Use flexible generics nullness in RestClient ExchangeFunction

Closes gh-35180

Author: sdeleuze

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

@@ -561,7 +561,7 @@ public ResponseSpec retrieve() {
 		}
 
 		@Override
-		public <T> @Nullable T exchange(ExchangeFunction<T> exchangeFunction, boolean close) {
+		public <T extends @Nullable Object> T exchange(ExchangeFunction<T> exchangeFunction, boolean close) {
 			return exchangeInternal(exchangeFunction, close);
 		}
 
@@ -572,7 +572,7 @@ public <T> T exchangeForRequiredValue(RequiredValueExchangeFunction<T> exchangeF
 			return value;
 		}
 
-		private <T> @Nullable T exchangeInternal(ExchangeFunction<T> exchangeFunction, boolean close) {
+		private <T extends @Nullable Object> T exchangeInternal(ExchangeFunction<T> exchangeFunction, boolean close) {
 			Assert.notNull(exchangeFunction, "ExchangeFunction must not be null");
 
 			ClientHttpResponse clientResponse = null;
@@ -803,11 +803,13 @@ private ResponseSpec onStatusInternal(StatusHandler statusHandler) {
 		}
 
 		@Override
+		@SuppressWarnings("NullAway") // See https://github.com/uber/NullAway/issues/1075
 		public <T> @Nullable T body(Class<T> bodyType) {
 			return executeAndExtract((request, response) -> readBody(request, response, bodyType, bodyType, this.hints));
 		}
 
 		@Override
+		@SuppressWarnings("NullAway") // See https://github.com/uber/NullAway/issues/1075
 		public <T> @Nullable T body(ParameterizedTypeReference<T> bodyType) {
 			Type type = bodyType.getType();
 			Class<T> bodyClass = bodyClass(type);
@@ -880,7 +882,7 @@ private Map<String, Object> getHints() {
 			return hints;
 		}
 
-		public <T> @Nullable T executeAndExtract(RequestHeadersSpec.ExchangeFunction<T> exchangeFunction) {
+		public <T extends @Nullable Object> T executeAndExtract(RequestHeadersSpec.ExchangeFunction<T> exchangeFunction) {
 			return this.requestHeadersSpec.exchange(exchangeFunction);
 		}
 

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

@@ -29,6 +29,7 @@
 import java.util.function.Predicate;
 
 import io.micrometer.observation.ObservationRegistry;
+import org.jspecify.annotations.NonNull;
 import org.jspecify.annotations.Nullable;
 
 import org.springframework.core.ParameterizedTypeReference;
@@ -714,7 +715,7 @@ interface RequestHeadersSpec<S extends RequestHeadersSpec<S>> {
 		 * @return the value returned from the exchange function, potentially {@code null}
 		 * @see RequestHeadersSpec#exchangeForRequiredValue(RequiredValueExchangeFunction)
 		 */
-		default <T> @Nullable T exchange(ExchangeFunction<T> exchangeFunction) {
+		default <T extends @Nullable Object> T exchange(ExchangeFunction<T> exchangeFunction) {
 			return exchange(exchangeFunction, true);
 		}
 
@@ -775,7 +776,7 @@ default <T> T exchangeForRequiredValue(RequiredValueExchangeFunction<T> exchange
 		 * @return the value returned from the exchange function, potentially {@code null}
 		 * @see RequestHeadersSpec#exchangeForRequiredValue(RequiredValueExchangeFunction, boolean)
 		 */
-		<T> @Nullable T exchange(ExchangeFunction<T> exchangeFunction, boolean close);
+		<T extends @Nullable Object> T exchange(ExchangeFunction<T> exchangeFunction, boolean close);
 
 		/**
 		 * Exchange the {@link ClientHttpResponse} for a value of type {@code T}.
@@ -814,7 +815,7 @@ default <T> T exchangeForRequiredValue(RequiredValueExchangeFunction<T> exchange
 		 * @param <T> the type the response will be transformed to
 		 */
 		@FunctionalInterface
-		interface ExchangeFunction<T> {
+		interface ExchangeFunction<T extends @Nullable Object> {
 
 			/**
 			 * Exchange the given response into a value of type {@code T}.
@@ -823,7 +824,7 @@ interface ExchangeFunction<T> {
 			 * @return the exchanged value, potentially {@code null}
 			 * @throws IOException in case of I/O errors
 			 */
-			@Nullable T exchange(HttpRequest clientRequest, ConvertibleClientHttpResponse clientResponse) throws IOException;
+			T exchange(HttpRequest clientRequest, ConvertibleClientHttpResponse clientResponse) throws IOException;
 		}
 
 		/**
@@ -833,7 +834,7 @@ interface ExchangeFunction<T> {
 		 * @param <T> the type the response will be transformed to
 		 */
 		@FunctionalInterface
-		interface RequiredValueExchangeFunction<T> extends ExchangeFunction<T> {
+		interface RequiredValueExchangeFunction<T> extends ExchangeFunction<@NonNull T> {
 
 			/**
 			 * Exchange the given response into a value of type {@code T}.