Perform explicit class checks in ReactiveAdapterRegistry

In order to allow Spring Framework applications running as GraalVM
native images, ReactiveAdapterRegistry should perform explicit class
checks instead of catching Throwable in order to avoid
UnsupportedFeatureError errors.

Issue: SPR-17000

Author: sdeleuze

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

@@ -63,41 +63,32 @@ public class ReactiveAdapterRegistry {
 	 */
 	public ReactiveAdapterRegistry() {
 
+		ClassLoader classLoader = ReactiveAdapterRegistry.class.getClassLoader();
+
 		// Reactor
 		boolean reactorRegistered = false;
-		try {
+		if (ClassUtils.isPresent("reactor.core.publisher.Flux", classLoader)) {
 			new ReactorRegistrar().registerAdapters(this);
 			reactorRegistered = true;
 		}
-		catch (Throwable ex) {
-			// Ignore
-		}
 		this.reactorPresent = reactorRegistered;
 
 		// RxJava1
-		try {
+		if (ClassUtils.isPresent("rx.Observable", classLoader)) {
 			new RxJava1Registrar().registerAdapters(this);
 		}
-		catch (Throwable ex) {
-			// Ignore
-		}
 
 		// RxJava2
-		try {
+		if (ClassUtils.isPresent("io.reactivex.Flowable", classLoader)) {
 			new RxJava2Registrar().registerAdapters(this);
 		}
-		catch (Throwable ex) {
-			// Ignore
-		}
 
 		// Java 9+ Flow.Publisher
-		try {
+		if (ClassUtils.isPresent("java.util.concurrent.Flow.Publisher", classLoader)) {
 			new ReactorJdkFlowAdapterRegistrar().registerAdapter(this);
 		}
-		catch (Throwable ex) {
-			// Ignore for the time being...
-			// We can fall back on "reactive-streams-flow-bridge" (once released)
-		}
+		// If not present, do nothing for the time being...
+		// We can fall back on "reactive-streams-flow-bridge" (once released)
 	}
 
 
@@ -276,24 +267,29 @@ void registerAdapters(ReactiveAdapterRegistry registry) {
 
 	private static class ReactorJdkFlowAdapterRegistrar {
 
-		void registerAdapter(ReactiveAdapterRegistry registry) throws Exception {
+		void registerAdapter(ReactiveAdapterRegistry registry) {
 			// TODO: remove reflection when build requires JDK 9+
 
-			String publisherName = "java.util.concurrent.Flow.Publisher";
-			Class<?> publisherClass = ClassUtils.forName(publisherName, getClass().getClassLoader());
+			try {
+				String publisherName = "java.util.concurrent.Flow.Publisher";
+				Class<?> publisherClass = ClassUtils.forName(publisherName, getClass().getClassLoader());
 
-			String adapterName = "reactor.adapter.JdkFlowAdapter";
-			Class<?> flowAdapterClass = ClassUtils.forName(adapterName,  getClass().getClassLoader());
+				String adapterName = "reactor.adapter.JdkFlowAdapter";
+				Class<?> flowAdapterClass = ClassUtils.forName(adapterName,  getClass().getClassLoader());
 
-			Method toFluxMethod = flowAdapterClass.getMethod("flowPublisherToFlux", publisherClass);
-			Method toFlowMethod = flowAdapterClass.getMethod("publisherToFlowPublisher", Publisher.class);
-			Object emptyFlow = ReflectionUtils.invokeMethod(toFlowMethod, null, Flux.empty());
+				Method toFluxMethod = flowAdapterClass.getMethod("flowPublisherToFlux", publisherClass);
+				Method toFlowMethod = flowAdapterClass.getMethod("publisherToFlowPublisher", Publisher.class);
+				Object emptyFlow = ReflectionUtils.invokeMethod(toFlowMethod, null, Flux.empty());
 
-			registry.registerReactiveType(
-					ReactiveTypeDescriptor.multiValue(publisherClass, () -> emptyFlow),
-					source -> (Publisher<?>) ReflectionUtils.invokeMethod(toFluxMethod, null, source),
-					publisher -> ReflectionUtils.invokeMethod(toFlowMethod, null, publisher)
-			);
+				registry.registerReactiveType(
+						ReactiveTypeDescriptor.multiValue(publisherClass, () -> emptyFlow),
+						source -> (Publisher<?>) ReflectionUtils.invokeMethod(toFluxMethod, null, source),
+						publisher -> ReflectionUtils.invokeMethod(toFlowMethod, null, publisher)
+				);
+			}
+			catch (Throwable ex) {
+				// Ignore
+			}
 		}
 	}