Support setCharacterEncoding(null) in MockHttpServletResponse

Closes gh-30341

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

@@ -33,6 +33,7 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Date;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
@@ -201,15 +202,42 @@ public class MockHttpServletResponse implements HttpServletResponse {
 	}
 
 	@Override
-	public void setCharacterEncoding(String characterEncoding) {
+	public void setCharacterEncoding(@Nullable String characterEncoding) {
 		setExplicitCharacterEncoding(characterEncoding);
 		updateContentTypePropertyAndHeader();
 	}
 
-	private void setExplicitCharacterEncoding(String characterEncoding) {
-		Assert.notNull(characterEncoding, "'characterEncoding' must not be null");
-		this.characterEncoding = characterEncoding;
-		this.characterEncodingSet = true;
+	private void setExplicitCharacterEncoding(@Nullable String characterEncoding) {
+		if (characterEncoding == null) {
+			this.characterEncoding = this.defaultCharacterEncoding;
+			this.characterEncodingSet = false;
+			if (this.contentType != null) {
+				try {
+					MediaType mediaType = MediaType.parseMediaType(this.contentType);
+					if (mediaType.getCharset() != null) {
+						Map<String, String> parameters = new LinkedHashMap<>(mediaType.getParameters());
+						parameters.remove("charset");
+						mediaType = new MediaType(mediaType.getType(), mediaType.getSubtype(), parameters);
+						this.contentType = mediaType.toString();
+					}
+				}
+				catch (Exception ignored) {
+					String value = this.contentType;
+					int charsetIndex = value.toLowerCase().indexOf(CHARSET_PREFIX);
+					if (charsetIndex != -1) {
+						value = value.substring(0, charsetIndex).trim();
+						if (value.endsWith(";")) {
+							value = value.substring(0, value.length() - 1);
+						}
+						this.contentType = value;
+					}
+				}
+			}
+		}
+		else {
+			this.characterEncoding = characterEncoding;
+			this.characterEncodingSet = true;
+		}
 	}
 
 	private void updateContentTypePropertyAndHeader() {

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

@@ -195,6 +195,18 @@ class MockHttpServletResponseTests {
 		assertThat(response.getCharacterEncoding()).isEqualTo("UTF-8");
 	}
 
+	@Test
+	void setCharacterEncodingNull() {
+		response.setContentType("test/plain");
+		response.setCharacterEncoding("UTF-8");
+		assertThat(response.getContentType()).isEqualTo("test/plain;charset=UTF-8");
+		assertThat(response.getHeader(CONTENT_TYPE)).isEqualTo("test/plain;charset=UTF-8");
+		response.setCharacterEncoding(null);
+		assertThat(response.getContentType()).isEqualTo("test/plain");
+		assertThat(response.getHeader(CONTENT_TYPE)).isEqualTo("test/plain");
+		assertThat(response.getCharacterEncoding()).isEqualTo(WebUtils.DEFAULT_CHARACTER_ENCODING);
+	}
+
 	@Test
 	void defaultCharacterEncoding() {
 		assertThat(response.isCharset()).isFalse();

spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java

@@ -33,6 +33,7 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Date;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
@@ -201,15 +202,42 @@ public class MockHttpServletResponse implements HttpServletResponse {
 	}
 
 	@Override
-	public void setCharacterEncoding(String characterEncoding) {
+	public void setCharacterEncoding(@Nullable String characterEncoding) {
 		setExplicitCharacterEncoding(characterEncoding);
 		updateContentTypePropertyAndHeader();
 	}
 
-	private void setExplicitCharacterEncoding(String characterEncoding) {
-		Assert.notNull(characterEncoding, "'characterEncoding' must not be null");
-		this.characterEncoding = characterEncoding;
-		this.characterEncodingSet = true;
+	private void setExplicitCharacterEncoding(@Nullable String characterEncoding) {
+		if (characterEncoding == null) {
+			this.characterEncoding = this.defaultCharacterEncoding;
+			this.characterEncodingSet = false;
+			if (this.contentType != null) {
+				try {
+					MediaType mediaType = MediaType.parseMediaType(this.contentType);
+					if (mediaType.getCharset() != null) {
+						Map<String, String> parameters = new LinkedHashMap<>(mediaType.getParameters());
+						parameters.remove("charset");
+						mediaType = new MediaType(mediaType.getType(), mediaType.getSubtype(), parameters);
+						this.contentType = mediaType.toString();
+					}
+				}
+				catch (Exception ignored) {
+					String value = this.contentType;
+					int charsetIndex = value.toLowerCase().indexOf(CHARSET_PREFIX);
+					if (charsetIndex != -1) {
+						value = value.substring(0, charsetIndex).trim();
+						if (value.endsWith(";")) {
+							value = value.substring(0, value.length() - 1);
+						}
+						this.contentType = value;
+					}
+				}
+			}
+		}
+		else {
+			this.characterEncoding = characterEncoding;
+			this.characterEncodingSet = true;
+		}
 	}
 
 	private void updateContentTypePropertyAndHeader() {