Support Optional in UriComponentsBuilder#queryParam

Closes gh-25951

spring-web/src/main/java/org/springframework/web/util/UriComponentsBuilder.java

@@ -698,7 +698,7 @@ public class UriComponentsBuilder implements UriBuilder, Cloneable {
 		Assert.notNull(name, "Name must not be null");
 		if (!ObjectUtils.isEmpty(values)) {
 			for (Object value : values) {
-				String valueAsString = (value != null ? value.toString() : null);
+				String valueAsString = getQueryParamValue(value);
 				this.queryParams.add(name, valueAsString);
 			}
 		}
@@ -709,6 +709,16 @@ public class UriComponentsBuilder implements UriBuilder, Cloneable {
 		return this;
 	}
 
+	@Nullable
+	private String getQueryParamValue(@Nullable Object value) {
+		if (value != null) {
+			return (value instanceof Optional ?
+					((Optional<?>) value).map(Object::toString).orElse(null) :
+					value.toString());
+		}
+		return null;
+	}
+
 	@Override
 	public UriComponentsBuilder queryParam(String name, @Nullable Collection<?> values) {
 		return queryParam(name, (CollectionUtils.isEmpty(values) ? EMPTY_VALUES : values.toArray()));

spring-web/src/test/java/org/springframework/web/util/UriComponentsBuilderTests.java

@@ -749,14 +749,24 @@ class UriComponentsBuilderTests {
 
 	@Test
 	void queryParamWithList() {
-		UriComponentsBuilder builder = UriComponentsBuilder.newInstance();
-		UriComponents result = builder.queryParam("baz", Arrays.asList("qux", 42)).build();
+		List<String> values = Arrays.asList("qux", "42");
+		UriComponents result = UriComponentsBuilder.newInstance().queryParam("baz", values).build();
 
 		assertThat(result.getQuery()).isEqualTo("baz=qux&baz=42");
-		MultiValueMap<String, String> expectedQueryParams = new LinkedMultiValueMap<>(2);
-		expectedQueryParams.add("baz", "qux");
-		expectedQueryParams.add("baz", "42");
-		assertThat(result.getQueryParams()).isEqualTo(expectedQueryParams);
+		assertThat(result.getQueryParams()).containsOnlyKeys("baz").containsEntry("baz", values);
+	}
+
+	@Test
+	void queryParamWithOptionalValue() {
+		UriComponents result = UriComponentsBuilder.newInstance()
+				.queryParam("foo", Optional.empty())
+				.queryParam("baz", Optional.of("qux"), 42)
+				.build();
+
+		assertThat(result.getQuery()).isEqualTo("foo&baz=qux&baz=42");
+		assertThat(result.getQueryParams()).containsOnlyKeys("foo", "baz")
+				.containsEntry("foo", Collections.singletonList(null))
+				.containsEntry("baz", Arrays.asList("qux", "42"));
 	}
 
 	@Test