Allow explicitly sending no API version

Closes gh-35566

spring-test/src/main/java/org/springframework/test/web/servlet/client/DefaultRestTestClient.java

@@ -248,7 +248,7 @@ class DefaultRestTestClient implements RestTestClient {
 		}
 
 		@Override
-		public RequestBodySpec apiVersion(Object version) {
+		public RequestBodySpec apiVersion(@Nullable Object version) {
 			this.requestHeadersUriSpec.apiVersion(version);
 			return this;
 		}

spring-test/src/main/java/org/springframework/test/web/servlet/client/RestTestClient.java

@@ -479,14 +479,19 @@ public interface RestTestClient {
 
 		/**
 		 * Set an API version for the request. The version is inserted into the
-		 * request by the {@linkplain Builder#apiVersionInserter(ApiVersionInserter)
+		 * request through the {@link Builder#apiVersionInserter(ApiVersionInserter)
 		 * configured} {@code ApiVersionInserter}.
-		 * @param version the API version of the request; this can be a String or
-		 * some Object that can be formatted by the inserter — for example,
-		 * through an {@link ApiVersionFormatter}
+		 * <p>If no version is set, the
+		 * {@link Builder#defaultApiVersion(Object) defaultApiVersion} is used,
+		 * if configured.
+		 * <p>If {@code null} is passed, then an API version is not inserted
+		 * irrespective of default version settings.
+		 * @param version the API version for the request; this can be a String
+		 * or some Object that can be formatted the inserter, e.g. through an
+		 * {@link ApiVersionFormatter}.
 		 * @return this spec for further declaration of the request
 		 */
-		S apiVersion(Object version);
+		S apiVersion(@Nullable Object version);
 
 		/**
 		 * Set the attribute with the given name to the given value.

spring-web/src/main/java/org/springframework/web/client/DefaultRestClient.java

@@ -293,6 +293,8 @@ final class DefaultRestClient implements RestClient {
 
 	private class DefaultRequestBodyUriSpec implements RequestBodyUriSpec {
 
+		private static final Object NO_VERSION = new Object();
+
 		private final HttpMethod httpMethod;
 
 		private @Nullable URI uri;
@@ -430,8 +432,8 @@ final class DefaultRestClient implements RestClient {
 		}
 
 		@Override
-		public RequestBodySpec apiVersion(Object version) {
-			this.apiVersion = version;
+		public RequestBodySpec apiVersion(@Nullable Object version) {
+			this.apiVersion = (version != null ? version : NO_VERSION);
 			return this;
 		}
 
@@ -646,7 +648,15 @@ final class DefaultRestClient implements RestClient {
 		}
 
 		private @Nullable Object getApiVersionOrDefault() {
-			return (this.apiVersion != null ? this.apiVersion : DefaultRestClient.this.defaultApiVersion);
+			if (this.apiVersion == null) {
+				return DefaultRestClient.this.defaultApiVersion;
+			}
+			else if (this.apiVersion == NO_VERSION) {
+				return null;
+			}
+			else {
+				return this.apiVersion;
+			}
 		}
 
 		private @Nullable String serializeCookies() {

spring-web/src/main/java/org/springframework/web/client/RestClient.java

@@ -642,14 +642,19 @@ public interface RestClient {
 
 		/**
 		 * Set an API version for the request. The version is inserted into the
-		 * request by the {@link Builder#apiVersionInserter(ApiVersionInserter)
+		 * request through the {@link Builder#apiVersionInserter(ApiVersionInserter)
 		 * configured} {@code ApiVersionInserter}.
-		 * @param version the API version of the request; this can be a String or
-		 * some Object that can be formatted the inserter, e.g. through an
+		 * <p>If no version is set, the
+		 * {@link Builder#defaultApiVersion(Object) defaultApiVersion} is used,
+		 * if configured.
+		 * <p>If {@code null} is passed, then an API version is not inserted
+		 * irrespective of default version settings.
+		 * @param version the API version for the request; this can be a String
+		 * or some Object that can be formatted the inserter, e.g. through an
 		 * {@link ApiVersionFormatter}.
 		 * @since 7.0
 		 */
-		S apiVersion(Object version);
+		S apiVersion(@Nullable Object version);
 
 		/**
 		 * Set the attribute with the given name to the given value.

spring-web/src/test/java/org/springframework/web/client/RestClientVersionTests.java

@@ -108,6 +108,15 @@ public class RestClientVersionTests {
 		expectRequest(request -> assertThat(request.getHeaders().get("API-Version")).isEqualTo("1.2"));
 	}
 
+	@Test
+	void noVersion() {
+		ApiVersionInserter inserter = ApiVersionInserter.useHeader("API-Version");
+		RestClient restClient = restClientBuilder.defaultApiVersion(1.2).apiVersionInserter(inserter).build();
+		restClient.get().uri("/path").apiVersion(null).retrieve().body(String.class);
+
+		expectRequest(request -> assertThat(request.getHeaders().get("API-Version")).isNull());
+	}
+
 	private void performRequest(ApiVersionInserter versionInserter) {
 		restClientBuilder.apiVersionInserter(versionInserter).build()
 				.post().uri("/path")

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java

@@ -206,6 +206,8 @@ final class DefaultWebClient implements WebClient {
 
 	private class DefaultRequestBodyUriSpec implements RequestBodyUriSpec {
 
+		private static final Object NO_VERSION = new Object();
+
 		private final HttpMethod httpMethod;
 
 		private @Nullable URI uri;
@@ -335,8 +337,8 @@ final class DefaultWebClient implements WebClient {
 		}
 
 		@Override
-		public DefaultRequestBodyUriSpec apiVersion(Object version) {
-			this.apiVersion = version;
+		public DefaultRequestBodyUriSpec apiVersion(@Nullable Object version) {
+			this.apiVersion = (version != null ? version : NO_VERSION);
 			return this;
 		}
 
@@ -503,7 +505,15 @@ final class DefaultWebClient implements WebClient {
 		}
 
 		private @Nullable Object getApiVersionOrDefault() {
-			return (this.apiVersion != null ? this.apiVersion : DefaultWebClient.this.defaultApiVersion);
+			if (this.apiVersion == null) {
+				return DefaultWebClient.this.defaultApiVersion;
+			}
+			else if (this.apiVersion == NO_VERSION) {
+				return null;
+			}
+			else {
+				return this.apiVersion;
+			}
 		}
 
 		private void initHeaders(HttpHeaders out) {

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/WebClient.java

@@ -497,14 +497,19 @@ public interface WebClient {
 
 		/**
 		 * Set an API version for the request. The version is inserted into the
-		 * request by the {@link Builder#apiVersionInserter(ApiVersionInserter)
+		 * request through the {@link Builder#apiVersionInserter(ApiVersionInserter)
 		 * configured} {@code ApiVersionInserter}.
-		 * @param version the API version of the request; this can be a String or
-		 * some Object that can be formatted the inserter, e.g. through an
+		 * <p>If no version is set, the
+		 * {@link Builder#defaultApiVersion(Object) defaultApiVersion} is used,
+		 * if configured.
+		 * <p>If {@code null} is passed, then an API version is not inserted
+		 * irrespective of default version settings.
+		 * @param version the API version for the request; this can be a String
+		 * or some Object that can be formatted the inserter, e.g. through an
 		 * {@link ApiVersionFormatter}.
 		 * @since 7.0
 		 */
-		S apiVersion(Object version);
+		S apiVersion(@Nullable Object version);
 
 		/**
 		 * Set the attribute with the given name to the given value.

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientVersionTests.java

@@ -99,6 +99,15 @@ public class WebClientVersionTests {
 		expectRequest(request -> assertThat(request.getHeaders().get("API-Version")).isEqualTo("1.2"));
 	}
 
+	@Test
+	void noVersion() {
+		ApiVersionInserter inserter = ApiVersionInserter.useHeader("API-Version");
+		WebClient webClient = webClientBuilder.defaultApiVersion(1.2).apiVersionInserter(inserter).build();
+		webClient.get().uri("/path").apiVersion(null).retrieve().bodyToMono(String.class).block();
+
+		expectRequest(request -> assertThat(request.getHeaders().get("API-Version")).isNull());
+	}
+
 	private void performRequest(ApiVersionInserter versionInserter) {
 		WebClient webClient = webClientBuilder.apiVersionInserter(versionInserter).build();
 		webClient.get().uri("/path").apiVersion(1.2).retrieve().bodyToMono(String.class).block();