Add multipart support for MockMvcTester

File uploads with MockMvc require a separate
MockHttpServletRequestBuilder implementation. This commit applies the
same change to support AssertJ on this builder, but for the multipart
version.

Any request builder can now use `multipart()` to "down cast" to a
dedicated multipart request builder that contains the settings
configured thus far.

Closes gh-33027

spring-test/src/test/java/org/springframework/test/web/servlet/assertj/MockMvcTesterIntegrationTests.java

@@ -20,6 +20,7 @@ import java.io.ByteArrayOutputStream;
 import java.io.PrintStream;
 import java.nio.charset.StandardCharsets;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
@@ -31,6 +32,7 @@ import jakarta.servlet.ServletException;
 import jakarta.servlet.http.Cookie;
 import jakarta.servlet.http.HttpServletRequest;
 import jakarta.servlet.http.HttpServletResponse;
+import jakarta.servlet.http.Part;
 import jakarta.validation.Valid;
 import jakarta.validation.constraints.Size;
 import org.junit.jupiter.api.AfterEach;
@@ -44,6 +46,8 @@ import org.springframework.core.io.ClassPathResource;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
+import org.springframework.mock.web.MockMultipartFile;
+import org.springframework.mock.web.MockPart;
 import org.springframework.stereotype.Controller;
 import org.springframework.test.context.junit.jupiter.web.SpringJUnitWebConfig;
 import org.springframework.test.web.Person;
@@ -54,13 +58,19 @@ import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.ModelAttribute;
 import org.springframework.web.bind.annotation.PathVariable;
 import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.PutMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.bind.annotation.RequestPart;
 import org.springframework.web.bind.annotation.RestController;
 import org.springframework.web.bind.annotation.SessionAttributes;
 import org.springframework.web.context.WebApplicationContext;
+import org.springframework.web.multipart.MultipartFile;
+import org.springframework.web.multipart.support.MissingServletRequestPartException;
 import org.springframework.web.server.ResponseStatusException;
 import org.springframework.web.servlet.DispatcherServlet;
 import org.springframework.web.servlet.HandlerInterceptor;
+import org.springframework.web.servlet.ModelAndView;
 import org.springframework.web.servlet.config.annotation.EnableWebMvc;
 import org.springframework.web.servlet.mvc.support.RedirectAttributes;
 
@@ -113,6 +123,41 @@ public class MockMvcTesterIntegrationTests {
 		}
 	}
 
+	@Nested
+	class MultipartTests {
+
+		private final MockMultipartFile JSON_PART_FILE = new MockMultipartFile("json", "json", "application/json", """
+				{
+					"name": "test"
+				}""".getBytes(StandardCharsets.UTF_8));
+
+		@Test
+		void multipartWithPut() {
+			MockMultipartFile part = new MockMultipartFile("file", "content.txt", null, "value".getBytes(StandardCharsets.UTF_8));
+			assertThat(mvc.put().uri("/multipart-put").multipart().file(part).file(JSON_PART_FILE))
+					.hasStatusOk()
+					.hasViewName("index")
+					.model().contains(entry("name", "file"));
+		}
+
+		@Test
+		void multipartWithMissingPart() {
+			assertThat(mvc.put().uri("/multipart-put").multipart().file(JSON_PART_FILE))
+					.hasStatus(HttpStatus.BAD_REQUEST)
+					.failure().isInstanceOfSatisfying(MissingServletRequestPartException.class,
+							ex -> assertThat(ex.getRequestPartName()).isEqualTo("file"));
+		}
+
+		@Test
+		void multipartWithNamedPart() {
+			MockPart part = new MockPart("part", "content.txt", "value".getBytes(StandardCharsets.UTF_8));
+			assertThat(mvc.post().uri("/part").multipart().part(part).file(JSON_PART_FILE))
+					.hasStatusOk()
+					.hasViewName("index")
+					.model().contains(entry("part", "content.txt"), entry("name", "test"));
+		}
+	}
+
 	@Nested
 	class CookieTests {
 
@@ -516,7 +561,7 @@ public class MockMvcTesterIntegrationTests {
 	@Configuration
 	@EnableWebMvc
 	@Import({ TestController.class, PersonController.class, AsyncController.class,
-			SessionController.class, ErrorController.class })
+			MultipartController.class, SessionController.class, ErrorController.class })
 	static class WebConfiguration {
 	}
 
@@ -564,6 +609,22 @@ public class MockMvcTesterIntegrationTests {
 		}
 	}
 
+	@Controller
+	static class MultipartController {
+
+		@PostMapping("/part")
+		ModelAndView part(@RequestPart Part part, @RequestPart Map<String, String> json) {
+			Map<String, Object> model = new HashMap<>(json);
+			model.put(part.getName(), part.getSubmittedFileName());
+			return new ModelAndView("index", model);
+		}
+
+		@PutMapping("/multipart-put")
+		public ModelAndView multiPartViaHttpPut(@RequestParam MultipartFile file) {
+			return new ModelAndView("index", Map.of("name", file.getName()));
+		}
+	}
+
 	@Controller
 	@SessionAttributes("locale")
 	static class SessionController {