
 import org.springframework.web.bind.support.WebDataBinderFactory;
@@ -115,7 +114,7 @@ public void initModel(NativeWebRequest request, ModelAndViewContainer container,
 			if (!container.containsAttribute(name)) {
 				Object value = this.sessionAttributesHandler.retrieveAttribute(request, name);
 				if (value == null) {
-					throw new HttpSessionRequiredException("Expected session attribute '" + name + "'", name);
+					throw new IllegalStateException("Expected session attribute '" + name + "'");
 				}
 				container.addAttribute(name, value);
 			}

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

@@ -27,7 +27,6 @@
 import org.springframework.ui.Model;
 import org.springframework.ui.ModelMap;
 import org.springframework.validation.BindingResult;
-import org.springframework.web.HttpSessionRequiredException;
 import org.springframework.web.bind.WebDataBinder;
 import org.springframework.web.bind.annotation.ModelAttribute;
 import org.springframework.web.bind.annotation.SessionAttributes;
@@ -43,7 +42,7 @@
 import org.springframework.web.testfixture.servlet.MockHttpServletRequest;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 import static org.mockito.BDDMockito.given;
 import static org.mockito.Mockito.mock;
 
@@ -151,7 +150,7 @@ void sessionAttribute() throws Exception {
 	void sessionAttributeNotPresent() throws Exception {
 		ModelFactory modelFactory = new ModelFactory(null, null, this.attributeHandler);
 		HandlerMethod handlerMethod = createHandlerMethod("handleSessionAttr", String.class);
-		assertThatExceptionOfType(HttpSessionRequiredException.class).isThrownBy(() ->
+		assertThatIllegalStateException().isThrownBy(() ->
 				modelFactory.initModel(this.webRequest, this.mavContainer, handlerMethod));
 
 		// Now add attribute and try again
@@ -164,7 +163,7 @@ void sessionAttributeNotPresent() throws Exception {
 	void sessionAttributeByType() throws Exception {
 		ModelFactory modelFactory = new ModelFactory(null, null, this.attributeHandler);
 		HandlerMethod handlerMethod = createHandlerMethod("handleTestBean", TestBean.class);
-		assertThatExceptionOfType(HttpSessionRequiredException.class).isThrownBy(() ->
+		assertThatIllegalStateException().isThrownBy(() ->
 				modelFactory.initModel(this.webRequest, this.mavContainer, handlerMethod));
 
 		// Now add attribute and try again