Actual caching of null values in retrieve(key, valueLoader)

See gh-31637

spring-context-support/src/main/java/org/springframework/cache/caffeine/CaffeineCache.java

@@ -148,7 +148,14 @@ public CompletableFuture<?> retrieve(Object key) {
 	@SuppressWarnings("unchecked")
 	@Override
 	public <T> CompletableFuture<T> retrieve(Object key, Supplier<CompletableFuture<T>> valueLoader) {
-		return (CompletableFuture<T>) getAsyncCache().get(key, (k, e) -> valueLoader.get());
+		if (isAllowNullValues()) {
+			return (CompletableFuture<T>) getAsyncCache()
+					.get(key, (k, e) -> valueLoader.get().thenApply(this::toStoreValue))
+					.thenApply(this::fromStoreValue);
+		}
+		else {
+			return (CompletableFuture<T>) getAsyncCache().get(key, (k, e) -> valueLoader.get());
+		}
 	}
 
 	@Override

spring-context-support/src/test/java/org/springframework/cache/caffeine/CaffeineCacheManagerTests.java

@@ -181,7 +181,9 @@ void asyncMode() {
 		assertThat(cache1.retrieve("key3", () -> CompletableFuture.completedFuture("value3")).join())
 				.isEqualTo("value3");
 		cache1.evict("key3");
+		assertThat(cache1.retrieve("key3")).isNull();
 		assertThat(cache1.retrieve("key3", () -> CompletableFuture.completedFuture(null)).join()).isNull();
+		assertThat(cache1.retrieve("key3").join()).isEqualTo(new SimpleValueWrapper(null));
 		assertThat(cache1.retrieve("key3", () -> CompletableFuture.completedFuture(null)).join()).isNull();
 	}
 

spring-context/src/main/java/org/springframework/cache/Cache.java

@@ -151,10 +151,10 @@ default CompletableFuture<?> retrieve(Object key) {
 	 * <p>If possible, implementations should ensure that the loading operation
 	 * is synchronized so that the specified {@code valueLoader} is only called
 	 * once in case of concurrent access on the same key.
-	 * <p>Null values are generally not supported by this method. The provided
-	 * {@link CompletableFuture} handle produces a value or raises an exception.
-	 * If the {@code valueLoader} raises an exception, it will be propagated
-	 * to the {@code CompletableFuture} handle returned from here.
+	 * <p>Null values always indicate a user-level {@code null} value with this
+	 * method. The provided {@link CompletableFuture} handle produces a value
+	 * or raises an exception. If the {@code valueLoader} raises an exception,
+	 * it will be propagated to the returned {@code CompletableFuture} handle.
 	 * @param key the key whose associated value is to be returned
 	 * @return the value to which this cache maps the specified key, contained
 	 * within a {@link CompletableFuture} which will never be {@code null}.