Serialize with type only collections in Jackson HttpMessageConverter

Issue: SPR-13318

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

@@ -258,7 +258,7 @@ public abstract class AbstractJackson2HttpMessageConverter extends AbstractGener
 			else {
 				objectWriter = this.objectMapper.writer();
 			}
-			if (javaType != null) {
+			if (javaType != null && javaType.isContainerType()) {
 				objectWriter = objectWriter.withType(javaType);
 			}
 			objectWriter.writeValue(generator, value);

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

@@ -315,8 +315,54 @@ public class MappingJackson2HttpMessageConverterTests {
 		assertThat(result, not(containsString("\"withoutView\":\"without\"")));
 	}
 
+	@Test  // SPR-13318
+	public void writeSubType() throws Exception {
+		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		MyBean bean = new MyBean();
+		bean.setString("Foo");
+		bean.setNumber(42);
+
+		this.converter.writeInternal(bean, MyInterface.class, outputMessage);
+
+		String result = outputMessage.getBodyAsString(Charset.forName("UTF-8"));
+		assertTrue(result.contains("\"string\":\"Foo\""));
+		assertTrue(result.contains("\"number\":42"));
+	}
+
+	@Test  // SPR-13318
+	public void writeSubTypeList() throws Exception {
+		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		List<MyBean> beans = new ArrayList<MyBean>();
+		MyBean foo = new MyBean();
+		foo.setString("Foo");
+		foo.setNumber(42);
+		beans.add(foo);
+		MyBean bar = new MyBean();
+		bar.setString("Bar");
+		bar.setNumber(123);
+		beans.add(bar);
+		ParameterizedTypeReference<List<MyInterface>> typeReference =
+				new ParameterizedTypeReference<List<MyInterface>>() {};
+
+		this.converter.writeInternal(beans, typeReference.getType(), outputMessage);
+
+		String result = outputMessage.getBodyAsString(Charset.forName("UTF-8"));
+		assertTrue(result.contains("\"string\":\"Foo\""));
+		assertTrue(result.contains("\"number\":42"));
+		assertTrue(result.contains("\"string\":\"Bar\""));
+		assertTrue(result.contains("\"number\":123"));
+	}
+
+
+	interface MyInterface {
+
+		String getString();
+
+		void setString(String string);
+	}
+
 
-	public static class MyBean {
+	public static class MyBean implements MyInterface {
 
 		private String string;
 

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/RequestResponseBodyMethodProcessorTests.java

@@ -24,6 +24,7 @@ import java.io.Serializable;
 import java.lang.reflect.Method;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 
@@ -543,7 +544,7 @@ public class RequestResponseBodyMethodProcessorTests {
 
 	@Test  // SPR-12811
 	public void jacksonTypeInfoList() throws Exception {
-		Method method = JacksonController.class.getMethod("handleList");
+		Method method = JacksonController.class.getMethod("handleTypeInfoList");
 		HandlerMethod handlerMethod = new HandlerMethod(new JacksonController(), method);
 		MethodParameter methodReturnType = handlerMethod.getReturnType();
 
@@ -551,7 +552,7 @@ public class RequestResponseBodyMethodProcessorTests {
 		converters.add(new MappingJackson2HttpMessageConverter());
 		RequestResponseBodyMethodProcessor processor = new RequestResponseBodyMethodProcessor(converters);
 
-		Object returnValue = new JacksonController().handleList();
+		Object returnValue = new JacksonController().handleTypeInfoList();
 		processor.handleReturnValue(returnValue, methodReturnType, this.mavContainer, this.webRequest);
 
 		String content = this.servletResponse.getContentAsString();
@@ -559,6 +560,44 @@ public class RequestResponseBodyMethodProcessorTests {
 		assertTrue(content.contains("\"type\":\"bar\""));
 	}
 
+	@Test  // SPR-13318
+	public void jacksonSubType() throws Exception {
+		Method method = JacksonController.class.getMethod("handleSubType");
+		HandlerMethod handlerMethod = new HandlerMethod(new JacksonController(), method);
+		MethodParameter methodReturnType = handlerMethod.getReturnType();
+
+		List<HttpMessageConverter<?>> converters = new ArrayList<HttpMessageConverter<?>>();
+		converters.add(new MappingJackson2HttpMessageConverter());
+		RequestResponseBodyMethodProcessor processor = new RequestResponseBodyMethodProcessor(converters);
+
+		Object returnValue = new JacksonController().handleSubType();
+		processor.handleReturnValue(returnValue, methodReturnType, this.mavContainer, this.webRequest);
+
+		String content = this.servletResponse.getContentAsString();
+		assertTrue(content.contains("\"id\":123"));
+		assertTrue(content.contains("\"name\":\"foo\""));
+	}
+
+	@Test  // SPR-13318
+	public void jacksonSubTypeList() throws Exception {
+		Method method = JacksonController.class.getMethod("handleSubTypeList");
+		HandlerMethod handlerMethod = new HandlerMethod(new JacksonController(), method);
+		MethodParameter methodReturnType = handlerMethod.getReturnType();
+
+		List<HttpMessageConverter<?>> converters = new ArrayList<HttpMessageConverter<?>>();
+		converters.add(new MappingJackson2HttpMessageConverter());
+		RequestResponseBodyMethodProcessor processor = new RequestResponseBodyMethodProcessor(converters);
+
+		Object returnValue = new JacksonController().handleSubTypeList();
+		processor.handleReturnValue(returnValue, methodReturnType, this.mavContainer, this.webRequest);
+
+		String content = this.servletResponse.getContentAsString();
+		assertTrue(content.contains("\"id\":123"));
+		assertTrue(content.contains("\"name\":\"foo\""));
+		assertTrue(content.contains("\"id\":456"));
+		assertTrue(content.contains("\"name\":\"bar\""));
+	}
+
 
 	String handle(
 			@RequestBody List<SimpleBean> list,
@@ -774,12 +813,33 @@ public class RequestResponseBodyMethodProcessorTests {
 
 		@RequestMapping
 		@ResponseBody
-		public List<ParentClass> handleList() {
+		public List<ParentClass> handleTypeInfoList() {
 			List<ParentClass> list = new ArrayList<>();
 			list.add(new Foo("foo"));
 			list.add(new Bar("bar"));
 			return list;
 		}
+
+		@RequestMapping
+		@ResponseBody
+		public Identifiable handleSubType() {
+			SimpleBean foo = new SimpleBean();
+			foo.setId(123L);
+			foo.setName("foo");
+			return foo;
+		}
+
+		@RequestMapping
+		@ResponseBody
+		public List<Identifiable> handleSubTypeList() {
+			SimpleBean foo = new SimpleBean();
+			foo.setId(123L);
+			foo.setName("foo");
+			SimpleBean bar = new SimpleBean();
+			bar.setId(456L);
+			bar.setName("bar");
+			return Arrays.asList(foo, bar);
+		}
 	}
 
 	private static class EmptyRequestBodyAdvice implements RequestBodyAdvice {