Fix URL decoding issue in reactive @RequestParam

%-encoded strings were injected undecoded into @RequestParam variables,
which does not coincide with spring-webmvc behaviour. This commit
fixes AbstractServerHttpRequest.getQueryParams() to correctly return
decoded name-value pairs.

Issue: SPR-15140

spring-web/src/main/java/org/springframework/http/server/reactive/AbstractServerHttpRequest.java

@@ -16,7 +16,10 @@
 
 package org.springframework.http.server.reactive;
 
+import java.io.UnsupportedEncodingException;
 import java.net.URI;
+import java.net.URLDecoder;
+import java.nio.charset.StandardCharsets;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -76,6 +79,22 @@ public abstract class AbstractServerHttpRequest implements ServerHttpRequest {
 		return this.queryParams;
 	}
 
+	/**
+	 * A method for decoding name and value string in a name-value pair.
+	 * <p>Note that the plus sign "+" is converted into a space character " ".</p>
+	 * @param encodedString the string to be decoded
+	 * @return the decoded string
+	 * @see java.net.URLDecoder#decode(String, String)
+	 */
+	private static String decodeQueryParam(final String encodedString) {
+		try {
+			return URLDecoder.decode(encodedString, StandardCharsets.UTF_8.name());
+		} catch (UnsupportedEncodingException e) {
+			// StandardCharsets are guaranteed to be available on every implementation of the Java platform, so this should never happen.
+			throw new IllegalStateException(e);
+		}
+	}
+
 	/**
 	 * A method for parsing of the query into name-value pairs. The return
 	 * value is turned into an immutable map and cached.
@@ -94,7 +113,8 @@ public abstract class AbstractServerHttpRequest implements ServerHttpRequest {
 				String eq = matcher.group(2);
 				String value = matcher.group(3);
 				value = (value != null ? value : (StringUtils.hasLength(eq) ? "" : null));
-				queryParams.add(name, value);
+				queryParams.add(decodeQueryParam(name),
+						value != null ? decodeQueryParam(value) : null);
 			}
 		}
 		return queryParams;

spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpRequestTests.java

@@ -64,6 +64,13 @@ public class ServerHttpRequestTests {
 		assertEquals(Arrays.asList("1", "2"), params.get("a"));
 	}
 
+	@Test
+	public void queryParamsWithUrlEncodedValue() throws Exception {
+		MultiValueMap<String, String> params = createHttpRequest("/path?a=%20%2B+%C3%A0").getQueryParams();
+		assertEquals(1, params.size());
+		assertEquals(Collections.singletonList(" + \u00e0"), params.get("a"));
+	}
+
 	@Test
 	public void queryParamsWithEmptyValue() throws Exception {
 		MultiValueMap<String, String> params = createHttpRequest("/path?a=").getQueryParams();