Improve support for generics in Jackson codecs

Closes gh-23791

spring-web/src/main/java/org/springframework/http/codec/json/AbstractJackson2Decoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -153,8 +153,10 @@ public abstract class AbstractJackson2Decoder extends Jackson2CodecSupport imple
 
 	private ObjectReader getObjectReader(ResolvableType elementType, @Nullable Map<String, Object> hints) {
 		Assert.notNull(elementType, "'elementType' must not be null");
-		MethodParameter param = getParameter(elementType);
+		Class<?> contextClass = getContextClass(elementType);
-		Class<?> contextClass = (param != null ? param.getContainingClass() : null);
+		if (contextClass == null && hints != null) {
+			contextClass = getContextClass((ResolvableType) hints.get(ACTUAL_TYPE_HINT));
+		}
 		JavaType javaType = getJavaType(elementType.getType(), contextClass);
 		Class<?> jsonView = (hints != null ? (Class<?>) hints.get(Jackson2CodecSupport.JSON_VIEW_HINT) : null);
 		return jsonView != null ?
@@ -162,6 +164,12 @@ public abstract class AbstractJackson2Decoder extends Jackson2CodecSupport imple
 				getObjectMapper().readerFor(javaType);
 	}
 
+	@Nullable
+	private Class<?> getContextClass(@Nullable ResolvableType elementType) {
+		MethodParameter param = (elementType != null ? getParameter(elementType)  : null);
+		return (param != null ? param.getContainingClass() : null);
+	}
+
 	private void logValue(@Nullable Object value, @Nullable Map<String, Object> hints) {
 		if (!Hints.isLoggingSuppressed(hints)) {
 			LogFormatUtils.traceDebug(logger, traceOn -> {

spring-web/src/main/java/org/springframework/http/codec/json/Jackson2CodecSupport.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,6 +20,7 @@ import java.lang.annotation.Annotation;
 import java.lang.reflect.Type;
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
@@ -34,6 +35,8 @@ import org.springframework.core.MethodParameter;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.codec.Hints;
 import org.springframework.http.HttpLogging;
+import org.springframework.http.server.reactive.ServerHttpRequest;
+import org.springframework.http.server.reactive.ServerHttpResponse;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.MimeType;
@@ -55,6 +58,15 @@ public abstract class Jackson2CodecSupport {
 	 */
 	public static final String JSON_VIEW_HINT = Jackson2CodecSupport.class.getName() + ".jsonView";
 
+	/**
+	 * The key for the hint to access the actual ResolvableType passed into
+	 * {@link org.springframework.http.codec.HttpMessageReader#read(ResolvableType, ResolvableType, ServerHttpRequest, ServerHttpResponse, Map)}
+	 * (server-side only). Currently set when the method argument has generics because
+	 * in case of reactive types, use of {@code ResolvableType.getGeneric()} means no
+	 * MethodParameter source and no knowledge of the containing class.
+	 */
+	static final String ACTUAL_TYPE_HINT = Jackson2CodecSupport.class.getName() + ".actualType";
+
 	private static final String JSON_VIEW_HINT_ERROR =
 			"@JsonView only supported for write hints with exactly 1 class argument: ";
 
@@ -106,11 +118,20 @@ public abstract class Jackson2CodecSupport {
 	protected Map<String, Object> getHints(ResolvableType resolvableType) {
 		MethodParameter param = getParameter(resolvableType);
 		if (param != null) {
+			Map<String, Object> hints = null;
+			if (resolvableType.hasGenerics()) {
+				hints = new HashMap<>(2);
+				hints.put(ACTUAL_TYPE_HINT, resolvableType);
+			}
 			JsonView annotation = getAnnotation(param, JsonView.class);
 			if (annotation != null) {
 				Class<?>[] classes = annotation.value();
 				Assert.isTrue(classes.length == 1, JSON_VIEW_HINT_ERROR + param);
-				return Hints.from(JSON_VIEW_HINT, classes[0]);
+				hints = (hints != null ? hints : new HashMap<>(1));
+				hints.put(JSON_VIEW_HINT, classes[0]);
+			}
+			if (hints != null) {
+				return hints;
 			}
 		}
 		return Hints.none();

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingMessageConversionIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,7 +28,6 @@ import javax.xml.bind.annotation.XmlRootElement;
 
 import io.reactivex.Flowable;
 import io.reactivex.Maybe;
-import org.junit.jupiter.api.Disabled;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -522,7 +521,6 @@ public class RequestMappingMessageConversionIntegrationTests extends AbstractReq
 		assertThat(getApplicationContext().getBean(PersonCreateController.class).persons.size()).isEqualTo(2);
 	}
 
-	@Disabled
 	@ParameterizedHttpServerTest // gh-23791
 	public void personCreateViaDefaultMethodWithGenerics(HttpServer httpServer) throws Exception {
 		startServer(httpServer);