Add shared instance of ReactiveAdapterRegistry

Issue: SPR-16218

Author: rstoyanchev

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

@@ -59,7 +59,7 @@ public abstract class Conventions {
 	}
 
 	private static final ReactiveAdapterRegistry reactiveAdapterRegistry =
-			new ReactiveAdapterRegistry();
+			ReactiveAdapterRegistry.getSharedInstance();
 
 
 	/**

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

@@ -33,7 +33,10 @@
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ReflectionUtils;
 
-import static org.springframework.core.ReactiveTypeDescriptor.*;
+import static org.springframework.core.ReactiveTypeDescriptor.multiValue;
+import static org.springframework.core.ReactiveTypeDescriptor.noValue;
+import static org.springframework.core.ReactiveTypeDescriptor.singleOptionalValue;
+import static org.springframework.core.ReactiveTypeDescriptor.singleRequiredValue;
 
 /**
  * A registry of adapters to adapt a Reactive Streams {@link Publisher} to/from
@@ -50,6 +53,9 @@
  */
 public class ReactiveAdapterRegistry {
 
+	@Nullable
+	private static volatile ReactiveAdapterRegistry sharedInstance;
+
 	private final boolean reactorPresent;
 
 	private final List<ReactiveAdapter> adapters = new ArrayList<>(32);
@@ -98,6 +104,31 @@ public ReactiveAdapterRegistry() {
 	}
 
 
+	/**
+	 * Return a shared default {@code ReactiveAdapterRegistry} instance, lazily
+	 * building it once needed.
+	 * <p><b>NOTE:</b> We highly recommend passing a long-lived, pre-configured
+	 * {@code ReactiveAdapterRegistry} instance for customization purposes.
+	 * This accessor is only meant as a fallback for code paths that want to
+	 * fall back on a default instance if one isn't provided.
+	 * @return the shared {@code ReactiveAdapterRegistry} instance (never {@code null})
+	 * @since 5.0.2
+	 */
+	public static ReactiveAdapterRegistry getSharedInstance() {
+		ReactiveAdapterRegistry ar = sharedInstance;
+		if (ar == null) {
+			synchronized (ReactiveAdapterRegistry.class) {
+				ar = sharedInstance;
+				if (ar == null) {
+					ar = new ReactiveAdapterRegistry();
+					sharedInstance = ar;
+				}
+			}
+		}
+		return ar;
+	}
+
+
 	/**
 	 * Whether the registry has any adapters which would be the case if any of
 	 * Reactor, RxJava 2, or RxJava 1 (+ RxJava Reactive Streams bridge) are

spring-core/src/test/java/org/springframework/core/convert/support/ReactiveAdapterRegistryTests.java

@@ -50,7 +50,7 @@
 @SuppressWarnings("unchecked")
 public class ReactiveAdapterRegistryTests {
 
-	private final ReactiveAdapterRegistry registry = new ReactiveAdapterRegistry();
+	private final ReactiveAdapterRegistry registry = ReactiveAdapterRegistry.getSharedInstance();
 
 
 	@Test

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/InvocableHandlerMethod.java

@@ -66,7 +66,7 @@ public class InvocableHandlerMethod extends HandlerMethod {
 
 	private ParameterNameDiscoverer parameterNameDiscoverer = new DefaultParameterNameDiscoverer();
 
-	private ReactiveAdapterRegistry reactiveAdapterRegistry = new ReactiveAdapterRegistry();
+	private ReactiveAdapterRegistry reactiveAdapterRegistry = ReactiveAdapterRegistry.getSharedInstance();
 
 
 	public InvocableHandlerMethod(HandlerMethod handlerMethod) {

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/AbstractMessageReaderArgumentResolver.java

@@ -81,7 +81,7 @@ public abstract class AbstractMessageReaderArgumentResolver extends HandlerMetho
 	 * @param readers readers to convert from the request body
 	 */
 	protected AbstractMessageReaderArgumentResolver(List<HttpMessageReader<?>> readers) {
-		this(readers, new ReactiveAdapterRegistry());
+		this(readers, ReactiveAdapterRegistry.getSharedInstance());
 	}
 
 	/**

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/AbstractMessageWriterResultHandler.java

@@ -60,7 +60,7 @@ public abstract class AbstractMessageWriterResultHandler extends HandlerResultHa
 	protected AbstractMessageWriterResultHandler(List<HttpMessageWriter<?>> messageWriters,
 			RequestedContentTypeResolver contentTypeResolver) {
 
-		this(messageWriters, contentTypeResolver, new ReactiveAdapterRegistry());
+		this(messageWriters, contentTypeResolver, ReactiveAdapterRegistry.getSharedInstance());
 	}
 
 	/**

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerAdapter.java

@@ -163,7 +163,7 @@ public void afterPropertiesSet() throws Exception {
 			this.argumentResolverConfigurer = new ArgumentResolverConfigurer();
 		}
 		if (this.reactiveAdapterRegistry == null) {
-			this.reactiveAdapterRegistry = new ReactiveAdapterRegistry();
+			this.reactiveAdapterRegistry = ReactiveAdapterRegistry.getSharedInstance();
 		}
 
 		this.methodResolver = new ControllerMethodResolver(this.argumentResolverConfigurer,

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ResponseBodyResultHandler.java

@@ -56,7 +56,7 @@ public class ResponseBodyResultHandler extends AbstractMessageWriterResultHandle
 	public ResponseBodyResultHandler(List<HttpMessageWriter<?>> writers,
 			RequestedContentTypeResolver resolver) {
 
-		this(writers, resolver, new ReactiveAdapterRegistry());
+		this(writers, resolver, ReactiveAdapterRegistry.getSharedInstance());
 	}
 
 	/**

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandler.java

@@ -63,7 +63,7 @@ public class ResponseEntityResultHandler extends AbstractMessageWriterResultHand
 	public ResponseEntityResultHandler(List<HttpMessageWriter<?>> writers,
 			RequestedContentTypeResolver resolver) {
 
-		this(writers, resolver, new ReactiveAdapterRegistry());
+		this(writers, resolver, ReactiveAdapterRegistry.getSharedInstance());
 	}
 
 	/**

spring-webflux/src/main/java/org/springframework/web/reactive/result/view/AbstractView.java

@@ -70,7 +70,7 @@ public abstract class AbstractView implements View, ApplicationContextAware {
 
 
 	public AbstractView() {
-		this(new ReactiveAdapterRegistry());
+		this(ReactiveAdapterRegistry.getSharedInstance());
 	}
 
 	public AbstractView(ReactiveAdapterRegistry registry) {