Polish ReactiveTypeHandler when checking wildcard ndjson subtype

This commit polishes the previous change, ensuring that if the request
uses a wildcard subtype with an ndjson suffix then the wildcard isn't
present in the response's Content-Type.

Insteand, in that case we would fall back to `applicatin/x-ndjson` (the
base ndjson media type), assuming that the requester is only interested
in the ndjson nature of the underlying representation and not in the
specific semantics of any subtype.

See commit 9332b3f
See gh-26817

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

@@ -104,6 +104,54 @@ public class ReactiveTypeHandlerTests {
 		assertThat(this.handler.isReactiveType(String.class)).isFalse();
 	}
 
+	@Test
+	void findsConcreteStreamingMediaType() {
+		final List<MediaType> accept = List.of(
+				MediaType.ALL,
+				MediaType.parseMediaType("application/*+x-ndjson"),
+				MediaType.parseMediaType("application/vnd.myapp.v1+x-ndjson"));
+
+		assertThat(ReactiveTypeHandler.findConcreteStreamingMediaType(accept))
+				.isEqualTo(MediaType.APPLICATION_NDJSON);
+	}
+
+	@Test
+	void findsConcreteStreamingMediaType_vendorFirst() {
+		final List<MediaType> accept = List.of(
+				MediaType.ALL,
+				MediaType.parseMediaType("application/vnd.myapp.v1+x-ndjson"),
+				MediaType.parseMediaType("application/*+x-ndjson"),
+				MediaType.APPLICATION_NDJSON);
+
+		assertThat(ReactiveTypeHandler.findConcreteStreamingMediaType(accept))
+				.hasToString("application/vnd.myapp.v1+x-ndjson");
+	}
+
+	@Test
+	void findsConcreteStreamingMediaType_plainNdJsonFirst() {
+		final List<MediaType> accept = List.of(
+				MediaType.ALL,
+				MediaType.APPLICATION_NDJSON,
+				MediaType.parseMediaType("application/*+x-ndjson"),
+				MediaType.parseMediaType("application/vnd.myapp.v1+x-ndjson"));
+
+		assertThat(ReactiveTypeHandler.findConcreteStreamingMediaType(accept))
+				.isEqualTo(MediaType.APPLICATION_NDJSON);
+	}
+
+	@SuppressWarnings("deprecation")
+	@Test
+	void findsConcreteStreamingMediaType_plainStreamingJsonFirst() {
+		final List<MediaType> accept = List.of(
+				MediaType.ALL,
+				MediaType.APPLICATION_STREAM_JSON,
+				MediaType.parseMediaType("application/*+x-ndjson"),
+				MediaType.parseMediaType("application/vnd.myapp.v1+x-ndjson"));
+
+		assertThat(ReactiveTypeHandler.findConcreteStreamingMediaType(accept))
+				.isEqualTo(MediaType.APPLICATION_STREAM_JSON);
+	}
+
 	@Test
 	public void deferredResultSubscriberWithOneValue() throws Exception {
 
@@ -251,7 +299,7 @@ public class ReactiveTypeHandlerTests {
 		sink.tryEmitNext(bar2);
 		sink.tryEmitComplete();
 
-		assertThat(message.getHeaders().getContentType().toString()).isEqualTo("application/x-ndjson");
+		assertThat(message.getHeaders().getContentType()).hasToString("application/x-ndjson");
 		assertThat(emitterHandler.getValues()).isEqualTo(Arrays.asList(bar1, "\n", bar2, "\n"));
 	}
 
@@ -277,7 +325,33 @@ public class ReactiveTypeHandlerTests {
 		sink.tryEmitNext(bar2);
 		sink.tryEmitComplete();
 
-		assertThat(message.getHeaders().getContentType().toString()).isEqualTo("application/vnd.myapp.v1+x-ndjson");
+		assertThat(message.getHeaders().getContentType()).hasToString("application/vnd.myapp.v1+x-ndjson");
+		assertThat(emitterHandler.getValues()).isEqualTo(Arrays.asList(bar1, "\n", bar2, "\n"));
+	}
+
+	@Test
+	public void writeStreamJsonWithWildcardSubtype() throws Exception {
+		this.servletRequest.addHeader("Accept", "application/*+x-ndjson");
+
+		Sinks.Many<Bar> sink = Sinks.many().unicast().onBackpressureBuffer();
+		ResponseBodyEmitter emitter = handleValue(sink.asFlux(), Flux.class, forClass(Bar.class));
+
+		assertThat(emitter).as("emitter").isNotNull();
+
+		EmitterHandler emitterHandler = new EmitterHandler();
+		emitter.initialize(emitterHandler);
+
+		ServletServerHttpResponse message = new ServletServerHttpResponse(this.servletResponse);
+		emitter.extendResponse(message);
+
+		Bar bar1 = new Bar("foo");
+		Bar bar2 = new Bar("bar");
+
+		sink.tryEmitNext(bar1);
+		sink.tryEmitNext(bar2);
+		sink.tryEmitComplete();
+
+		assertThat(message.getHeaders().getContentType()).hasToString("application/x-ndjson");
 		assertThat(emitterHandler.getValues()).isEqualTo(Arrays.asList(bar1, "\n", bar2, "\n"));
 	}