Remove charset parameter from Spring MVC JSON content type

This change is a follow-up to the ones done as part of gh-22788.

Closes gh-22954

Author: sdeleuze

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

@@ -130,10 +130,6 @@ private Charset getContentTypeCharset(@Nullable MediaType contentType) {
 		if (contentType != null && contentType.getCharset() != null) {
 			return contentType.getCharset();
 		}
-		else if (contentType != null && contentType.isCompatibleWith(MediaType.APPLICATION_JSON)) {
-			// Matching to AbstractJackson2HttpMessageConverter#DEFAULT_CHARSET
-			return StandardCharsets.UTF_8;
-		}
 		else {
 			Charset charset = getDefaultCharset();
 			Assert.state(charset != null, "No default charset");

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

@@ -48,7 +48,6 @@
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.http.converter.HttpMessageNotReadableException;
 import org.springframework.http.converter.HttpMessageNotWritableException;
-import org.springframework.http.server.ServletServerHttpResponse;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.TypeUtils;
@@ -251,9 +250,6 @@ private Object readJavaType(JavaType javaType, HttpInputMessage inputMessage) th
 	protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessage outputMessage)
 			throws IOException, HttpMessageNotWritableException {
 
-		if (getDefaultCharset() == null && outputMessage instanceof ServletServerHttpResponse) {
-			((ServletServerHttpResponse)outputMessage).getServletResponse().setCharacterEncoding("UTF-8");
-		}
 		MediaType contentType = outputMessage.getHeaders().getContentType();
 		JsonEncoding encoding = getJsonEncoding(contentType);
 		JsonGenerator generator = this.objectMapper.getFactory().createGenerator(outputMessage.getBody(), encoding);

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

@@ -293,7 +293,7 @@ public void handleReturnValueSortByQuality() throws Exception {
 
 		processor.writeWithMessageConverters("Foo", returnTypeString, request);
 
-		assertThat(servletResponse.getHeader("Content-Type")).isEqualTo("application/json;charset=UTF-8");
+		assertThat(servletResponse.getHeader("Content-Type")).isEqualTo(MediaType.APPLICATION_JSON_VALUE);
 	}
 
 	@Test
@@ -676,22 +676,6 @@ public void jacksonSubTypeList() throws Exception {
 		assertThat(content.contains("\"name\":\"bar\"")).isTrue();
 	}
 
-	@Test  // SPR-13631
-	public void defaultCharset() throws Exception {
-		Method method = JacksonController.class.getMethod("defaultCharset");
-		HandlerMethod handlerMethod = new HandlerMethod(new JacksonController(), method);
-		MethodParameter methodReturnType = handlerMethod.getReturnType();
-
-		List<HttpMessageConverter<?>> converters = new ArrayList<>();
-		converters.add(new MappingJackson2HttpMessageConverter());
-		RequestResponseBodyMethodProcessor processor = new RequestResponseBodyMethodProcessor(converters);
-
-		Object returnValue = new JacksonController().defaultCharset();
-		processor.handleReturnValue(returnValue, methodReturnType, this.container, this.request);
-
-		assertThat(this.servletResponse.getCharacterEncoding()).isEqualTo("UTF-8");
-	}
-
 	@Test  // SPR-14520
 	public void resolveArgumentTypeVariableWithGenericInterface() throws Exception {
 		this.servletRequest.setContent("\"foo\"".getBytes("UTF-8"));