Fix reactive retrieval of cached null value for empty Mono

Closes gh-31722

Author: jhoeller

spring-context/src/main/java/org/springframework/cache/interceptor/CacheAspectSupport.java

@@ -1068,13 +1068,13 @@ public Object findInCaches(CacheOperationContext context, Cache cache, Object ke
 				if (adapter.isMultiValue()) {
 					return adapter.fromPublisher(Flux.from(
 							Mono.fromFuture(cachedFuture)
-									.flatMap(value -> (Mono<?>) evaluate(Mono.just(unwrapCacheValue(value)), invoker, method, contexts)))
+									.flatMap(value -> (Mono<?>) evaluate(Mono.justOrEmpty(unwrapCacheValue(value)), invoker, method, contexts)))
 							.flatMap(v -> (v instanceof Iterable<?> iv ? Flux.fromIterable(iv) : Flux.just(v)))
 							.switchIfEmpty(Flux.defer(() -> (Flux<?>) evaluate(null, invoker, method, contexts))));
 				}
 				else {
 					return adapter.fromPublisher(Mono.fromFuture(cachedFuture)
-							.flatMap(value -> (Mono<?>) evaluate(Mono.just(unwrapCacheValue(value)), invoker, method, contexts))
+							.flatMap(value -> (Mono<?>) evaluate(Mono.justOrEmpty(unwrapCacheValue(value)), invoker, method, contexts))
 							.switchIfEmpty(Mono.defer(() -> (Mono) evaluate(null, invoker, method, contexts))));
 				}
 			}

spring-context/src/test/java/org/springframework/cache/CacheReproTests.java

@@ -202,6 +202,10 @@ void spr14235AdaptsToCompletableFuture() {
 		assertThat(bean.findById("tb2").join()).isNotSameAs(tb);
 		assertThat(cache.get("tb2")).isNull();
 
+		assertThat(bean.findByIdEmpty("").join()).isNull();
+		assertThat(bean.findByIdEmpty("").join()).isNull();
+		assertThat(cache.get("").get()).isNull();
+
 		context.close();
 	}
 
@@ -226,6 +230,10 @@ void spr14235AdaptsToCompletableFutureWithSync() throws Exception {
 		assertThat(bean.findById("tb1").get()).isSameAs(tb);
 		assertThat(cache.get("tb1").get()).isSameAs(tb);
 
+		assertThat(bean.findById("").join()).isNull();
+		assertThat(bean.findById("").join()).isNull();
+		assertThat(cache.get("").get()).isNull();
+
 		context.close();
 	}
 
@@ -257,6 +265,10 @@ void spr14235AdaptsToReactorMono() {
 		assertThat(bean.findById("tb2").block()).isNotSameAs(tb);
 		assertThat(cache.get("tb2")).isNull();
 
+		assertThat(bean.findByIdEmpty("").block()).isNull();
+		assertThat(bean.findByIdEmpty("").block()).isNull();
+		assertThat(cache.get("").get()).isNull();
+
 		context.close();
 	}
 
@@ -281,6 +293,10 @@ void spr14235AdaptsToReactorMonoWithSync() {
 		assertThat(bean.findById("tb1").block()).isSameAs(tb);
 		assertThat(cache.get("tb1").get()).isSameAs(tb);
 
+		assertThat(bean.findById("").block()).isNull();
+		assertThat(bean.findById("").block()).isNull();
+		assertThat(cache.get("").get()).isNull();
+
 		context.close();
 	}
 
@@ -312,6 +328,10 @@ void spr14235AdaptsToReactorFlux() {
 		assertThat(bean.findById("tb2").collectList().block()).isNotEqualTo(tb);
 		assertThat(cache.get("tb2")).isNull();
 
+		assertThat(bean.findByIdEmpty("").collectList().block()).isEmpty();
+		assertThat(bean.findByIdEmpty("").collectList().block()).isEmpty();
+		assertThat(cache.get("").get()).isEqualTo(Collections.emptyList());
+
 		context.close();
 	}
 
@@ -336,6 +356,10 @@ void spr14235AdaptsToReactorFluxWithSync() {
 		assertThat(bean.findById("tb1").collectList().block()).isEqualTo(tb);
 		assertThat(cache.get("tb1").get()).isEqualTo(tb);
 
+		assertThat(bean.findById("").collectList().block()).isEmpty();
+		assertThat(bean.findById("").collectList().block()).isEmpty();
+		assertThat(cache.get("").get()).isEqualTo(Collections.emptyList());
+
 		context.close();
 	}
 
@@ -568,6 +592,11 @@ public CompletableFuture<TestBean> findById(String id) {
 			return CompletableFuture.completedFuture(new TestBean(id));
 		}
 
+		@Cacheable(value = "itemCache")
+		public CompletableFuture<TestBean> findByIdEmpty(String id) {
+			return CompletableFuture.completedFuture(null);
+		}
+
 		@CachePut(cacheNames = "itemCache", key = "#item.name")
 		public CompletableFuture<TestBean> insertItem(TestBean item) {
 			return CompletableFuture.completedFuture(item);
@@ -584,7 +613,7 @@ public static class Spr14235FutureServiceSync {
 
 		@Cacheable(value = "itemCache", sync = true)
 		public CompletableFuture<TestBean> findById(String id) {
-			return CompletableFuture.completedFuture(new TestBean(id));
+			return CompletableFuture.completedFuture(id.isEmpty() ? null : new TestBean(id));
 		}
 
 		@CachePut(cacheNames = "itemCache", key = "#item.name")
@@ -601,6 +630,11 @@ public Mono<TestBean> findById(String id) {
 			return Mono.just(new TestBean(id));
 		}
 
+		@Cacheable(value = "itemCache")
+		public Mono<TestBean> findByIdEmpty(String id) {
+			return Mono.empty();
+		}
+
 		@CachePut(cacheNames = "itemCache", key = "#item.name")
 		public Mono<TestBean> insertItem(TestBean item) {
 			return Mono.just(item);
@@ -617,7 +651,7 @@ public static class Spr14235MonoServiceSync {
 
 		@Cacheable(value = "itemCache", sync = true)
 		public Mono<TestBean> findById(String id) {
-			return Mono.just(new TestBean(id));
+			return (id.isEmpty() ? Mono.empty() : Mono.just(new TestBean(id)));
 		}
 
 		@CachePut(cacheNames = "itemCache", key = "#item.name")
@@ -636,6 +670,11 @@ public Flux<TestBean> findById(String id) {
 			return Flux.just(new TestBean(id), new TestBean(id + (counter++)));
 		}
 
+		@Cacheable(value = "itemCache")
+		public Flux<TestBean> findByIdEmpty(String id) {
+			return Flux.empty();
+		}
+
 		@CachePut(cacheNames = "itemCache", key = "#id")
 		public Flux<TestBean> insertItem(String id, List<TestBean> item) {
 			return Flux.fromIterable(item);
@@ -654,7 +693,7 @@ public static class Spr14235FluxServiceSync {
 
 		@Cacheable(value = "itemCache", sync = true)
 		public Flux<TestBean> findById(String id) {
-			return Flux.just(new TestBean(id), new TestBean(id + (counter++)));
+			return (id.isEmpty() ? Flux.empty() : Flux.just(new TestBean(id), new TestBean(id + (counter++))));
 		}
 
 		@CachePut(cacheNames = "itemCache", key = "#id")