Infer UTF-8 encoding for JSON response with MockMvc's andDo(print(...))

Prior to this commit, the PrintingResultHandler in MockMvc -- typically
invoked via .andDo(print()) -- printed an `application/json` response
body using the default encoding (ISO-8859-1), which resulted in UTF-8
characters being garbled.

Since an `application/json` response is implicitly encoded using UTF-8,
the PrintingResultHandler now infers UTF-8 encoding for such response
bodies.

Closes gh-27926

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.test.web.servlet.result;
 
+import java.nio.charset.StandardCharsets;
 import java.util.Collections;
 import java.util.Enumeration;
 import java.util.Map;
@@ -27,6 +28,7 @@ import javax.servlet.http.HttpSession;
 
 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;
@@ -249,7 +251,9 @@ public class PrintingResultHandler implements ResultHandler {
 		this.printer.printValue("Error message", response.getErrorMessage());
 		this.printer.printValue("Headers", getResponseHeaders(response));
 		this.printer.printValue("Content type", response.getContentType());
-		this.printer.printValue("Body", response.getContentAsString());
+		String body = (MediaType.APPLICATION_JSON_VALUE.equals(response.getContentType()) ?
+				response.getContentAsString(StandardCharsets.UTF_8) : response.getContentAsString());
+		this.printer.printValue("Body", body);
 		this.printer.printValue("Forwarded URL", response.getForwardedUrl());
 		this.printer.printValue("Redirected URL", response.getRedirectedUrl());
 		printCookies(response.getCookies());

spring-test/src/test/java/org/springframework/test/web/servlet/samples/standalone/resulthandlers/PrintingResultHandlerIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -23,10 +23,12 @@ import javax.servlet.http.HttpServletResponse;
 
 import org.junit.jupiter.api.Test;
 
+import org.springframework.http.MediaType;
 import org.springframework.test.web.servlet.result.PrintingResultHandler;
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.RestController;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.fail;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
@@ -61,6 +63,22 @@ class PrintingResultHandlerIntegrationTests {
 			.contains("Headers = [Set-Cookie:\"enigma=42\", Content-Type:\"text/plain;charset=ISO-8859-1\", Content-Length:\"14\"]");
 	}
 
+	@Test
+	void printMvcResultsToWriterWithJsonResponseBodyInterpretedAsUtf8() throws Exception {
+		StringWriter writer = new StringWriter();
+
+		standaloneSetup(new SimpleController()).build()
+			// "Hallöchen" is German slang for "hello".
+			.perform(get("/utf8").accept(MediaType.APPLICATION_JSON).content("Hallöchen, Welt!".getBytes()).characterEncoding(UTF_8))
+			.andDo(print(writer))
+			// "Grüß dich!" is German for "greetings to you".
+			.andExpect(content().bytes("Grüß dich!".getBytes()));
+
+		assertThat(writer).asString()
+			.contains("Body = Hallöchen, Welt!")
+			.contains("Body = Grüß dich!");
+	}
+
 	@Test
 	void printMvcResultsToWriterWithFailingGlobalResultMatcher() throws Exception {
 		StringWriter writer = new StringWriter();
@@ -92,6 +110,11 @@ class PrintingResultHandlerIntegrationTests {
 			response.addCookie(new Cookie("enigma", "42"));
 			return "Hello Response";
 		}
+
+		@GetMapping("/utf8")
+		String utf8(HttpServletResponse response) {
+			return "Grüß dich!";
+		}
 	}
 
 }