Refine SingletonSupplier<T> static methods nullability

SingletonSupplier<T> supplier-based static methods nullability should
be refined to accept in a flexible way nullable or non-nullable T.

Closes gh-35559

spring-context-support/src/main/java/org/springframework/cache/jcache/config/AbstractJCacheConfiguration.java

@@ -41,11 +41,12 @@ import org.springframework.context.annotation.Role;
 @Configuration(proxyBeanMethods = false)
 public abstract class AbstractJCacheConfiguration extends AbstractCachingConfiguration {
 
-	protected @Nullable Supplier<@Nullable CacheResolver> exceptionCacheResolver;
+	@SuppressWarnings("NullAway.Init")
+	protected Supplier<@Nullable CacheResolver> exceptionCacheResolver;
 
 
 	@Override
-	@SuppressWarnings("NullAway") // https://github.com/uber/NullAway/issues/1128
+	@SuppressWarnings("NullAway") // See https://github.com/uber/NullAway/issues/1290
 	protected void useCachingConfigurer(CachingConfigurerSupplier cachingConfigurerSupplier) {
 		super.useCachingConfigurer(cachingConfigurerSupplier);
 		this.exceptionCacheResolver = cachingConfigurerSupplier.adapt(config -> {

spring-context-support/src/main/java/org/springframework/cache/jcache/interceptor/DefaultJCacheOperationSource.java

@@ -50,11 +50,11 @@ import org.springframework.util.function.SupplierUtils;
 public class DefaultJCacheOperationSource extends AnnotationJCacheOperationSource
 		implements BeanFactoryAware, SmartInitializingSingleton {
 
-	private @Nullable SingletonSupplier<CacheManager> cacheManager;
+	private @Nullable SingletonSupplier<@Nullable CacheManager> cacheManager;
 
-	private @Nullable SingletonSupplier<CacheResolver> cacheResolver;
+	private @Nullable SingletonSupplier<@Nullable CacheResolver> cacheResolver;
 
-	private @Nullable SingletonSupplier<CacheResolver> exceptionCacheResolver;
+	private @Nullable SingletonSupplier<@Nullable CacheResolver> exceptionCacheResolver;
 
 	private SingletonSupplier<KeyGenerator> keyGenerator;
 

spring-context/src/main/java/org/springframework/cache/annotation/AbstractCachingConfiguration.java

@@ -50,13 +50,17 @@ public abstract class AbstractCachingConfiguration implements ImportAware {
 
 	protected @Nullable AnnotationAttributes enableCaching;
 
-	protected @Nullable Supplier<@Nullable CacheManager> cacheManager;
+	@SuppressWarnings("NullAway.Init")
+	protected Supplier<@Nullable CacheManager> cacheManager;
 
-	protected @Nullable Supplier<@Nullable CacheResolver> cacheResolver;
+	@SuppressWarnings("NullAway.Init")
+	protected Supplier<@Nullable CacheResolver> cacheResolver;
 
-	protected @Nullable Supplier<@Nullable KeyGenerator> keyGenerator;
+	@SuppressWarnings("NullAway.Init")
+	protected Supplier<@Nullable KeyGenerator> keyGenerator;
 
-	protected @Nullable Supplier<@Nullable CacheErrorHandler> errorHandler;
+	@SuppressWarnings("NullAway.Init")
+	protected Supplier<@Nullable CacheErrorHandler> errorHandler;
 
 
 	@Override
@@ -101,7 +105,7 @@ public abstract class AbstractCachingConfiguration implements ImportAware {
 
 	protected static class CachingConfigurerSupplier {
 
-		private final SingletonSupplier<CachingConfigurer> supplier;
+		private final SingletonSupplier<@Nullable CachingConfigurer> supplier;
 
 		public CachingConfigurerSupplier(Supplier<@Nullable CachingConfigurer> supplier) {
 			this.supplier = SingletonSupplier.ofNullable(supplier);

spring-core/src/main/java/org/springframework/util/function/SingletonSupplier.java

@@ -143,7 +143,7 @@ public class SingletonSupplier<T extends @Nullable Object> implements Supplier<T
 	 * @return the singleton supplier, or {@code null} if the instance was {@code null}
 	 */
 	@Contract("null -> null; !null -> !null")
-	public static <T> @Nullable SingletonSupplier<T> ofNullable(@Nullable T instance) {
+	public static <T extends @Nullable Object> @Nullable SingletonSupplier<T> ofNullable(T instance) {
 		return (instance != null ? new SingletonSupplier<>(instance) : null);
 	}
 
@@ -152,7 +152,7 @@ public class SingletonSupplier<T extends @Nullable Object> implements Supplier<T
 	 * @param supplier the instance supplier (never {@code null})
 	 * @return the singleton supplier (never {@code null})
 	 */
-	public static <T> SingletonSupplier<T> of(Supplier<T> supplier) {
+	public static <T extends @Nullable Object> SingletonSupplier<T> of(Supplier<T> supplier) {
 		return new SingletonSupplier<>(supplier);
 	}
 
@@ -162,7 +162,7 @@ public class SingletonSupplier<T extends @Nullable Object> implements Supplier<T
 	 * @return the singleton supplier, or {@code null} if the instance supplier was {@code null}
 	 */
 	@Contract("null -> null; !null -> !null")
-	public static <T> @Nullable SingletonSupplier<T> ofNullable(@Nullable Supplier<@Nullable T> supplier) {
+	public static <T extends @Nullable Object> @Nullable SingletonSupplier<T> ofNullable(@Nullable Supplier<T> supplier) {
 		return (supplier != null ? new SingletonSupplier<>(supplier) : null);
 	}
 

spring-core/src/main/java/org/springframework/util/function/SupplierUtils.java

@@ -38,7 +38,7 @@ public abstract class SupplierUtils {
 	 * @return the supplier's result, or {@code null} if none
 	 */
 	@Contract("null -> null")
-	public static <T> @Nullable T resolve(@Nullable Supplier<T> supplier) {
+	public static <T extends @Nullable Object> @Nullable T resolve(@Nullable Supplier<T> supplier) {
 		return (supplier != null ? supplier.get() : null);
 	}