Spring Boot WebFlux + Server-sent events example
By：Roy.LiuLast updated：2019-08-11
In this article, we will show you how to develop a reactive web application, using Server-sent events
- Spring Boot 2.1.2.RELEASE
- Spring WebFlux 5.1.4.RELEASE
- Thymeleaf 3.0.11.RELEASE
- JUnit 5.3.2
- Maven 3
In Spring, returns JSON and header MediaType.TEXT_EVENT_STREAM_VALUE
@RestController
public class CommentController {
@GetMapping(path = "/comment/stream",
produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<Comment> feed() {
//...
In Javascript, uses EventSource to send a request to the above endpoint.
function loadComments () {
this.source = null;
this.start = function () {
this.source = new EventSource("/comment/stream");
this.source.addEventListener("message", function (event) {
var comment = JSON.parse(event.data);
//... update somewhere
});
this.source.onerror = function () {
this.close();
};
};
this.stop = function() {
this.source.close();
comment = new loadComments();
window.onload = function() {
comment.start();
};
window.onbeforeunload = function() {
comment.stop();
1. Project Directory
2. Maven
pom.xml
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.mkyong.spring.reactive</groupId>
<artifactId>webflux-thymeleaf-sse</artifactId>
<version>1.0</version>
<properties>
<java.version>1.8</java.version>
<junit-jupiter.version>5.3.2</junit-jupiter.version>
</properties>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.1.2.RELEASE</version>
</parent>
<dependencies>
<!-- webflux reactive -->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-webflux</artifactId>
</dependency>
<!-- thymeleaf -->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-thymeleaf</artifactId>
</dependency>
<!-- exclude junit 4, prefer junit 5 -->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
<exclusions>
<exclusion>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
</exclusion>
</exclusions>
</dependency>
<!-- junit 5 -->
<dependency>
<groupId>org.junit.jupiter</groupId>
<artifactId>junit-jupiter-engine</artifactId>
<version>${junit-jupiter.version}</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-devtools</artifactId>
<optional>true</optional>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-surefire-plugin</artifactId>
<version>2.22.0</version>
</plugin>
</plugins>
</build>
</project>
Display the project dependencies.
$ mvn dependency:tree [INFO] com.mkyong.spring.reactive:webflux-thymeleaf-sse:jar:1.0 [INFO] +- org.springframework.boot:spring-boot-starter-webflux:jar:2.1.2.RELEASE:compile [INFO] | +- org.springframework.boot:spring-boot-starter:jar:2.1.2.RELEASE:compile [INFO] | | +- org.springframework.boot:spring-boot-starter-logging:jar:2.1.2.RELEASE:compile [INFO] | | | +- ch.qos.logback:logback-classic:jar:1.2.3:compile [INFO] | | | | \- ch.qos.logback:logback-core:jar:1.2.3:compile [INFO] | | | +- org.apache.logging.log4j:log4j-to-slf4j:jar:2.11.1:compile [INFO] | | | | \- org.apache.logging.log4j:log4j-api:jar:2.11.1:compile [INFO] | | | \- org.slf4j:jul-to-slf4j:jar:1.7.25:compile [INFO] | | +- javax.annotation:javax.annotation-api:jar:1.3.2:compile [INFO] | | \- org.yaml:snakeyaml:jar:1.23:runtime [INFO] | +- org.springframework.boot:spring-boot-starter-json:jar:2.1.2.RELEASE:compile [INFO] | | +- com.fasterxml.jackson.core:jackson-databind:jar:2.9.8:compile [INFO] | | | +- com.fasterxml.jackson.core:jackson-annotations:jar:2.9.0:compile [INFO] | | | \- com.fasterxml.jackson.core:jackson-core:jar:2.9.8:compile [INFO] | | +- com.fasterxml.jackson.datatype:jackson-datatype-jdk8:jar:2.9.8:compile [INFO] | | +- com.fasterxml.jackson.datatype:jackson-datatype-jsr310:jar:2.9.8:compile [INFO] | | \- com.fasterxml.jackson.module:jackson-module-parameter-names:jar:2.9.8:compile [INFO] | +- org.springframework.boot:spring-boot-starter-reactor-netty:jar:2.1.2.RELEASE:compile [INFO] | | \- io.projectreactor.netty:reactor-netty:jar:0.8.4.RELEASE:compile [INFO] | | +- io.netty:netty-codec-http:jar:4.1.31.Final:compile [INFO] | | | \- io.netty:netty-codec:jar:4.1.31.Final:compile [INFO] | | +- io.netty:netty-codec-http2:jar:4.1.31.Final:compile [INFO] | | +- io.netty:netty-handler:jar:4.1.31.Final:compile [INFO] | | | +- io.netty:netty-buffer:jar:4.1.31.Final:compile [INFO] | | | \- io.netty:netty-transport:jar:4.1.31.Final:compile [INFO] | | | \- io.netty:netty-resolver:jar:4.1.31.Final:compile [INFO] | | +- io.netty:netty-handler-proxy:jar:4.1.31.Final:compile [INFO] | | | \- io.netty:netty-codec-socks:jar:4.1.31.Final:compile [INFO] | | \- io.netty:netty-transport-native-epoll:jar:linux-x86_64:4.1.31.Final:compile [INFO] | | +- io.netty:netty-common:jar:4.1.31.Final:compile [INFO] | | \- io.netty:netty-transport-native-unix-common:jar:4.1.31.Final:compile [INFO] | +- org.hibernate.validator:hibernate-validator:jar:6.0.14.Final:compile [INFO] | | +- javax.validation:validation-api:jar:2.0.1.Final:compile [INFO] | | +- org.jboss.logging:jboss-logging:jar:3.3.2.Final:compile [INFO] | | \- com.fasterxml:classmate:jar:1.4.0:compile [INFO] | +- org.springframework:spring-web:jar:5.1.4.RELEASE:compile [INFO] | | \- org.springframework:spring-beans:jar:5.1.4.RELEASE:compile [INFO] | +- org.springframework:spring-webflux:jar:5.1.4.RELEASE:compile [INFO] | | \- io.projectreactor:reactor-core:jar:3.2.5.RELEASE:compile [INFO] | | \- org.reactivestreams:reactive-streams:jar:1.0.2:compile [INFO] | \- org.synchronoss.cloud:nio-multipart-parser:jar:1.1.0:compile [INFO] | +- org.slf4j:slf4j-api:jar:1.7.25:compile [INFO] | \- org.synchronoss.cloud:nio-stream-storage:jar:1.1.3:compile [INFO] +- org.springframework.boot:spring-boot-starter-thymeleaf:jar:2.1.2.RELEASE:compile [INFO] | +- org.thymeleaf:thymeleaf-spring5:jar:3.0.11.RELEASE:compile [INFO] | | \- org.thymeleaf:thymeleaf:jar:3.0.11.RELEASE:compile [INFO] | | +- org.attoparser:attoparser:jar:2.0.5.RELEASE:compile [INFO] | | \- org.unbescape:unbescape:jar:1.1.6.RELEASE:compile [INFO] | \- org.thymeleaf.extras:thymeleaf-extras-java8time:jar:3.0.2.RELEASE:compile [INFO] +- org.springframework.boot:spring-boot-starter-test:jar:2.1.2.RELEASE:test [INFO] | +- org.springframework.boot:spring-boot-test:jar:2.1.2.RELEASE:test [INFO] | +- org.springframework.boot:spring-boot-test-autoconfigure:jar:2.1.2.RELEASE:test [INFO] | +- com.jayway.jsonpath:json-path:jar:2.4.0:test [INFO] | | \- net.minidev:json-smart:jar:2.3:test [INFO] | | \- net.minidev:accessors-smart:jar:1.2:test [INFO] | | \- org.ow2.asm:asm:jar:5.0.4:test [INFO] | +- org.assertj:assertj-core:jar:3.11.1:test [INFO] | +- org.mockito:mockito-core:jar:2.23.4:test [INFO] | | +- net.bytebuddy:byte-buddy:jar:1.9.7:test [INFO] | | +- net.bytebuddy:byte-buddy-agent:jar:1.9.7:test [INFO] | | \- org.objenesis:objenesis:jar:2.6:test [INFO] | +- org.hamcrest:hamcrest-core:jar:1.3:test [INFO] | +- org.hamcrest:hamcrest-library:jar:1.3:test [INFO] | +- org.skyscreamer:jsonassert:jar:1.5.0:test [INFO] | | \- com.vaadin.external.google:android-json:jar:0.0.20131108.vaadin1:test [INFO] | +- org.springframework:spring-core:jar:5.1.4.RELEASE:compile [INFO] | | \- org.springframework:spring-jcl:jar:5.1.4.RELEASE:compile [INFO] | +- org.springframework:spring-test:jar:5.1.4.RELEASE:test [INFO] | \- org.xmlunit:xmlunit-core:jar:2.6.2:test [INFO] | \- javax.xml.bind:jaxb-api:jar:2.3.1:test [INFO] | \- javax.activation:javax.activation-api:jar:1.2.0:test [INFO] +- org.junit.jupiter:junit-jupiter-engine:jar:5.3.2:test [INFO] | +- org.apiguardian:apiguardian-api:jar:1.0.0:test [INFO] | +- org.junit.platform:junit-platform-engine:jar:1.3.2:test [INFO] | | +- org.junit.platform:junit-platform-commons:jar:1.3.2:test [INFO] | | \- org.opentest4j:opentest4j:jar:1.1.1:test [INFO] | \- org.junit.jupiter:junit-jupiter-api:jar:5.3.2:test [INFO] \- org.springframework.boot:spring-boot-devtools:jar:2.1.2.RELEASE:compile (optional) [INFO] +- org.springframework.boot:spring-boot:jar:2.1.2.RELEASE:compile [INFO] | \- org.springframework:spring-context:jar:5.1.4.RELEASE:compile [INFO] | +- org.springframework:spring-aop:jar:5.1.4.RELEASE:compile [INFO] | \- org.springframework:spring-expression:jar:5.1.4.RELEASE:compile [INFO] \- org.springframework.boot:spring-boot-autoconfigure:jar:2.1.2.RELEASE:compile
3. Spring Boot + Spring WebFlux
3.1 Spring WebFlux annotation based controller. To enable data streaming. write produces = MediaType.TEXT_EVENT_STREAM_VALUE
CommentController.java
package com.mkyong.reactive.controller;
import com.mkyong.reactive.model.Comment;
import com.mkyong.reactive.repository.CommentRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import reactor.core.publisher.Flux;
@RestController
public class CommentController {
@Autowired
private CommentRepository commentRepository;
@GetMapping(path = "/comment/stream",
produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<Comment> feed() {
return this.commentRepository.findAll();
MainController.java
package com.mkyong.reactive.controller;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
@Controller
public class MainController {
@GetMapping("/")
public String index(final Model model) {
return "index";
3.2 In repository, return a Flux object.
CommentRepository.java
package com.mkyong.reactive.repository;
import com.mkyong.reactive.model.Comment;
import reactor.core.publisher.Flux;
public interface CommentRepository {
Flux<Comment> findAll();
ReactiveCommentRepository.java
package com.mkyong.reactive.repository;
import com.mkyong.reactive.model.Comment;
import com.mkyong.reactive.utils.CommentGenerator;
import org.springframework.stereotype.Repository;
import reactor.core.publisher.Flux;
import java.time.Duration;
import java.util.Arrays;
import java.util.List;
@Repository
public class ReactiveCommentRepository implements CommentRepository {
@Override
public Flux<Comment> findAll() {
//simulate data streaming every 1 second.
return Flux.interval(Duration.ofSeconds(1))
.onBackpressureDrop()
.map(this::generateComment)
.flatMapIterable(x -> x);
private List<Comment> generateComment(long interval) {
Comment obj = new Comment(
CommentGenerator.randomAuthor(),
CommentGenerator.randomMessage(),
CommentGenerator.getCurrentTimeStamp());
return Arrays.asList(obj);
3.3 A utils class to generate random comments.
CommentGenerator.java
package com.mkyong.reactive.utils;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Arrays;
import java.util.List;
import java.util.Random;
public class CommentGenerator {
private static final DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");
private static final Random RANDOM = new Random(System.currentTimeMillis());
private static final List<String> COMMENT_AUTHOR =
Arrays.asList(
"Mkyong", "Oliver", "Jack", "Harry", "Jacob",
"Isla", "Emily", "Poppy", "Ava", "Isabella");
private static final List<String> COMMENT_MESSAGE =
Arrays.asList(
"I Love this!",
"Me too!",
"Wow",
"True!",
"Hello everyone here?",
"Good!");
public static String randomAuthor() {
return COMMENT_AUTHOR.get(RANDOM.nextInt(COMMENT_AUTHOR.size()));
public static String randomMessage() {
return COMMENT_MESSAGE.get(RANDOM.nextInt(COMMENT_MESSAGE.size()));
public static String getCurrentTimeStamp() {
return dtf.format(LocalDateTime.now());
3.4 Comment model.
Movie.java
package com.mkyong.reactive.model;
public class Comment {
private String author;
private String message;
private String timestamp;
//getter, setter and constructor
3.5 Start Spring Boot.
CommentWebApplication.java
package com.mkyong.reactive;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class CommentWebApplication {
public static void main(String[] args) {
SpringApplication.run(CommentWebApplication.class, args);
4. Thymeleaf
There is no special reactive tag in thymeleaf template, just uses the normal loop.
resources/templates/index.html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link data-th-href="@{/css/bootstrap.min.css}" rel="stylesheet">
<link data-th-href="@{/css/main.css}" rel="stylesheet">
</head>
<body>
<div class="container">
<div class="row">
<div id="title">
<h1>Spring WebFlux + Server Sent Events</h1>
</div>
<table id="comments" class="table table-striped">
<thead>
<tr>
<th width="10%">Author</th>
<th width="60%">Message</th>
<th width="30%">Date</th>
</tr>
</thead>
<tbody>
<tr class="result" data-th-each="comment : ${comments}">
<td>[[${comment.author}]]</td>
<td>[[${comment.message}]]</td>
<td>[[${comment.timestamp}]]</td>
</tr>
</tbody>
</table>
</div>
</div>
<script data-th-src="@{/js/main.js}"></script>
</body>
</html>
5. JavaScript EventSource.
The key is to use the Javascript EventSource class to send a request and listen to the message event, and update the streaming data into a table reactively.
resources/static/js/main.js
function loadComments () {
this.source = null;
this.start = function () {
var commentTable = document.getElementById("comments");
this.source = new EventSource("/comment/stream");
this.source.addEventListener("message", function (event) {
// These events are JSON, so parsing and DOM fiddling are needed
var comment = JSON.parse(event.data);
var row = commentTable.getElementsByTagName("tbody")[0].insertRow(0);
var cell0 = row.insertCell(0);
var cell1 = row.insertCell(1);
var cell2 = row.insertCell(2);
cell0.className = "author-style";
cell0.innerHTML = comment.author;
cell1.className = "text";
cell1.innerHTML = comment.message;
cell2.className = "date";
cell2.innerHTML = comment.timestamp;
});
this.source.onerror = function () {
this.close();
};
};
this.stop = function() {
this.source.close();
comment = new loadComments();
/*
* Register callbacks for starting and stopping the SSE controller.
*/
window.onload = function() {
comment.start();
};
window.onbeforeunload = function() {
comment.stop();
6. Unit Test
WebTestClient to unit test the Streaming Responses
TestCommentWebApplication.java
package com.mkyong.reactive;
import com.mkyong.reactive.model.Comment;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.http.MediaType;
import org.springframework.test.web.reactive.server.WebTestClient;
import java.util.List;
import static org.junit.jupiter.api.Assertions.assertEquals;
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
public class TestCommentWebApplication {
@Autowired
private WebTestClient webClient;
@Test
public void testCommentStream() {
List<Comment> comments = webClient
.get().uri("/comment/stream")
.accept(MediaType.valueOf(MediaType.TEXT_EVENT_STREAM_VALUE))
.exchange()
.expectStatus().isOk()
.returnResult(Comment.class)
.getResponseBody()
.take(3) // take 3 comment objects
.collectList()
.block();
comments.forEach(x -> System.out.println(x));
assertEquals(3, comments.size());
7.Demo
$ mvn spring-boot:run 2019-02-11 15:41:17.657 INFO 257192 --- [ restartedMain] o.s.b.web.embedded.netty.NettyWebServer : Netty started on port(s): 8080
URL = http://localhost:8080
The data are streaming, and random comment will be displayed every 1 second.
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