Defensively register default adapters behind Reactive Streams guard

See gh-31000

spring-core/src/main/java/org/springframework/core/ReactiveAdapterRegistry.java

@@ -60,6 +60,8 @@ public class ReactiveAdapterRegistry {
 	@Nullable
 	private static volatile ReactiveAdapterRegistry sharedInstance;
 
+	private static final boolean reactiveStreamsPresent;
+
 	private static final boolean reactorPresent;
 
 	private static final boolean rxjava3Present;
@@ -70,6 +72,7 @@ public class ReactiveAdapterRegistry {
 
 	static {
 		ClassLoader classLoader = ReactiveAdapterRegistry.class.getClassLoader();
+		reactiveStreamsPresent = ClassUtils.isPresent("org.reactivestreams.Publisher", classLoader);
 		reactorPresent = ClassUtils.isPresent("reactor.core.publisher.Flux", classLoader);
 		rxjava3Present = ClassUtils.isPresent("io.reactivex.rxjava3.core.Flowable", classLoader);
 		kotlinCoroutinesPresent = ClassUtils.isPresent("kotlinx.coroutines.reactor.MonoKt", classLoader);
@@ -85,6 +88,11 @@ public class ReactiveAdapterRegistry {
 	 */
 	@SuppressWarnings("unchecked")
 	public ReactiveAdapterRegistry() {
+		// Defensive guard for the Reactive Streams API itself
+		if (!reactiveStreamsPresent) {
+			return;
+		}
+
 		// Reactor
 		if (reactorPresent) {
 			new ReactorRegistrar().registerAdapters(this);
@@ -107,21 +115,14 @@ public class ReactiveAdapterRegistry {
 
 		// Simple Flow.Publisher bridge if Reactor is not present
 		if (!reactorPresent) {
-			registerReactiveType(
+			this.adapters.add(new ReactiveAdapter(
 					ReactiveTypeDescriptor.multiValue(Flow.Publisher.class, () -> PublisherToRS.EMPTY_FLOW),
 					source -> new PublisherToRS<>((Flow.Publisher<Object>) source),
-					source -> new PublisherToFlow<>((Publisher<Object>) source));
+					source -> new PublisherToFlow<>((Publisher<Object>) source)));
 		}
 	}
 
 
-	/**
-	 * Whether the registry has any adapters.
-	 */
-	public boolean hasAdapters() {
-		return !this.adapters.isEmpty();
-	}
-
 	/**
 	 * Register a reactive type along with functions to adapt to and from a
 	 * Reactive Streams {@link Publisher}. The function arguments assume that
@@ -138,6 +139,13 @@ public class ReactiveAdapterRegistry {
 		}
 	}
 
+	/**
+	 * Return whether the registry has any adapters.
+	 */
+	public boolean hasAdapters() {
+		return !this.adapters.isEmpty();
+	}
+
 	/**
 	 * Get the adapter for the given reactive type.
 	 * @return the corresponding adapter, or {@code null} if none available