SSE (Server-Sent Events) is a W3C standard. It is just another variation of the "HTTP Streaming" technique which we saw in the last tutorial, except that, with SSE, events pushed from the server are formatted according to the W3C Server-Sent Events specification.
Spring class SseEmitter provides support of SSE. It is a subclass of ResponseBodyEmitter. Let's see a quick example how to use this class:
Example
The controller
package com.logicbig.example;
import org.springframework.http.MediaType;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.servlet.mvc.method.annotation.SseEmitter;
import java.time.LocalTime;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
@Controller
public class MyWebController {
@GetMapping("/test")
public SseEmitter handleRequest() {
final SseEmitter emitter = new SseEmitter();
ExecutorService service = Executors.newSingleThreadExecutor();
service.execute(() -> {
for (int i = 0; i < 10; i++) {
try {
emitter.send(String.valueOf(i),
MediaType.TEXT_PLAIN);
Thread.sleep(100);
} catch (Exception e) {
e.printStackTrace();
emitter.completeWithError(e);
return;
}
}
emitter.complete();
});
return emitter;
}
}
Running the example
To try examples, run embedded Jetty (configured in pom.xml of example project below):
mvn jetty:run Using curl
$ curl -s "http://localhost:8080/test"
data:0
data:1
data:2
data:3
data:4
data:5
data:6
data:7
data:8
data:9
This GIF shows how the curl command streams output in real time:
Using SseEmitter with JavaScript
SSE specification defines client side scripting handling as well. The real advantage of using SseEmitter can be seen when used with JavaScript.
In the following example we are going to add a static web page to initiate the SSE request from javascript.
src/main/webapp/static/index.html<!DOCTYPE html>
<html>
<body>
<h1>Sse client</h1>
<a href="javascript:void" onclick="sendRequest()">Send Request</a>
<div id="sseDiv"></div>
<script>
function sendRequest(){
if(typeof(EventSource) !== "undefined") {
var source = new EventSource("/test");
source.onmessage = function(event) {
document.getElementById("sseDiv").innerHTML += event.data + " - ";
};
} else {
document.getElementById("sseDiv").innerHTML =
"Your browser does not support server-sent events.";
}
}
</script>
</body>
</html>
There's no change with our controller from the previous example.
Output
On Clicking 'Send Request':
Note that, just like ResponseBodyEmitter, can be wrapped in ResponseEntity:
package com.logicbig.example;
import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.servlet.mvc.method.annotation.SseEmitter;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
@Controller
public class MyWebController2 {
@GetMapping("/test2")
public ResponseEntity<SseEmitter> handleRequest() {
final SseEmitter emitter = new SseEmitter();
ExecutorService service = Executors.newSingleThreadExecutor();
service.execute(() -> {
for (int i = 0; i < 10; i++) {
try {
emitter.send(String.valueOf(i), MediaType.TEXT_PLAIN);
Thread.sleep(100);
} catch (Exception e) {
e.printStackTrace();
emitter.completeWithError(e);
return;
}
}
emitter.complete();
});
return new ResponseEntity(emitter, HttpStatus.OK);
}
}
Example ProjectDependencies and Technologies Used: - spring-webmvc 7.0.6 (Spring Web MVC)
Version Compatibility: 4.2.0.RELEASE - 7.0.6 Version compatibilities of spring-webmvc with this example: Versions in green have been tested.
- jakarta.servlet-api 6.1.0 (Jakarta Servlet API documentation)
- spring-test 7.0.6 (Spring TestContext Framework)
- junit-jupiter-engine 6.0.3 (Module "junit-jupiter-engine" of JUnit)
- hamcrest 3.0 (Core API and libraries of hamcrest matcher framework)
- JDK 25
- Maven 3.9.11
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