GH-1043 Add support for communicating type via MimeType parameter for JsonMessageConverter

Resolves #1043

docs/src/main/asciidoc/spring-cloud-function.adoc

@@ -608,7 +608,7 @@ up to the actual implementation of the `MessageConverter` to support multiple ty
 As mentioned earlier, the framework already provides a stack of `MessageConverters` to handle most common use cases.
 The following list describes the provided `MessageConverters`, in order of precedence (the first `MessageConverter` that works is used):
 
-. `JsonMessageConverter`:  Supports conversion of the payload of the `Message` to/from POJO for cases when `contentType` is `application/json` using Jackson or Gson libraries (DEFAULT).
+. `JsonMessageConverter`:  Supports conversion of the payload of the `Message` to/from POJO for cases when `contentType` is `application/json` using Jackson (DEFAULT) or Gson libraries. This message converter also aware of `type` parameter (e.g., _application/json;type=foo.bar.Person_). This is useful for cases where types may noyt be known at the time when function is developed hecn function signature may look like `Function<?, ?>` or `Function` or `Function<Object, Object>`. In other words for type conversion we typically derive type from function signature. Having, mime-type parameter allows you to communicate type in a more dynamic way.  
 . `ByteArrayMessageConverter`: Supports conversion of the payload of the `Message` from `byte[]` to `byte[]` for cases when `contentType` is `application/octet-stream`. It is essentially a pass through and exists primarily for backward compatibility.
 . `StringMessageConverter`: Supports conversion of any type to a `String` when `contentType` is `text/plain`.
 

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/config/JsonMessageConverter.java

@@ -31,6 +31,7 @@ import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.converter.AbstractMessageConverter;
 import org.springframework.messaging.converter.MessageConverter;
 import org.springframework.util.MimeType;
+import org.springframework.util.StringUtils;
 
 /**
  * Implementation of {@link MessageConverter} which uses Jackson or Gson libraries to do the
@@ -71,7 +72,15 @@ public class JsonMessageConverter extends AbstractMessageConverter {
 
 	@Override
 	protected boolean canConvertFrom(Message<?> message, @Nullable Class<?> targetClass) {
-		if (targetClass == null || !supportsMimeType(message.getHeaders())) {
+		return supportsMimeType(message.getHeaders()) && this.canDiscoverConvertToType(message, targetClass);
+	}
+
+	private boolean canDiscoverConvertToType(Message<?> message, Class<?> targetClass) {
+		if (targetClass == null || targetClass == Object.class) {
+			MimeType mimeType = getMimeType(message.getHeaders());
+			if (StringUtils.hasText(mimeType.getParameter("type"))) {
+				return true;
+			}
 			return false;
 		}
 		return true;
@@ -83,8 +92,20 @@ public class JsonMessageConverter extends AbstractMessageConverter {
 			conversionHint = ((ParameterizedTypeReference<?>) conversionHint).getType();
 		}
 		Type convertToType = this.getResolvedType(targetClass, conversionHint);
-		if (convertToType == Object.class) {
-			return message.getPayload();
+		if (convertToType == null || convertToType == Object.class) {
+			MimeType mimeType = getMimeType(message.getHeaders());
+			String type = mimeType.getParameter("type");
+			if (StringUtils.hasText(type)) {
+				try {
+					convertToType = Thread.currentThread().getContextClassLoader().loadClass(type);
+				}
+				catch (ClassNotFoundException e) {
+					throw new IllegalArgumentException("Failed to load class `" + type + "` specified by the provided content-type: " + mimeType, e);
+				}
+			}
+			else {
+				return message.getPayload();
+			}
 		}
 		if (targetClass == byte[].class && message.getPayload() instanceof String) {
 			return ((String) message.getPayload()).getBytes(StandardCharsets.UTF_8);

spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/config/JsonMessageConverterTests.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2023-2023 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.function.context.config;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.cloud.function.json.JacksonMapper;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageHeaders;
+import org.springframework.messaging.support.MessageBuilder;
+import org.springframework.util.MimeTypeUtils;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ *
+ * @author Oleg Zhurakousky
+ *
+ */
+public class JsonMessageConverterTests {
+
+	@Test
+	public void testTypeInference() {
+		JsonMessageConverter converter = new JsonMessageConverter(new JacksonMapper(new ObjectMapper()));
+
+		Message<String> message = MessageBuilder.withPayload("{\"name\":\"bill\"}").build();
+		assertThat(converter.canConvertFrom(message, Person.class)).isTrue();
+
+		message = MessageBuilder.withPayload("{\"name\":\"bill\"}").setHeader(MessageHeaders.CONTENT_TYPE, MimeTypeUtils.APPLICATION_JSON).build();
+		assertThat(converter.canConvertFrom(message, Person.class)).isTrue();
+		assertThat(converter.canConvertFrom(message, Object.class)).isFalse();
+		assertThat(converter.canConvertFrom(message, null)).isFalse();
+		assertThat(converter.convertFromInternal(message, Person.class, null)).isInstanceOf(Person.class);
+
+		message = MessageBuilder.withPayload("{\"name\":\"bill\"}")
+				.setHeader(MessageHeaders.CONTENT_TYPE, MimeTypeUtils.APPLICATION_JSON.toString() + ";type=" + Person.class.getName()).build();
+		assertThat(converter.canConvertFrom(message, Object.class)).isTrue();
+		assertThat(converter.canConvertFrom(message, null)).isTrue();
+		assertThat(converter.convertFromInternal(message, Person.class, null)).isInstanceOf(Person.class);
+		assertThat(converter.convertFromInternal(message, Object.class, null)).isInstanceOf(Person.class);
+		assertThat(converter.convertFromInternal(message, null, null)).isInstanceOf(Person.class);
+	}
+
+	public static class Person {
+		private String name;
+
+		public String getName() {
+			return name;
+		}
+
+		public void setName(String name) {
+			this.name = name;
+		}
+	}
+}