Support Optional in AbstractJackson2HttpMessageConverter

This commit introduces support for Optional in the
AbstractJackson2HttpMessageConverter, similar the existing support for
collection types were supported.

Closes gh-24498

spring-web/src/main/java/org/springframework/http/converter/json/AbstractJackson2HttpMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -30,6 +30,7 @@ import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
 
@@ -471,7 +472,7 @@ public abstract class AbstractJackson2HttpMessageConverter extends AbstractGener
 			if (filters != null) {
 				objectWriter = objectWriter.with(filters);
 			}
-			if (javaType != null && javaType.isContainerType()) {
+			if (javaType != null && (javaType.isContainerType() || javaType.isTypeOrSubTypeOf(Optional.class))) {
 				objectWriter = objectWriter.forType(javaType);
 			}
 			SerializationConfig config = objectWriter.getConfig();

spring-web/src/test/java/org/springframework/http/converter/json/MappingJackson2HttpMessageConverterTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -24,8 +24,11 @@ import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 
 import com.fasterxml.jackson.annotation.JsonFilter;
+import com.fasterxml.jackson.annotation.JsonSubTypes;
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
 import com.fasterxml.jackson.annotation.JsonView;
 import com.fasterxml.jackson.databind.DeserializationFeature;
 import com.fasterxml.jackson.databind.JavaType;
@@ -345,6 +348,18 @@ public class MappingJackson2HttpMessageConverterTests {
 		JSONAssert.assertEquals(body, outputMessage.getBodyAsString(StandardCharsets.UTF_8), true);
 	}
 
+	// gh-24498
+	@Test
+	public void writeOptional() throws IOException {
+		ParameterizedTypeReference<Optional<MyParent>> optionalParent = new ParameterizedTypeReference<>() {};
+		Optional<MyParent> result = Optional.of(new Impl1());
+		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		converter.write(result, optionalParent.getType(), MediaType.APPLICATION_JSON, outputMessage);
+
+		assertThat(outputMessage.getBodyAsString(StandardCharsets.UTF_8))
+				.contains("@type");
+	}
+
 	@Test
 	public void prettyPrint() throws Exception {
 		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
@@ -773,6 +788,18 @@ public class MappingJackson2HttpMessageConverterTests {
 		}
 	}
 
+	@JsonTypeInfo(use = JsonTypeInfo.Id.NAME)
+	@JsonSubTypes(value = {@JsonSubTypes.Type(value = Impl1.class),
+			@JsonSubTypes.Type(value = Impl2.class)})
+	public static interface MyParent {
+	}
+
+	public static class Impl1 implements MyParent {
+	}
+
+	public static class Impl2 implements MyParent {
+	}
+
 	private static class MappingJackson2HttpMessageConverterWithCustomization extends MappingJackson2HttpMessageConverter {
 
 		@Override