Use UTF-8 with JSON in MockHttpServletResponse

This commit makes MockHttpServletResponse consistent with the other
parts of the framework where the body of a response is read as an UTF-8
String when the content type is application/json or similar, overriding
the ISO-8859-1 default Servlet encoding.

Closes gh-33019

spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java

@@ -24,6 +24,7 @@
 import java.io.UnsupportedEncodingException;
 import java.io.Writer;
 import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 import java.text.DateFormat;
 import java.text.ParseException;
 import java.text.SimpleDateFormat;
@@ -72,6 +73,8 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	private static final TimeZone GMT = TimeZone.getTimeZone("GMT");
 
+	private static final MediaType APPLICATION_PLUS_JSON = new MediaType("application", "*+json");
+
 
 	//---------------------------------------------------------------------
 	// ServletResponse properties
@@ -348,6 +351,10 @@ public void setContentType(@Nullable String contentType) {
 				if (mediaType.getCharset() != null) {
 					setExplicitCharacterEncoding(mediaType.getCharset().name());
 				}
+				else if (mediaType.isCompatibleWith(MediaType.APPLICATION_JSON) ||
+						mediaType.isCompatibleWith(APPLICATION_PLUS_JSON)) {
+						this.characterEncoding = StandardCharsets.UTF_8.name();
+				}
 			}
 			catch (Exception ex) {
 				// Try to get charset value anyway

spring-test/src/main/java/org/springframework/test/web/servlet/result/ContentResultMatchers.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -257,7 +257,7 @@ public ResultMatcher json(String jsonContent, JsonCompareMode compareMode) {
 	 */
 	public ResultMatcher json(String jsonContent, JsonComparator comparator) {
 		return result -> {
-			String content = result.getResponse().getContentAsString(StandardCharsets.UTF_8);
+			String content = result.getResponse().getContentAsString();
 			comparator.assertIsMatch(jsonContent, content);
 		};
 	}

spring-test/src/main/java/org/springframework/test/web/servlet/result/JsonPathResultMatchers.java

@@ -17,7 +17,6 @@
 package org.springframework.test.web.servlet.result;
 
 import java.io.UnsupportedEncodingException;
-import java.nio.charset.StandardCharsets;
 
 import com.jayway.jsonpath.JsonPath;
 import org.hamcrest.Matcher;
@@ -238,7 +237,7 @@ public ResultMatcher isMap() {
 	}
 
 	private String getContent(MvcResult result) throws UnsupportedEncodingException {
-		String content = result.getResponse().getContentAsString(StandardCharsets.UTF_8);
+		String content = result.getResponse().getContentAsString();
 		if (StringUtils.hasLength(this.prefix)) {
 			try {
 				String reason = String.format("Expected a JSON payload prefixed with \"%s\" but found: %s",

spring-test/src/main/java/org/springframework/test/web/servlet/result/PrintingResultHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -16,7 +16,6 @@
 
 package org.springframework.test.web.servlet.result;
 
-import java.nio.charset.StandardCharsets;
 import java.util.Collections;
 import java.util.Enumeration;
 import java.util.Map;
@@ -28,7 +27,6 @@
 
 import org.springframework.core.style.ToStringCreator;
 import org.springframework.http.HttpHeaders;
-import org.springframework.http.MediaType;
 import org.springframework.lang.Nullable;
 import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.mock.web.MockHttpServletResponse;
@@ -250,9 +248,7 @@ protected void printResponse(MockHttpServletResponse response) throws Exception
 		this.printer.printValue("Error message", response.getErrorMessage());
 		this.printer.printValue("Headers", getResponseHeaders(response));
 		this.printer.printValue("Content type", response.getContentType());
-		String body = (MediaType.APPLICATION_JSON_VALUE.equals(response.getContentType()) ?
-				response.getContentAsString(StandardCharsets.UTF_8) : response.getContentAsString());
-		this.printer.printValue("Body", body);
+		this.printer.printValue("Body", response.getContentAsString());
 		this.printer.printValue("Forwarded URL", response.getForwardedUrl());
 		this.printer.printValue("Redirected URL", response.getRedirectedUrl());
 		printCookies(response.getCookies());

spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java

@@ -30,6 +30,7 @@
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.ValueSource;
 
+import org.springframework.http.MediaType;
 import org.springframework.web.util.WebUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -620,4 +621,12 @@ void resetResetsCharset() {
 		assertThat(contentTypeHeader).isEqualTo("text/plain");
 	}
 
+	@Test  // gh-33019
+	void contentAsStringEncodingWithJson() throws IOException {
+		String content = "{\"name\": \"Jürgen\"}";
+		response.setContentType(MediaType.APPLICATION_JSON_VALUE);
+		response.getWriter().write(content);
+		assertThat(response.getContentAsString()).isEqualTo(content);
+	}
+
 }