Flush JSON stream after each element

Issue: SPR-15104

spring-core/src/main/java/org/springframework/core/codec/AbstractEncoder.java

@@ -31,6 +31,13 @@ import org.springframework.util.MimeType;
  */
 public abstract class AbstractEncoder<T> implements Encoder<T> {
 
+	/**
+	 * Hint key to use with a {@link FlushingStrategy} value.
+	 */
+	public static final String FLUSHING_STRATEGY_HINT = AbstractEncoder.class.getName() + ".flushingStrategy";
+
+	public enum FlushingStrategy { AUTO, AFTER_EACH_ELEMENT }
+
 	private final List<MimeType> encodableMimeTypes;
 
 

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

@@ -21,6 +21,8 @@ import java.util.List;
 import java.util.Map;
 
 import org.reactivestreams.Publisher;
+import static org.springframework.core.codec.AbstractEncoder.FLUSHING_STRATEGY_HINT;
+import static org.springframework.core.codec.AbstractEncoder.FlushingStrategy.AFTER_EACH_ELEMENT;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
@@ -102,7 +104,8 @@ public class EncoderHttpMessageWriter<T> implements HttpMessageWriter<T> {
 
 		DataBufferFactory bufferFactory = outputMessage.bufferFactory();
 		Flux<DataBuffer> body = this.encoder.encode(inputStream, bufferFactory, elementType, mediaType, hints);
-		return outputMessage.writeWith(body);
+		return (hints.get(FLUSHING_STRATEGY_HINT) == AFTER_EACH_ELEMENT ?
+				outputMessage.writeAndFlushWith(body.map(Flux::just)) : outputMessage.writeWith(body));
 	}
 
 	/**

spring-web/src/main/java/org/springframework/http/codec/Jackson2ServerHttpMessageWriter.java

@@ -21,19 +21,24 @@ import java.util.HashMap;
 import java.util.Map;
 
 import com.fasterxml.jackson.annotation.JsonView;
+import org.reactivestreams.Publisher;
+import static org.springframework.core.codec.AbstractEncoder.FLUSHING_STRATEGY_HINT;
+import static org.springframework.core.codec.AbstractEncoder.FlushingStrategy.AFTER_EACH_ELEMENT;
+import reactor.core.publisher.Mono;
 
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ResolvableType;
+import org.springframework.core.codec.AbstractEncoder;
 import org.springframework.core.codec.Encoder;
 import org.springframework.http.MediaType;
+import org.springframework.http.ReactiveHttpOutputMessage;
 import org.springframework.http.codec.json.AbstractJackson2Codec;
 import org.springframework.http.server.reactive.ServerHttpRequest;
+import org.springframework.http.server.reactive.ServerHttpResponse;
 
 /**
- * {@link ServerHttpMessageWriter} that resolves those annotation or request based Jackson 2 hints:
- * <ul>
- *   <li>{@code @JsonView} annotated handler method</li>
- * </ul>
+ * Jackson {@link ServerHttpMessageWriter} that resolves {@code @JsonView} annotated handler
+ * method and deals with {@link AbstractEncoder#FLUSHING_STRATEGY_HINT}.
  *
  * @author Sebastien Deleuze
  * @since 5.0
@@ -72,4 +77,27 @@ public class Jackson2ServerHttpMessageWriter extends AbstractServerHttpMessageWr
 		return hints;
 	}
 
+	@Override
+	public Mono<Void> write(Publisher<?> inputStream, ResolvableType elementType, MediaType mediaType,
+			ReactiveHttpOutputMessage outputMessage, Map<String, Object> hints) {
+
+		if ((mediaType != null) && mediaType.isCompatibleWith(MediaType.APPLICATION_STREAM_JSON)) {
+			Map<String, Object> hintsWithFlush =  new HashMap<>(hints);
+			hintsWithFlush.put(FLUSHING_STRATEGY_HINT, AFTER_EACH_ELEMENT);
+			return super.write(inputStream, elementType, mediaType, outputMessage, hintsWithFlush);
+		}
+		return super.write(inputStream, elementType, mediaType, outputMessage, hints);
+	}
+
+	@Override
+	public Mono<Void> write(Publisher<?> inputStream, ResolvableType streamType, ResolvableType elementType,
+			MediaType mediaType, ServerHttpRequest request, ServerHttpResponse response, Map<String, Object> hints) {
+
+		if ((mediaType != null) && mediaType.isCompatibleWith(MediaType.APPLICATION_STREAM_JSON)) {
+			Map<String, Object> hintsWithFlush =  new HashMap<>(hints);
+			hintsWithFlush.put(FLUSHING_STRATEGY_HINT, AFTER_EACH_ELEMENT);
+			return super.write(inputStream, streamType, elementType, mediaType, request, response, hintsWithFlush);
+		}
+		return super.write(inputStream, streamType, elementType, mediaType, request, response, hints);
+	}
 }

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/DefaultHandlerStrategiesBuilder.java

@@ -37,6 +37,7 @@ import org.springframework.http.codec.EncoderHttpMessageWriter;
 import org.springframework.http.codec.FormHttpMessageReader;
 import org.springframework.http.codec.HttpMessageReader;
 import org.springframework.http.codec.HttpMessageWriter;
+import org.springframework.http.codec.Jackson2ServerHttpMessageWriter;
 import org.springframework.http.codec.ResourceHttpMessageWriter;
 import org.springframework.http.codec.ServerSentEventHttpMessageWriter;
 import org.springframework.http.codec.json.Jackson2JsonDecoder;
@@ -97,7 +98,7 @@ class DefaultHandlerStrategiesBuilder implements HandlerStrategies.Builder {
 		if (jackson2Present) {
 			messageReader(new DecoderHttpMessageReader<>(new Jackson2JsonDecoder()));
 			Jackson2JsonEncoder jsonEncoder = new Jackson2JsonEncoder();
-			messageWriter(new EncoderHttpMessageWriter<>(jsonEncoder));
+			messageWriter(new Jackson2ServerHttpMessageWriter(jsonEncoder));
 			messageWriter(
 					new ServerSentEventHttpMessageWriter(Collections.singletonList(jsonEncoder)));
 		}

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/JsonStreamingIntegrationTests.java

@@ -19,7 +19,6 @@ package org.springframework.web.reactive.result.method.annotation;
 import java.time.Duration;
 
 import org.junit.Before;
-import org.junit.Ignore;
 import org.junit.Test;
 import reactor.core.publisher.Flux;
 import reactor.test.StepVerifier;
@@ -42,7 +41,6 @@ import static org.springframework.http.MediaType.APPLICATION_STREAM_JSON_VALUE;
 /**
  * @author Sebastien Deleuze
  */
-@Ignore
 public class JsonStreamingIntegrationTests extends AbstractHttpHandlerIntegrationTests {
 
 	private AnnotationConfigApplicationContext wac;
@@ -78,7 +76,8 @@ public class JsonStreamingIntegrationTests extends AbstractHttpHandlerIntegratio
 		StepVerifier.create(result)
 				.expectNext(new Person("foo 0"))
 				.expectNext(new Person("foo 1"))
-				.verifyComplete();
+				.thenCancel()
+				.verify();
 	}
 
 	@RestController
@@ -87,7 +86,7 @@ public class JsonStreamingIntegrationTests extends AbstractHttpHandlerIntegratio
 
 		@RequestMapping(value = "/stream", produces = APPLICATION_STREAM_JSON_VALUE)
 		Flux<Person> person() {
-			return Flux.interval(Duration.ofMillis(100)).map(l -> new Person("foo " + l)).take(2);
+			return Flux.interval(Duration.ofMillis(100)).map(l -> new Person("foo " + l));
 		}
 
 	}