Home Spring MVC Implementation of Server-Sent Events Using Spring Boot

Implementation of Server-Sent Events Using Spring Boot

Server-Sent Events (SSE)

Below is the step-by-step implementation of Server-Sent Events in Spring.

Step 1: Setting up a Spring Boot Project in Spring Initializer

Go to this link and create a Spring Boot project by pre-filling some information such as:

  • Specify the project metadata:
    • Group: Project group ID
    • Artifact: Project’s artifact ID
    • Name: Name of your project
  • Choosing the appropriate version of Spring Boot.
  • Select the language (Java)
  • Adding necessary dependencies:
    • Spring Reactive Web (for building reactive, non-blocking web applications).
    • Spring boot actuator (for monitoring and managing web applications).
    • Spring web (for building web applications).
  • At last, click on GENERATE button.
  • Download your Spring Boot project in zip format.
  • Now extract the folder into your local machine.

Step 2: Adding Dependencies in pom.xml

All dependencies are specified in pom.xml file.

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 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>3.2.3</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.article</groupId>
    <artifactId>server-sent-events-example</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>server-sent-events-example</name>
    <description>Demo project for server sent events</description>
    <properties>
        <java.version>17</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-webflux</artifactId>
        </dependency>
  
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>io.projectreactor</groupId>
            <artifactId>reactor-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>
  
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
  
</project> 






                        









																																						

																								

																										
Step 3: Creating a Server-Sent Events Controller

																																						

																								

																										
Here is the ServerEventsController class.

																																						

																								

																										




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package com.article.spring; 


  


import org.springframework.http.MediaType; 


import org.springframework.http.codec.ServerSentEvent; 


import org.springframework.web.bind.annotation.GetMapping; 


import org.springframework.web.bind.annotation.RequestMapping; 


import org.springframework.web.bind.annotation.RestController; 


import reactor.core.publisher.Flux; 


import java.io.IOException; 


import java.nio.file.Files; 


import java.nio.file.Path; 


import java.time.Duration; 


import java.util.concurrent.atomic.AtomicInteger; 


import java.util.stream.Stream; 


  


@RestController


@RequestMapping("/server-events") 


public class ServerEventsController { 


  


    @GetMapping(produces = MediaType.TEXT_EVENT_STREAM_VALUE) 


    public Flux<ServerSentEvent<String>>getEvents() throws IOException { 


  


        // Read lines from a file 


        Stream<String> lines=Files.lines(Path.of( 


          "C:\\Users\\prade\\Downloads\\server-sent-events-example\\server-sent-events-example\\pom.xml")); 


  


        // Counter for event IDs 


        AtomicInteger counter=new AtomicInteger(1); 


  


        // Convert lines to Server-Sent Events 


        return Flux.fromStream(lines) 


                // Filter out blank lines 


                .filter(line -> !line.isBlank()) 


                // Map each line to a Server-Sent Event 


                .map(line -> ServerSentEvent.<String>builder() 


                        // Assign an ID to the event 


                        .id(String.valueOf(counter.getAndIncrement())) 


                        // Set the data of the event to the line content 


                        .data(line) 


                        // Set the event type 


                        .event("lineEvent") 


                        // Set the retry duration 


                        .retry(Duration.ofMillis(1000)) 


                        // Build the Server-Sent Event 


                        .build()) 


                // Introduce a delay between sending each event 


                .delayElements(Duration.ofMillis(300)); 


                } 


  


    } 



















                        







                        









																																						

																								

																										
The ServerEventsController is responsible for handling HTTP requests related to Server-Sent Events (SSE). It reads lines from a file and converts them into SSEs.

																																						

																								

																										
    

  • Endpoint Mapping: The controller is mapped to “/server-events” endpoint which will handle requests made to this URL.
    • 

  • Reading from file: It reads lines from a file specified by the file path.
    • 

  • Event Creation: Each line from the file is converted into a Server-Sent Events (SSE) using a builder method. The ID is assigned sequentially using an “AtomicInteger” and the line content becomes the data of the event.
    • 

  • Retry Duration: The Server-Sent Events are given a retry duration of 1 second.
    • 

  • Streaming and Delay: The Server-Sent Events are streamed to clients with a delay of 300 milliseconds between each event.
    • 


																																						

																								

																										
Step 4: Run the Application.

																																						

																								

																										
To run the Spring Boot application, simply run this ServerSentEventsExampleApplication class .

																																						

																								

																										




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package com.article.spring; 


  


import org.springframework.boot.SpringApplication; 


import org.springframework.boot.autoconfigure.SpringBootApplication; 


  


@SpringBootApplication


public class ServerSentEventsExampleApplication { 


  


    public static void main(String[] args) { 


          


        SpringApplication.run(ServerSentEventsExampleApplication.class, args); 


    } 


  


} 



















                        







                        









																																						

																								

																										
Output Using CURL:

																																						

																								

																										
In the terminal execute the following command,

																																						

																								

																										
curl http://localhost:8080/server-events -v



																																						

																								

																										
After executing the above command, we will see the below output in terminal.

																																						

																								

																										

																																						

																								

																										
Output Using Browser:

																																						

																								

																										
To see the Server-Sent Events in action, open browser and go to the following URL:

																																						

																								

																										
http://localhost:8080/server-events


																																						

																								

																										
After hitting the above link in browser, we will get the below output.

																																						

																								

																										

																																						

																								

																										

																																						

																								

																										

																																						

																								

																										

																																						

																							

											
											

												
Server – Sent Events in Spring

																								

																										
When we design web applications, we generally have a client and a server model, where the client is always the initiator of the transaction or a request to the server. The server responds to these requests. However, in certain scenarios, there arises a need for the server to respond much more frequently or give additional information to the client. This requirement leads us to techniques such as long polling, WebSockets, and Server-Sent Events (SSE).

																																						

																								

																										
In this article, we are going to look at Server-Sent Events (SSE) using Spring Boot and how we can implement them.

																																						

																							

										


										
										

											
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