Add support for "application/jsonl" JSON lines

Prior to this commit, Spring web frameworks were using the
"application/x-ndjson" media type for streaming JSON payloads delimited
with newlines.

The "application/jsonl" media type seems to gain popularity in the
broader ecosystem and could supersede NDJSON in the future. This commit
adds support for JSON Lines as an alternative.

Closes gh-36485

framework-docs/modules/ROOT/pages/testing/webtestclient.adoc

@@ -580,8 +580,8 @@ Kotlin::
 [[webtestclient-stream]]
 ==== Streaming Responses
 
-To test potentially infinite streams such as `"text/event-stream"` or
-`"application/x-ndjson"`, start by verifying the response status and headers, and then
+To test potentially infinite streams such as `"text/event-stream"`,
+`"application/jsonl"` or `"application/x-ndjson"`, start by verifying the response status and headers, and then
 obtain a `FluxExchangeResult`:
 
 [tabs]

framework-docs/modules/ROOT/pages/web/webflux/reactive-spring.adoc

@@ -485,8 +485,8 @@ The `JacksonJsonEncoder` works as follows:
 * For a multi-value publisher with `application/json`, by default collect the values with
 `Flux#collectToList()` and then serialize the resulting collection.
 * For a multi-value publisher with a streaming media type such as
-`application/x-ndjson` or `application/stream+x-jackson-smile`, encode, write, and
-flush each value individually using a
+`application/jsonl`, `application/x-ndjson` or `application/stream+x-jackson-smile`,
+encode, write, and flush each value individually using a
 https://en.wikipedia.org/wiki/JSON_streaming[line-delimited JSON] format. Other
 streaming media types may be registered with the encoder.
 * For SSE the `JacksonJsonEncoder` is invoked per event and the output is flushed to ensure
@@ -598,7 +598,7 @@ To configure all three in WebFlux, you'll need to supply a pre-configured instan
 [.small]#xref:web/webmvc/mvc-ann-async.adoc#mvc-ann-async-http-streaming[See equivalent in the Servlet stack]#
 
 When streaming to the HTTP response (for example, `text/event-stream`,
-`application/x-ndjson`), it is important to send data periodically, in order to
+`application/jsonl`, `application/x-ndjson`), it is important to send data periodically, in order to
 reliably detect a disconnected client sooner rather than later. Such a send could be a
 comment-only, empty SSE event or any other "no-op" data that would effectively serve as
 a heartbeat.

framework-docs/modules/ROOT/pages/web/webmvc/mvc-ann-async.adoc

@@ -423,8 +423,8 @@ Reactive return values are handled as follows:
 
 * A single-value promise is adapted to, similar to using `DeferredResult`. Examples
 include `CompletionStage` (JDK), `Mono` (Reactor), and `Single` (RxJava).
-* A multi-value stream with a streaming media type (such as `application/x-ndjson`
-or `text/event-stream`) is adapted to, similar to using `ResponseBodyEmitter` or
+* A multi-value stream with a streaming media type (such as `application/jsonl`,
+`application/x-ndjson` or `text/event-stream`) is adapted to, similar to using `ResponseBodyEmitter` or
 `SseEmitter`. Examples include `Flux` (Reactor) or `Observable` (RxJava).
 Applications can also return `Flux<ServerSentEvent>` or `Observable<ServerSentEvent>`.
 * A multi-value stream with any other media type (such as `application/json`) is adapted

spring-web/src/main/java/org/springframework/http/MediaType.java

@@ -209,6 +209,19 @@ public class MediaType extends MimeType implements Serializable {
 	 */
 	public static final String APPLICATION_NDJSON_VALUE = "application/x-ndjson";
 
+	/**
+	 * Media type for {@code application/jsonl} (JSON Lines).
+	 * @since 7.1
+	 * @see <a href="https://jsonlines.org/">JSON Lines</a>
+	 */
+	public static final MediaType APPLICATION_JSONL;
+
+	/**
+	 * A String equivalent of {@link MediaType#APPLICATION_JSONL}.
+	 * @since 7.1
+	 */
+	public static final String APPLICATION_JSONL_VALUE = "application/jsonl";
+
 	/**
 	 * Media type for {@code application/xhtml+xml}.
 	 */
@@ -372,6 +385,7 @@ public class MediaType extends MimeType implements Serializable {
 		APPLICATION_GRAPHQL_RESPONSE = new MediaType("application", "graphql-response+json");
 		APPLICATION_JSON = new MediaType("application", "json");
 		APPLICATION_NDJSON = new MediaType("application", "x-ndjson");
+		APPLICATION_JSONL = new MediaType("application", "jsonl");
 		APPLICATION_OCTET_STREAM = new MediaType("application", "octet-stream");
 		APPLICATION_PDF = new MediaType("application", "pdf");
 		APPLICATION_PROBLEM_JSON = new MediaType("application", "problem+json");

spring-web/src/main/java/org/springframework/http/codec/json/GsonEncoder.java

@@ -58,7 +58,8 @@ public class GsonEncoder extends AbstractEncoder<Object> implements HttpMessageE
 	private static final MimeType[] DEFAULT_JSON_MIME_TYPES = new MimeType[] {
 			MediaType.APPLICATION_JSON,
 			new MediaType("application", "*+json"),
-			MediaType.APPLICATION_NDJSON
+			MediaType.APPLICATION_NDJSON,
+			MediaType.APPLICATION_JSONL
 	};
 
 	private final Gson gson;
@@ -68,11 +69,12 @@ public class GsonEncoder extends AbstractEncoder<Object> implements HttpMessageE
 	/**
 	 * Construct a new encoder using a default {@link Gson} instance
 	 * and the {@code "application/json"} and {@code "application/*+json"}
-	 * MIME types. The {@code "application/x-ndjson"} is configured for streaming.
+	 * MIME types. The {@code "application/jsonl"} and {@code "application/x-ndjson"}
+	 * are configured for streaming.
 	 */
 	public GsonEncoder() {
 		this(new Gson(), DEFAULT_JSON_MIME_TYPES);
-		setStreamingMediaTypes(List.of(MediaType.APPLICATION_NDJSON));
+		setStreamingMediaTypes(List.of(MediaType.APPLICATION_NDJSON, MediaType.APPLICATION_JSONL));
 	}
 
 	/**

spring-web/src/main/java/org/springframework/http/codec/json/Jackson2CodecSupport.java

@@ -81,7 +81,8 @@ public abstract class Jackson2CodecSupport {
 	private static final List<MimeType> defaultMimeTypes = List.of(
 			MediaType.APPLICATION_JSON,
 			new MediaType("application", "*+json"),
-			MediaType.APPLICATION_NDJSON);
+			MediaType.APPLICATION_NDJSON,
+			MediaType.APPLICATION_JSONL);
 
 
 	protected final Log logger = HttpLogging.forLogName(getClass());

spring-web/src/main/java/org/springframework/http/codec/json/Jackson2JsonEncoder.java

@@ -63,7 +63,7 @@ public class Jackson2JsonEncoder extends AbstractJackson2Encoder {
 
 	public Jackson2JsonEncoder(ObjectMapper mapper, MimeType... mimeTypes) {
 		super(mapper, mimeTypes);
-		setStreamingMediaTypes(Arrays.asList(MediaType.APPLICATION_NDJSON));
+		setStreamingMediaTypes(Arrays.asList(MediaType.APPLICATION_NDJSON, MediaType.APPLICATION_JSONL));
 		this.ssePrettyPrinter = initSsePrettyPrinter();
 	}
 

spring-web/src/main/java/org/springframework/http/codec/json/JacksonJsonDecoder.java

@@ -55,7 +55,8 @@ public class JacksonJsonDecoder extends AbstractJacksonDecoder<JsonMapper> {
 	private static final MimeType[] DEFAULT_JSON_MIME_TYPES = new MimeType[] {
 			MediaType.APPLICATION_JSON,
 			new MediaType("application", "*+json"),
-			MediaType.APPLICATION_NDJSON
+			MediaType.APPLICATION_NDJSON,
+			MediaType.APPLICATION_JSONL
 	};
 
 

spring-web/src/main/java/org/springframework/http/codec/json/JacksonJsonEncoder.java

@@ -55,7 +55,8 @@ public class JacksonJsonEncoder extends AbstractJacksonEncoder<JsonMapper> {
 	private static final MimeType[] DEFAULT_JSON_MIME_TYPES = new MimeType[] {
 			MediaType.APPLICATION_JSON,
 			new MediaType("application", "*+json"),
-			MediaType.APPLICATION_NDJSON
+			MediaType.APPLICATION_NDJSON,
+			MediaType.APPLICATION_JSONL
 	};
 
 
@@ -99,7 +100,7 @@ public class JacksonJsonEncoder extends AbstractJacksonEncoder<JsonMapper> {
 	 */
 	public JacksonJsonEncoder(JsonMapper.Builder builder, MimeType... mimeTypes) {
 		super(builder.addMixIn(ProblemDetail.class, ProblemDetailJacksonMixin.class), mimeTypes);
-		setStreamingMediaTypes(List.of(MediaType.APPLICATION_NDJSON));
+		setStreamingMediaTypes(List.of(MediaType.APPLICATION_NDJSON, MediaType.APPLICATION_JSONL));
 		this.ssePrettyPrinter = initSsePrettyPrinter();
 	}
 
@@ -110,7 +111,7 @@ public class JacksonJsonEncoder extends AbstractJacksonEncoder<JsonMapper> {
 	 */
 	public JacksonJsonEncoder(JsonMapper mapper, MimeType... mimeTypes) {
 		super(mapper, mimeTypes);
-		setStreamingMediaTypes(List.of(MediaType.APPLICATION_NDJSON));
+		setStreamingMediaTypes(List.of(MediaType.APPLICATION_NDJSON, MediaType.APPLICATION_JSONL));
 		this.ssePrettyPrinter = initSsePrettyPrinter();
 	}
 

spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonDecoder.java

@@ -56,7 +56,8 @@ public class KotlinSerializationJsonDecoder extends KotlinSerializationStringDec
 	private static final MimeType[] DEFAULT_JSON_MIME_TYPES = new MimeType[] {
 			MediaType.APPLICATION_JSON,
 			new MediaType("application", "*+json"),
-			MediaType.APPLICATION_NDJSON
+			MediaType.APPLICATION_NDJSON,
+			MediaType.APPLICATION_JSONL
 	};
 
 	/**

spring-web/src/main/java/org/springframework/http/codec/json/KotlinSerializationJsonEncoder.java

@@ -36,7 +36,7 @@ import org.springframework.util.MimeType;
 /**
  * Encode from an {@code Object} stream to a byte stream of JSON objects using
  * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
- * It supports {@code application/json}, {@code application/x-ndjson} and {@code application/*+json} with
+ * It supports {@code application/json}, {@code application/x-ndjson}, {@code application/jsonl} and {@code application/*+json} with
  * various character sets, {@code UTF-8} being the default.
  *
  * <p>As of Spring Framework 7.0, by default it only encodes types annotated with
@@ -59,7 +59,8 @@ public class KotlinSerializationJsonEncoder extends KotlinSerializationStringEnc
 	private static final MimeType[] DEFAULT_JSON_MIME_TYPES = new MimeType[] {
 			MediaType.APPLICATION_JSON,
 			new MediaType("application", "*+json"),
-			MediaType.APPLICATION_NDJSON
+			MediaType.APPLICATION_NDJSON,
+			MediaType.APPLICATION_JSONL
 	};
 
 	/**
@@ -87,7 +88,7 @@ public class KotlinSerializationJsonEncoder extends KotlinSerializationStringEnc
 	 */
 	public KotlinSerializationJsonEncoder(Json json) {
 		super(json, DEFAULT_JSON_MIME_TYPES);
-		setStreamingMediaTypes(List.of(MediaType.APPLICATION_NDJSON));
+		setStreamingMediaTypes(List.of(MediaType.APPLICATION_NDJSON, MediaType.APPLICATION_JSONL));
 	}
 
 	/**
@@ -97,7 +98,7 @@ public class KotlinSerializationJsonEncoder extends KotlinSerializationStringEnc
 	 */
 	public KotlinSerializationJsonEncoder(Json json, Predicate<ResolvableType> typePredicate) {
 		super(json, typePredicate, DEFAULT_JSON_MIME_TYPES);
-		setStreamingMediaTypes(List.of(MediaType.APPLICATION_NDJSON));
+		setStreamingMediaTypes(List.of(MediaType.APPLICATION_NDJSON, MediaType.APPLICATION_JSONL));
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/http/codec/protobuf/ProtobufJsonEncoder.java

@@ -80,7 +80,7 @@ public class ProtobufJsonEncoder implements HttpMessageEncoder<Message> {
 
 	@Override
 	public List<MediaType> getStreamingMediaTypes() {
-		return List.of(MediaType.APPLICATION_NDJSON);
+		return List.of(MediaType.APPLICATION_NDJSON, MediaType.APPLICATION_JSONL);
 	}
 
 	@Override

spring-webflux/src/test/java/org/springframework/web/reactive/result/view/HttpMessageWriterViewTests.java

@@ -55,7 +55,8 @@ class HttpMessageWriterViewTests {
 		assertThat(this.view.getSupportedMediaTypes()).containsExactly(
 				MediaType.APPLICATION_JSON,
 				MediaType.parseMediaType("application/*+json"),
-				MediaType.APPLICATION_NDJSON);
+				MediaType.APPLICATION_NDJSON,
+				MediaType.APPLICATION_JSONL);
 	}
 
 	@Test

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ReactiveTypeHandler.java

@@ -194,11 +194,11 @@ class ReactiveTypeHandler {
 	}
 
 	/**
-	 * Attempts to find a concrete {@code MediaType} that can be streamed (as json separated
-	 * by newlines in the response body). This method considers two concrete types
-	 * {@code APPLICATION_NDJSON} and {@code APPLICATION_STREAM_JSON}) as well as any
-	 * subtype of application that has the {@code +x-ndjson} suffix. In the later case,
-	 * the media type MUST be concrete for it to be considered.
+	 * Attempts to find a concrete {@code MediaType} that can be streamed (as JSON payloads
+	 * separated by newlines in the response body). This method considers {@code APPLICATION_JSONL},
+	 * {@code APPLICATION_NDJSON}, and any subtype of application
+	 * that has the {@code +x-ndjson} suffix. In the latter case, the media type MUST be
+	 * concrete for it to be considered.
 	 *
 	 * <p>For example {@code application/vnd.myapp+x-ndjson} is considered a streaming type
 	 * while {@code application/*+x-ndjson} isn't.
@@ -223,6 +223,9 @@ class ReactiveTypeHandler {
 			else if (MediaType.APPLICATION_NDJSON.includes(acceptedType)) {
 				return MediaType.APPLICATION_NDJSON;
 			}
+			else if (MediaType.APPLICATION_JSONL.includes(acceptedType)) {
+				return MediaType.APPLICATION_JSONL;
+			}
 		}
 		return null; // not a concrete streaming type
 	}

spring-webmvc/src/main/resources/org/springframework/web/servlet/config/spring-mvc.xsd

@@ -213,7 +213,7 @@
 	By default, a SimpleAsyncTaskExecutor is used which does not re-use threads and is not recommended for production.
 
 	As of 5.0 this executor is also used when a controller returns a reactive type that does streaming
-	(for example, "text/event-stream" or "application/x-ndjson") for the blocking writes to the
+	(for example, "text/event-stream", "application/x-ndjson" or "application/jsonl") for the blocking writes to the
 	"jakarta.servlet.ServletOutputStream".
 
 								]]></xsd:documentation>

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ReactiveTypeHandlerTests.java

@@ -35,6 +35,8 @@ import jakarta.servlet.http.HttpServletRequest;
 import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.core.publisher.Sinks;
@@ -287,10 +289,11 @@ class ReactiveTypeHandlerTests {
 		assertThat(emitterHandler.getValuesAsText()).isEqualTo("id:1\ndata:foo\n\nid:2\ndata:bar\n\nid:3\ndata:baz\n\n");
 	}
 
-	@Test
-	void writeStreamJson() throws Exception {
+	@ParameterizedTest
+	@ValueSource(strings = {"application/jsonl", "application/x-ndjson"})
+	void writeStreamJson(String mediaType) throws Exception {
 
-		this.servletRequest.addHeader("Accept", "application/x-ndjson");
+		this.servletRequest.addHeader("Accept", mediaType);
 
 		Sinks.Many<Bar> sink = Sinks.many().unicast().onBackpressureBuffer();
 		ResponseBodyEmitter emitter = handleValue(sink.asFlux(), Flux.class, forClass(Bar.class));
@@ -308,7 +311,7 @@ class ReactiveTypeHandlerTests {
 		sink.tryEmitNext(bar2);
 		sink.tryEmitComplete();
 
-		assertThat(message.getHeaders().getContentType()).hasToString("application/x-ndjson");
+		assertThat(message.getHeaders().getContentType()).hasToString(mediaType);
 		assertThat(emitterHandler.getValues()).isEqualTo(Arrays.asList(bar1, "\n", bar2, "\n"));
 	}