Support Map payloads in FormHttpMessageConverter

Prior to this commit, the `FormHttpMessageConverter` would only support
`MultiValueMap` payloads for reading and writing. While this can be
useful when web forms contain multiple values under the same key, this
prevent developers from using a common `Map` type and factory methods
like `Map.of(...)`.

This commit relaxes constraints in `FormHttpMessageConverter` and
ensures that `Map` types are supported for reading and writing URL
encoded forms.
Note that when reading forms to a `Map`, only the first value for each
key will be considered and other values will be dropped if they exist.

Closes gh-36408

framework-docs/modules/ROOT/pages/web/webmvc/message-converters.adoc

@@ -26,6 +26,7 @@ By default, this converter supports all text media types(`text/{asterisk}`) and
 | An `HttpMessageConverter` implementation that can read and write URL encoded forms.
 By default, this converter reads and writes the `application/x-www-form-urlencoded` media type.
 Form data is read from and written into a `MultiValueMap<String, String>`.
+`Map<String, String>` is also supported, but multiple values under the same key will be ignored.
 
 | `MultipartHttpMessageConverter`
 | An `HttpMessageConverter` implementation that can read and write multipart messages.

spring-test/src/main/java/org/springframework/test/web/client/match/ContentRequestMatchers.java

@@ -176,12 +176,13 @@ public class ContentRequestMatchers {
 		return formData(multiValueMap, false);
 	}
 
+	@SuppressWarnings("unchecked")
 	private RequestMatcher formData(MultiValueMap<String, String> expectedMap, boolean containsExactly) {
 		return request -> {
 			MockClientHttpRequest mockRequest = (MockClientHttpRequest) request;
 			MockHttpInputMessage message = new MockHttpInputMessage(mockRequest.getBodyAsBytes());
 			message.getHeaders().putAll(mockRequest.getHeaders());
-			MultiValueMap<String, String> actualMap = new FormHttpMessageConverter()
+			MultiValueMap<String, String> actualMap = (MultiValueMap<String, String>) new FormHttpMessageConverter()
 					.read(ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class), message, null);
 			if (containsExactly) {
 				assertEquals("Form data", expectedMap, actualMap);

spring-test/src/main/java/org/springframework/test/web/servlet/request/AbstractMockHttpServletRequestBuilder.java

@@ -1022,6 +1022,7 @@ public abstract class AbstractMockHttpServletRequestBuilder<B extends AbstractMo
 		}
 	}
 
+	@SuppressWarnings("unchecked")
 	private MultiValueMap<String, String> parseFormData(MediaType mediaType) {
 		HttpInputMessage message = new HttpInputMessage() {
 			@Override
@@ -1038,7 +1039,7 @@ public abstract class AbstractMockHttpServletRequestBuilder<B extends AbstractMo
 		};
 
 		try {
-			return new FormHttpMessageConverter()
+			return (MultiValueMap<String, String>) new FormHttpMessageConverter()
 					.read(ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class), message, null);
 		}
 		catch (IOException ex) {

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

@@ -35,6 +35,7 @@ import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
+import org.springframework.util.ObjectUtils;
 import org.springframework.util.StreamUtils;
 import org.springframework.util.StringUtils;
 
@@ -76,11 +77,17 @@ import org.springframework.util.StringUtils;
  * @since 3.0
  * @see org.springframework.util.MultiValueMap
  */
-public class FormHttpMessageConverter implements SmartHttpMessageConverter<MultiValueMap<String, String>> {
+public class FormHttpMessageConverter implements SmartHttpMessageConverter<Object> {
 
 	/** The default charset used by the converter. */
 	public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;
 
+	private static final ResolvableType MULTIVALUE_TYPE =
+			ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class);
+
+	private static final ResolvableType MAP_TYPE =
+			ResolvableType.forClassWithGenerics(Map.class, String.class, String.class);
+
 	private Charset charset = DEFAULT_CHARSET;
 
 
@@ -106,28 +113,20 @@ public class FormHttpMessageConverter implements SmartHttpMessageConverter<Multi
 
 	@Override
 	public boolean canRead(ResolvableType type, @Nullable MediaType mediaType) {
-		if (!MultiValueMap.class.isAssignableFrom(type.toClass()) ||
-				(!type.hasUnresolvableGenerics() &&
-				!String.class.isAssignableFrom(type.getGeneric(1).toClass()))) {
-			return false;
-		}
-		for (MediaType supportedMediaType : getSupportedMediaTypes()) {
-			if (supportedMediaType.includes(mediaType)) {
-				return true;
-			}
-		}
-		return false;
+		return canConvert(type, mediaType);
 	}
 
 	@Override
 	public boolean canWrite(ResolvableType targetType, Class<?> valueClass, @Nullable MediaType mediaType) {
-		if (!MultiValueMap.class.isAssignableFrom(targetType.toClass()) ||
-				(!targetType.hasUnresolvableGenerics() &&
-				!String.class.isAssignableFrom(targetType.getGeneric(1).toClass()))) {
+		return canConvert(targetType, mediaType);
+	}
+
+	private boolean canConvert(ResolvableType targetType, @Nullable MediaType mediaType) {
+		if (!Map.class.isAssignableFrom(targetType.toClass())) {
 			return false;
 		}
 		for (MediaType supportedMediaType : getSupportedMediaTypes()) {
-			if (supportedMediaType.isCompatibleWith(mediaType)) {
+			if (supportedMediaType.includes(mediaType)) {
 				return true;
 			}
 		}
@@ -135,7 +134,7 @@ public class FormHttpMessageConverter implements SmartHttpMessageConverter<Multi
 	}
 
 	@Override
-	public MultiValueMap<String, String> read(ResolvableType type, HttpInputMessage inputMessage, @Nullable Map<String, Object> hints)
+	public Object read(ResolvableType type, HttpInputMessage inputMessage, @Nullable Map<String, Object> hints)
 			throws IOException, HttpMessageNotReadableException {
 
 		MediaType contentType = inputMessage.getHeaders().getContentType();
@@ -161,20 +160,31 @@ public class FormHttpMessageConverter implements SmartHttpMessageConverter<Multi
 		catch (IllegalArgumentException ex) {
 			throw new HttpMessageNotReadableException("Could not decode HTTP form payload", ex, inputMessage);
 		}
+		if (!MultiValueMap.class.isAssignableFrom(type.toClass())) {
+			return result.asSingleValueMap();
+		}
 		return result;
 	}
 
 	@Override
 	@SuppressWarnings("unchecked")
-	public void write(MultiValueMap<String, String> formData, ResolvableType type, @Nullable MediaType contentType,
+	public void write(Object data, ResolvableType type, @Nullable MediaType contentType,
 					HttpOutputMessage outputMessage, @Nullable Map<String, Object> hints) throws IOException, HttpMessageNotWritableException {
 
+		Assert.isInstanceOf(Map.class, data, "data must be of type Map or MultiValueMap");
+
 		contentType = getFormContentType(contentType);
 		outputMessage.getHeaders().setContentType(contentType);
-
 		Charset charset = (contentType.getCharset() != null ? contentType.getCharset() : this.charset);
 
-		byte[] bytes = serializeForm(formData, charset).getBytes(charset);
+		String serializedForm = "";
+		if (data instanceof MultiValueMap<?,?> formData) {
+			serializedForm = serializeForm((MultiValueMap<String, String>) formData, charset);
+		}
+		else if (data instanceof Map<?,?> formData) {
+			serializedForm = serializeForm((Map<String, String>) formData, charset);
+		}
+		byte[] bytes = serializedForm.getBytes(charset);
 		outputMessage.getHeaders().setContentLength(bytes.length);
 
 		if (outputMessage instanceof StreamingHttpOutputMessage streamingOutputMessage) {
@@ -204,6 +214,18 @@ public class FormHttpMessageConverter implements SmartHttpMessageConverter<Multi
 		return contentType;
 	}
 
+	protected String serializeForm(Map<String, String> formData, Charset charset) {
+		StringBuilder builder = new StringBuilder();
+		formData.forEach((name, value) -> {
+			if (name == null) {
+				Assert.isTrue(ObjectUtils.isEmpty(value), () -> "Null name in form data: " + formData);
+				return;
+			}
+			serializeValue(builder, name, value, charset);
+		});
+		return builder.toString();
+	}
+
 	protected String serializeForm(MultiValueMap<String, String> formData, Charset charset) {
 		StringBuilder builder = new StringBuilder();
 		formData.forEach((name, values) -> {
@@ -211,19 +233,20 @@ public class FormHttpMessageConverter implements SmartHttpMessageConverter<Multi
 					Assert.isTrue(CollectionUtils.isEmpty(values), () -> "Null name in form data: " + formData);
 					return;
 				}
-				values.forEach(value -> {
-					if (builder.length() != 0) {
-						builder.append('&');
-					}
-					builder.append(URLEncoder.encode(name, charset));
-					if (value != null) {
-						builder.append('=');
-						builder.append(URLEncoder.encode(value, charset));
-					}
-				});
+				values.forEach(value -> serializeValue(builder, name, value, charset));
 		});
-
 		return builder.toString();
 	}
 
+	private void serializeValue(StringBuilder builder, String name, String value, Charset charset) {
+		if (builder.length() != 0) {
+			builder.append('&');
+		}
+		builder.append(URLEncoder.encode(name, charset));
+		if (value != null) {
+			builder.append('=');
+			builder.append(URLEncoder.encode(value, charset));
+		}
+	}
+
 }

spring-web/src/main/java/org/springframework/web/filter/FormContentFilter.java

@@ -94,6 +94,7 @@ public class FormContentFilter extends OncePerRequestFilter {
 		}
 	}
 
+	@SuppressWarnings("unchecked")
 	private @Nullable MultiValueMap<String, String> parseIfNecessary(HttpServletRequest request) throws IOException {
 		if (!shouldParse(request)) {
 			return null;
@@ -105,7 +106,7 @@ public class FormContentFilter extends OncePerRequestFilter {
 				return request.getInputStream();
 			}
 		};
-		return this.formConverter.read(ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class), inputMessage, null);
+		return (MultiValueMap<String, String>) this.formConverter.read(ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class), inputMessage, null);
 	}
 
 	private boolean shouldParse(HttpServletRequest request) {

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

@@ -18,7 +18,9 @@ package org.springframework.http.converter;
 
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 
 import org.junit.jupiter.api.Test;
 
@@ -44,6 +46,7 @@ import static org.springframework.http.MediaType.MULTIPART_RELATED;
  * @author Rossen Stoyanchev
  * @author Sam Brannen
  * @author Sebastien Deleuze
+ * @author Brian Clozel
  */
 class FormHttpMessageConverterTests {
 
@@ -53,35 +56,57 @@ class FormHttpMessageConverterTests {
 	private static final ResolvableType MULTI_VALUE_MAP =
 			ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class);
 
-	private final FormHttpMessageConverter converter = new FormHttpMessageConverter();
+	private static final ResolvableType MAP =
+			ResolvableType.forClassWithGenerics(Map.class, String.class, String.class);
 
+	private final FormHttpMessageConverter converter = new FormHttpMessageConverter();
 
+	@Test
+	void cannotReadToMapsWhenMediaTypeMissing() {
+		assertCannotRead(MAP, null);
+		assertCannotRead(MULTI_VALUE_MAP, null);
+		assertCannotRead(LINKED_MULTI_VALUE_MAP, null);
+		// without generics
+		assertCannotRead(ResolvableType.forClass(Map.class), null);
+		assertCannotRead(ResolvableType.forClass(MultiValueMap.class), null);
+		assertCannotRead(ResolvableType.forClass(LinkedMultiValueMap.class), null);
+	}
 
 	@Test
-	void canRead() {
-		assertCanRead(APPLICATION_FORM_URLENCODED);
+	void canReadToMapTypes() {
+		assertCanRead(MAP, APPLICATION_FORM_URLENCODED);
+		assertCanRead(MULTI_VALUE_MAP, APPLICATION_FORM_URLENCODED);
 		assertCanRead(LINKED_MULTI_VALUE_MAP, APPLICATION_FORM_URLENCODED);
+		// without generics
+		assertCanRead(ResolvableType.forClass(Map.class), APPLICATION_FORM_URLENCODED);
+		assertCanRead(ResolvableType.forClass(MultiValueMap.class), APPLICATION_FORM_URLENCODED);
 		assertCanRead(ResolvableType.forClass(LinkedMultiValueMap.class), APPLICATION_FORM_URLENCODED);
-
-		ResolvableType mapStringObject = ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, Object.class);
-		assertCannotRead(mapStringObject, null);
-		assertCannotRead(mapStringObject, APPLICATION_FORM_URLENCODED);
 	}
 
 	@Test
 	void cannotReadMultipart() {
 		// Without custom multipart types supported
-		asssertCannotReadMultipart();
+		assertCannotReadMultipart();
 
 		// Should still be the case with custom multipart types supported
-		asssertCannotReadMultipart();
+		assertCannotReadMultipart();
 	}
 
 	@Test
 	void canWrite() {
 		assertCanWrite(APPLICATION_FORM_URLENCODED);
-		assertCanWrite(MediaType.ALL);
+		assertCannotWrite(MediaType.ALL);
+	}
+
+
+	@Test
+	void canWriteMapTypes() {
+		assertCanWrite(MAP, APPLICATION_FORM_URLENCODED);
+		assertCanWrite(MULTI_VALUE_MAP, APPLICATION_FORM_URLENCODED);
 		assertCanWrite(LINKED_MULTI_VALUE_MAP, APPLICATION_FORM_URLENCODED);
+		// without generics
+		assertCanWrite(ResolvableType.forClass(Map.class), APPLICATION_FORM_URLENCODED);
+		assertCanWrite(ResolvableType.forClass(MultiValueMap.class), APPLICATION_FORM_URLENCODED);
 		assertCanWrite(ResolvableType.forClass(LinkedMultiValueMap.class), APPLICATION_FORM_URLENCODED);
 	}
 
@@ -95,18 +120,38 @@ class FormHttpMessageConverterTests {
 	}
 
 	@Test
-	void readForm() throws Exception {
+	@SuppressWarnings("unchecked")
+	void readFormAsMultiValueMap() throws Exception {
 		String body = "name+1=value+1&name+2=value+2%2B1&name+2=value+2%2B2&name+3";
 		MockHttpInputMessage inputMessage = new MockHttpInputMessage(body.getBytes(StandardCharsets.ISO_8859_1));
 		inputMessage.getHeaders().setContentType(
 				new MediaType("application", "x-www-form-urlencoded", StandardCharsets.ISO_8859_1));
-		MultiValueMap<String, String> result = this.converter.read(null, inputMessage);
+		Object result = this.converter.read(ResolvableType.forClass(MultiValueMap.class), inputMessage, null);
 
-		assertThat(result).as("Invalid result").hasSize(3);
-		assertThat(result.getFirst("name 1")).as("Invalid result").isEqualTo("value 1");
-		List<String> values = result.get("name 2");
+		assertThat(result).isInstanceOf(MultiValueMap.class);
+		MultiValueMap<String, String> form = (MultiValueMap<String, String>) result;
+		assertThat(form).as("Invalid result").hasSize(3);
+		assertThat(form.getFirst("name 1")).as("Invalid result").isEqualTo("value 1");
+		List<String> values = form.get("name 2");
 		assertThat(values).as("Invalid result").containsExactly("value 2+1", "value 2+2");
-		assertThat(result.getFirst("name 3")).as("Invalid result").isNull();
+		assertThat(form.getFirst("name 3")).as("Invalid result").isNull();
+	}
+
+	@Test
+	@SuppressWarnings("unchecked")
+	void readFormAsMap() throws Exception {
+		String body = "name+1=value+1&name+2=value+2&name+3";
+		MockHttpInputMessage inputMessage = new MockHttpInputMessage(body.getBytes(StandardCharsets.ISO_8859_1));
+		inputMessage.getHeaders().setContentType(
+				new MediaType("application", "x-www-form-urlencoded", StandardCharsets.ISO_8859_1));
+		Object result = this.converter.read(ResolvableType.forClass(Map.class), inputMessage, null);
+
+		assertThat(result).isInstanceOf(Map.class);
+		Map<String, String> form = (Map<String, String>) result;
+		assertThat(form).as("Invalid result").hasSize(3);
+		assertThat(form.get("name 1")).as("Invalid result").isEqualTo("value 1");
+		assertThat(form.get("name 2")).as("Invalid result").isEqualTo("value 2");
+		assertThat(form.get("name 3")).as("Invalid result").isNull();
 	}
 
 	@Test
@@ -131,7 +176,7 @@ class FormHttpMessageConverterTests {
 	}
 
 	@Test
-	void writeForm() throws IOException {
+	void writeFormFromMultiValueMap() throws IOException {
 		MultiValueMap<String, String> body = new LinkedMultiValueMap<>();
 		body.set("name 1", "value 1");
 		body.add("name 2", "value 2+1");
@@ -148,6 +193,22 @@ class FormHttpMessageConverterTests {
 				.as("Invalid content-length").isEqualTo(outputMessage.getBodyAsBytes().length);
 	}
 
+	@Test
+	void writeFormFromMap() throws IOException {
+		Map<String, String> body = new HashMap<>();
+		body.put("name 1", "value 1");
+		body.put("name 2", "value 2");
+		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		this.converter.write(body, APPLICATION_FORM_URLENCODED, outputMessage);
+
+		assertThat(outputMessage.getBodyAsString(UTF_8))
+				.as("Invalid result").isEqualTo("name+2=value+2&name+1=value+1");
+		assertThat(outputMessage.getHeaders().getContentType())
+				.as("Invalid content-type").isEqualTo(APPLICATION_FORM_URLENCODED);
+		assertThat(outputMessage.getHeaders().getContentLength())
+				.as("Invalid content-length").isEqualTo(outputMessage.getBodyAsBytes().length);
+	}
+
 	private void assertCanRead(MediaType mediaType) {
 		assertCanRead(MULTI_VALUE_MAP, mediaType);
 	}
@@ -156,7 +217,7 @@ class FormHttpMessageConverterTests {
 		assertThat(this.converter.canRead(type, mediaType)).as(type.toClass().getSimpleName() + " : " + mediaType).isTrue();
 	}
 
-	private void asssertCannotReadMultipart() {
+	private void assertCannotReadMultipart() {
 		assertCannotRead(new MediaType("multipart", "*"));
 		assertCannotRead(MULTIPART_FORM_DATA);
 		assertCannotRead(MULTIPART_MIXED);