Spring Boot 3 - server-sent events example using Spring MVC
In this article, we would like to show how to use server-sent events with Spring MVC under Spring Boot 3.
In the example project we used HTTP/2 protocol for better performance.
Warning from MDN:
When not used over HTTP/2, SSE suffers from a limitation to the maximum number of open connections, which can be specially painful when opening various tabs as the limit is per browser and set to a very low number (6). The issue has been marked as "Won't fix" in Chrome and Firefox. This limit is per browser + domain, so that means that you can open 6 SSE connections across all of the tabs to
www.example1.comand another 6 SSE connections towww.example2.com.(from Stackoverflow). When using HTTP/2, the maximum number of simultaneous HTTP streams is negotiated between the server and the client (defaults to 100).Source: https://developer.mozilla.org/en-US/docs/Web/API/EventSource
Project was generated using this configuration (by using GENERATE button it is possible to download the project).
To generate src/main/resources/cert.pem and src/main/resources/key.pem files use the following command:
xxxxxxxxxxopenssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365 -nodes -subj '/CN=localhost'Hint: to know how to generate certificate files under Windows read this article.
src/main/resources/application.properties file:
xxxxxxxxxxserver.port=8443server.ssl.enabled=trueserver.ssl.certificate=classpath:cert.pemserver.ssl.certificate-private-key=classpath:key.pemserver.http2.enabled=true
src/main/resources/static/index.html file:
xxxxxxxxxx<html><body> <script> const source = new EventSource('/events'); // OR: https://localhost:8443/events source.onopen = (e) => console.log('Event source stream open!'); source.onerror = (e) => console.error('Event source stream error!'); // Handling specific events: source.addEventListener('my-message', (e) => console.log('Message: ' + e.data)); // Add here more source.addEventListener(...) calls for different events ... </script></body></html>Note: event source can be closed by
source.close()method call.
src/main/java/com/example/demo/DemoApplication.java file:
xxxxxxxxxxpackage com.example.demo;import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;public class DemoApplication { public static void main(String[] args) { SpringApplication.run(DemoApplication.class, args); }}
src/main/java/com/example/demo/MainController.java file:
xxxxxxxxxxpackage com.example.demo;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import org.springframework.stereotype.Controller;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.RequestMethod;import org.springframework.web.servlet.mvc.method.annotation.SseEmitter;import org.springframework.web.servlet.mvc.method.annotation.SseEmitter.SseEventBuilder;public class MainController { private ExecutorService executor = Executors.newCachedThreadPool(); ( value = "/", method = RequestMethod.GET ) public String index() { return "index.html"; } ( value = "/events", method = RequestMethod.GET ) public SseEmitter events() { SseEmitter emitter = new SseEmitter(); // 1 second timeout emitter.onCompletion(() -> System.out.println("Event source stream closed!")); emitter.onTimeout(() -> System.out.println("Event source stream timeout!")); emitter.onError((Throwable ex) -> System.out.println("Event source stream error!")); this.executor.execute(() -> { int counter = 0; try { while (true) { String id = Long.toString(++counter); SseEventBuilder event = SseEmitter.event() .id(id) .name("my-message") .data("Some text message here ..."); emitter.send(event); Thread.sleep(1000); } } catch (Exception ex) { emitter.completeWithError(ex); } }); return emitter; }}
pom.xml file:
xxxxxxxxxx <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 https://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <parent> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-parent</artifactId> <version>3.2.0</version> <relativePath/> </parent> <groupId>com.example</groupId> <artifactId>server-sent-events</artifactId> <version>0.0.1-SNAPSHOT</version> <name>server-sent-events</name> <description>Server-sent events example using Spring Boot project.</description> <properties> <java.version>17</java.version> </properties> <dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-test</artifactId> <scope>test</scope> </dependency> <dependency> <groupId>org.projectlombok</groupId> <artifactId>lombok</artifactId> <optional>true</optional> </dependency> </dependencies> <build> <plugins> <plugin> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-maven-plugin</artifactId> <configuration> <excludes> <exclude> <groupId>org.projectlombok</groupId> <artifactId>lombok</artifactId> </exclude> </excludes> </configuration> </plugin> </plugins> </build></project>
To download dependencies and compile project use the following command:
xxxxxxxxxxmvn clean compile
To run server use the following command:
xxxxxxxxxxmvn exec:java -Dexec.mainClass=com.example.demo.DemoApplicationThen, to see the final effect, open the link in the web browser: https://localhost:8443
Example preview:
