Fix ServletRequestDataBinder ctor binding with `[]`-indexed query params

This change ensures that a request containing query parameters in the
array format `someArray[]=value` can be bound into a simple array in
constructors, even for cases where the array values don't have nested
properties.

The value resolver is directly called in the constructor case, before
any mutable properties are considered or even cleared (see
`WebDataBinder#adaptEmptyArrayIndices` method). As a result, we need to
accommodate the possibility that the request stores array elements under
the `name[]` key rather than `name`. This change attempts a secondary
lookup with the `[]` suffix if the type is a list or array, and the key
doesn't include an index.

Closes gh-34121

spring-test/src/test/java/org/springframework/test/web/servlet/samples/spr/ServletRequestDataBinderIntegrationTests.java

@@ -19,7 +19,6 @@ package org.springframework.test.web.servlet.samples.spr;
 import java.util.List;
 import java.util.Map;
 
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.test.context.junit.jupiter.web.SpringJUnitWebConfig;
@@ -57,7 +56,6 @@ class ServletRequestDataBinderIntegrationTests {
 				.andExpect(content().string("valueB"));
 	}
 
-	@Disabled("see gh-34121")
 	@Test // gh-34121
 	void postArrayWithEmptyIndex(WebApplicationContext wac) throws Exception {
 		MockMvc mockMvc = webAppContextSetup(wac).build();
@@ -88,7 +86,6 @@ class ServletRequestDataBinderIntegrationTests {
 				.andExpect(content().string("valueB"));
 	}
 
-	@Disabled("see gh-34121")
 	@Test // gh-34121
 	void postListWithEmptyIndex(WebApplicationContext wac) throws Exception {
 		MockMvc mockMvc = webAppContextSetup(wac).build();

spring-web/src/main/java/org/springframework/web/bind/ServletRequestDataBinder.java

@@ -244,6 +244,10 @@ public class ServletRequestDataBinder extends WebDataBinder {
 		@Nullable
 		protected Object getRequestParameter(String name, Class<?> type) {
 			Object value = this.request.getParameterValues(name);
+			if (value == null && !name.endsWith ("[]") &&
+					(List.class.isAssignableFrom(type) || type.isArray())) {
+				value = this.request.getParameterValues(name + "[]");
+			}
 			return (ObjectUtils.isArray(value) && Array.getLength(value) == 1 ? Array.get(value, 0) : value);
 		}
 

spring-web/src/test/java/org/springframework/web/bind/ServletRequestDataBinderTests.java

@@ -93,6 +93,32 @@ class ServletRequestDataBinderTests {
 		assertThat(target.isPostProcessed()).isFalse();
 	}
 
+	@Test
+	public void testFieldWithArrayIndex() {
+		TestBean target = new TestBean();
+		ServletRequestDataBinder binder = new ServletRequestDataBinder(target);
+		binder.setIgnoreUnknownFields(false);
+
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		request.addParameter("stringArray[0]", "ONE");
+		request.addParameter("stringArray[1]", "TWO");
+		binder.bind(request);
+		assertThat(target.getStringArray()).containsExactly("ONE", "TWO");
+	}
+
+	@Test
+	public void testFieldWithEmptyArrayIndex() {
+		TestBean target = new TestBean();
+		ServletRequestDataBinder binder = new ServletRequestDataBinder(target);
+		binder.setIgnoreUnknownFields(false);
+
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		request.addParameter("stringArray[]", "ONE");
+		request.addParameter("stringArray[]", "TWO");
+		binder.bind(request);
+		assertThat(target.getStringArray()).containsExactly("ONE", "TWO");
+	}
+
 	@Test
 	void testFieldDefault() {
 		TestBean target = new TestBean();