RequestParam resolver supports empty array suffix

Closes gh-32577

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -179,6 +179,9 @@ public class RequestParamMethodArgumentResolver extends AbstractNamedValueMethod
 		}
 		if (arg == null) {
 			String[] paramValues = request.getParameterValues(name);
+			if (paramValues == null) {
+				paramValues = request.getParameterValues(name + "[]");
+			}
 			if (paramValues != null) {
 				arg = (paramValues.length == 1 ? paramValues[0] : paramValues);
 			}

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

@@ -66,11 +66,11 @@ class RequestParamMethodArgumentResolverTests {
 
 	private RequestParamMethodArgumentResolver resolver = new RequestParamMethodArgumentResolver(null, true);
 
-	private MockHttpServletRequest request = new MockHttpServletRequest();
+	private final MockHttpServletRequest request = new MockHttpServletRequest();
 
 	private NativeWebRequest webRequest = new ServletWebRequest(request, new MockHttpServletResponse());
 
-	private ResolvableMethod testMethod = ResolvableMethod.on(getClass()).named("handle").build();
+	private final ResolvableMethod testMethod = ResolvableMethod.on(getClass()).named("handle").build();
 
 
 	@Test
@@ -167,6 +167,19 @@ class RequestParamMethodArgumentResolverTests {
 		assertThat((String[]) result).as("Invalid result").isEqualTo(expected);
 	}
 
+	@Test // gh-32577
+	void resolveStringArrayWithEmptyArraySuffix() throws Exception {
+		String[] expected = new String[] {"foo", "bar"};
+		request.addParameter("name[]", expected[0]);
+		request.addParameter("name[]", expected[1]);
+
+		MethodParameter param = this.testMethod.annotPresent(RequestParam.class).arg(String[].class);
+		Object result = resolver.resolveArgument(param, null, webRequest, null);
+		boolean condition = result instanceof String[];
+		assertThat(condition).isTrue();
+		assertThat((String[]) result).isEqualTo(expected);
+	}
+
 	@Test
 	void resolveMultipartFile() throws Exception {
 		MockMultipartHttpServletRequest request = new MockMultipartHttpServletRequest();

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/RequestParamMethodArgumentResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -100,8 +100,11 @@ public class RequestParamMethodArgumentResolver extends AbstractNamedValueSyncAr
 	@Override
 	@Nullable
 	protected Object resolveNamedValue(String name, MethodParameter parameter, ServerWebExchange exchange) {
-		List<String> paramValues = exchange.getRequest().getQueryParams().get(name);
 		Object result = null;
+		List<String> paramValues = exchange.getRequest().getQueryParams().get(name);
+		if (paramValues == null) {
+			paramValues = exchange.getRequest().getQueryParams().get(name + "[]");
+		}
 		if (paramValues != null) {
 			result = (paramValues.size() == 1 ? paramValues.get(0) : paramValues);
 		}

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestParamMethodArgumentResolverTests.java

@@ -53,7 +53,7 @@ class RequestParamMethodArgumentResolverTests {
 
 	private BindingContext bindContext;
 
-	private ResolvableMethod testMethod = ResolvableMethod.on(getClass()).named("handle").build();
+	private final ResolvableMethod testMethod = ResolvableMethod.on(getClass()).named("handle").build();
 
 
 	@BeforeEach
@@ -130,6 +130,16 @@ class RequestParamMethodArgumentResolverTests {
 		assertThat((String[]) result).isEqualTo(new String[] {"foo", "bar"});
 	}
 
+	@Test // gh-32577
+	void resolveStringArrayWithEmptyArraySuffix() {
+		MethodParameter param = this.testMethod.annotPresent(RequestParam.class).arg(String[].class);
+		MockServerHttpRequest request = MockServerHttpRequest.get("/path?name[]=foo&name[]=bar").build();
+		Object result = resolve(param, MockServerWebExchange.from(request));
+		boolean condition = result instanceof String[];
+		assertThat(condition).isTrue();
+		assertThat((String[]) result).isEqualTo(new String[] {"foo", "bar"});
+	}
+
 	@Test
 	void resolveDefaultValue() {
 		MethodParameter param = this.testMethod.annot(requestParam().notRequired("bar")).arg(String.class);