Introduce JSON streaming support

This commit introduces JSON streaming support which
consists of serializing HTTP request with
application/stream+json media type as line delimited JSON.

It also optimize Flux serialization for application/json by
using flux.collectList() and a single Jackson invocation
instead of one call per element previous strategy.
This change result in a x4 throughput improvement
for collection with a lot of small elements.

Issues: SPR-15095, SPR-15104

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

@@ -0,0 +1,148 @@
+/*
+ * Copyright 2002-2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.reactive.result.method.annotation;
+
+import java.time.Duration;
+
+import org.junit.Before;
+import org.junit.Test;
+import static org.springframework.http.MediaType.APPLICATION_STREAM_JSON;
+import static org.springframework.http.MediaType.APPLICATION_STREAM_JSON_VALUE;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
+import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.http.server.reactive.AbstractHttpHandlerIntegrationTests;
+import org.springframework.http.server.reactive.HttpHandler;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+import org.springframework.web.reactive.DispatcherHandler;
+import org.springframework.web.reactive.config.EnableWebFlux;
+import org.springframework.web.reactive.function.client.WebClient;
+import org.springframework.web.server.adapter.WebHttpHandlerBuilder;
+
+/**
+ * @author Sebastien Deleuze
+ */
+public class JsonStreamingIntegrationTests extends AbstractHttpHandlerIntegrationTests {
+
+	private AnnotationConfigApplicationContext wac;
+
+	private WebClient webClient;
+
+
+	@Override
+	@Before
+	public void setup() throws Exception {
+		super.setup();
+		this.webClient = WebClient.create("http://localhost:" + this.port);
+	}
+
+
+	@Override
+	protected HttpHandler createHttpHandler() {
+		this.wac = new AnnotationConfigApplicationContext();
+		this.wac.register(TestConfiguration.class);
+		this.wac.refresh();
+
+		return WebHttpHandlerBuilder.webHandler(new DispatcherHandler(this.wac)).build();
+	}
+
+	@Test
+	public void jsonStreaming() throws Exception {
+		Flux<Person> result = this.webClient.get()
+				.uri("/stream")
+				.accept(APPLICATION_STREAM_JSON)
+				.exchange()
+				.flatMap(response -> response.bodyToFlux(Person.class));
+
+		StepVerifier.create(result)
+				.expectNext(new Person("foo 0"))
+				.expectNext(new Person("foo 1"))
+				.verifyComplete();
+	}
+
+	@RestController
+	@SuppressWarnings("unused")
+	static class JsonStreamingController {
+
+		@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);
+		}
+
+	}
+
+	@Configuration
+	@EnableWebFlux
+	@SuppressWarnings("unused")
+	static class TestConfiguration {
+
+		@Bean
+		public JsonStreamingController jsonStreamingController() {
+			return new JsonStreamingController();
+		}
+	}
+
+	private static class Person {
+
+		private String name;
+
+		@SuppressWarnings("unused")
+		public Person() {
+		}
+
+		public Person(String name) {
+			this.name = name;
+		}
+
+		public String getName() {
+			return name;
+		}
+
+		public void setName(String name) {
+			this.name = name;
+		}
+
+		@Override
+		public boolean equals(Object o) {
+			if (this == o) {
+				return true;
+			}
+			if (o == null || getClass() != o.getClass()) {
+				return false;
+			}
+			Person person = (Person) o;
+			return !(this.name != null ? !this.name.equals(person.name) : person.name != null);
+		}
+
+		@Override
+		public int hashCode() {
+			return this.name != null ? this.name.hashCode() : 0;
+		}
+
+		@Override
+		public String toString() {
+			return "Person{" +
+					"name='" + name + '\'' +
+					'}';
+		}
+	}
+
+}