Introduce addSupportedMediaType() in FormHttpMessageConverter

Closes gh-23203

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

@@ -88,6 +88,7 @@ import org.springframework.util.StringUtils;
  * @author Arjen Poutsma
  * @author Rossen Stoyanchev
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 3.0
  * @see org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter
  * @see org.springframework.util.MultiValueMap
@@ -127,9 +128,23 @@ public class FormHttpMessageConverter implements HttpMessageConverter<MultiValue
 
 	/**
 	 * Set the list of {@link MediaType} objects supported by this converter.
+	 * @see #addSupportedMediaType(MediaType)
+	 * @see #getSupportedMediaTypes()
 	 */
 	public void setSupportedMediaTypes(List<MediaType> supportedMediaTypes) {
-		this.supportedMediaTypes = supportedMediaTypes;
+		Assert.notNull(supportedMediaTypes, "'supportedMediaTypes' must not be null");
+		// Ensure internal list is mutable.
+		this.supportedMediaTypes = new ArrayList<>(supportedMediaTypes);
+	}
+
+	/**
+	 * Add a {@link MediaType} to be supported by this converter.
+	 * @since 5.2
+	 * @see #setSupportedMediaTypes(List)
+	 */
+	public void addSupportedMediaType(MediaType supportedMediaType) {
+		Assert.notNull(supportedMediaType, "'supportedMediaType' must not be null");
+		this.supportedMediaTypes.add(supportedMediaType);
 	}
 
 	@Override

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

@@ -58,36 +58,58 @@ import static org.mockito.Mockito.verify;
  */
 public class FormHttpMessageConverterTests {
 
+	protected static final MediaType MULTIPART_MIXED = new MediaType("multipart", "mixed");
+	protected static final MediaType MULTIPART_RELATED = new MediaType("multipart", "related");
+
 	private final FormHttpMessageConverter converter = new AllEncompassingFormHttpMessageConverter();
 
 
 	@Test
 	public void canRead() {
-		assertThat(this.converter.canRead(MultiValueMap.class, new MediaType("application", "x-www-form-urlencoded"))).isTrue();
-		assertThat(this.converter.canRead(MultiValueMap.class, new MediaType("multipart", "form-data"))).isFalse();
+		assertThat(this.converter.canRead(MultiValueMap.class, MediaType.APPLICATION_FORM_URLENCODED)).isTrue();
+		assertThat(this.converter.canRead(MultiValueMap.class, MediaType.MULTIPART_FORM_DATA)).isFalse();
 	}
 
 	@Test
 	public void canWrite() {
-		assertCanWrite(MultiValueMap.class, new MediaType("application", "x-www-form-urlencoded"));
-		assertCanWrite(MultiValueMap.class, new MediaType("multipart", "form-data"));
-		assertCanWrite(MultiValueMap.class, new MediaType("multipart", "form-data", StandardCharsets.UTF_8));
-		assertCanWrite(MultiValueMap.class, MediaType.ALL);
+		assertCanWrite(MediaType.APPLICATION_FORM_URLENCODED);
+		assertCanWrite(MediaType.MULTIPART_FORM_DATA);
+		assertCanWrite(new MediaType("multipart", "form-data", StandardCharsets.UTF_8));
+		assertCanWrite(MediaType.ALL);
 	}
 
 	@Test
-	public void canWriteMultipartMixedAndMultipartRelated() {
+	public void setSupportedMediaTypes() {
+		assertCannotWrite(MULTIPART_MIXED);
+		assertCannotWrite(MULTIPART_RELATED);
+
 		List<MediaType> supportedMediaTypes = new ArrayList<>(this.converter.getSupportedMediaTypes());
-		supportedMediaTypes.add(new MediaType("multipart", "mixed"));
-		supportedMediaTypes.add(new MediaType("multipart", "related"));
+		supportedMediaTypes.add(MULTIPART_MIXED);
+		supportedMediaTypes.add(MULTIPART_RELATED);
 		this.converter.setSupportedMediaTypes(supportedMediaTypes);
 
-		assertCanWrite(MultiValueMap.class, new MediaType("multipart", "mixed"));
-		assertCanWrite(MultiValueMap.class, new MediaType("multipart", "related"));
+		assertCanWrite(MULTIPART_MIXED);
+		assertCanWrite(MULTIPART_RELATED);
+	}
+
+	@Test
+	public void addSupportedMediaType() {
+		assertCannotWrite(MULTIPART_MIXED);
+		assertCannotWrite(MULTIPART_RELATED);
+
+		this.converter.addSupportedMediaType(MULTIPART_MIXED);
+		this.converter.addSupportedMediaType(MULTIPART_RELATED);
+
+		assertCanWrite(MULTIPART_MIXED);
+		assertCanWrite(MULTIPART_RELATED);
+	}
+
+	private void assertCanWrite(MediaType mediaType) {
+		assertThat(this.converter.canWrite(MultiValueMap.class, mediaType)).isTrue();
 	}
 
-	private void assertCanWrite(Class<?> clazz, MediaType mediaType) {
-		assertThat(this.converter.canWrite(clazz, mediaType)).isTrue();
+	private void assertCannotWrite(MediaType mediaType) {
+		assertThat(this.converter.canWrite(MultiValueMap.class, mediaType)).isFalse();
 	}
 
 	@Test