Spring Boot Server-Sent Events Tutorial Using WebFlux

Welcome, in this tutorial, we will explain the server-sent events in a spring boot application using WebFlux. In this approach, we will use the Reactive SSE where the service returns a Flux of transactions. 1. Introduction Before going further in this Spring Boot Server-Sent Events Tutorial using WebFlux, we will look at the common terminology such as introduction to Spring Boot, Lombok, and Server-Sent Events (SSE). 1.1 Spring Boot

Spring boot is a module that provides rapid application development feature to the spring framework including auto-configuration, standalone-code, and production-ready code It creates applications that are packaged as jar and are directly started using embedded servlet container (such as Tomcat, Jetty or, Undertow). Thus, no need to deploy the war files It simplifies the maven configuration by providing the starter template and helps to resolve the dependency conflicts. It automatically identifies the required dependencies and imports them in the application It helps in removing the boilerplate code, extra annotations, and XML configurations It provides a powerful batch processing and manages the rest endpoints It provides an efficient JPA-starter library to effectively connect the application with the relational databases It offers a Microservice architecture and cloud configuration that manages all the application related configuration properties in a centralized manner

1.2 Lombok

Lombok is nothing but a small library which reduces the amount of boilerplate Java code from the project Automatically generates the getters and setters for the object by using the Lombok annotations Hooks in via the Annotation processor API Raw source code is passed to Lombok for code generation before the Java Compiler continues. Thus, produces properly compiled Java code in conjunction with the Java Compiler Under the target/classes folder you can view the compiled class files Can be used with Maven, Gradle IDE, etc.

1.2.1 Lombok features

Feature Details

val Local variables are declared as final

var Mutable local variables

@Slf4J Creates an SLF4J logger

@Cleanup Will call close() on the resource in the finally block

@Getter Creates getter methods for all properties

@Setter Creates setter for all non-final properties

@EqualsAndHashCode

Generates implementations of equals(Object other) and hashCode() By default will use all non-static, non-transient properties Can optionally exclude specific properties

@ToString

Generates String of class name, and each field separated by commas Optional parameter to include field names Optional parameter to include a call to the super toString method

@NoArgsConstructor

Generates no-args constructor Will cause compiler error if there are final fields Can optionally force, which will initialize final fields with 0/false/null var - mutable local variables

@RequiredArgsContructor

Generates a constructor for all fields that are final or marked @NonNull The constructor will throw a NullPointerException if any @NonNull fields are null val - local variables are declared final

@AllArgsConstructor

Generates a constructor for all properties of the class Any @NotNull properties will have null checks

@Data

Generates typical boilerplate code for POJOs Combines - @Getter, @Setter, @ToString, @EqualsAndHashCode, @RequiredArgsConstructor No constructor is generated if constructors have been explicitly declared

@Builder

Implements the Builder pattern for object creation

@Value

The immutable variant of @Data All fields are made private and final by default

1.3 Server-Sent Events (SSE) Server-Sent Events (SSE) is a web technology where a browser receives updates from a server via the HTTP connection and offers a better approach then polling as polling causes a lot of HTTP overhead. It is unidirectional (i.e. server to browser) and the events are sent over the traditional HTTP, hence no special implementation is required on the server. It is used to –

Replace long polling Enable applications that use the server to browser data communication

Spring Webflux is an async and non-blocking reactive web stack that enables the handles of a massive number of concurrent connections. It supports reactive streams backpressure and runs on servers such as Netty. Helps to enable vertical scaling of the service to handle a greater load. 2. Spring Boot Server-Sent Events Tutorial Using WebFlux Here is a systematic guide for implementing this tutorial but before going any further I’m assuming that you are aware of the Spring boot. 2.1 Application Pre-requisite To start with this tutorial, we are hoping that you at present have the Lombok plugin installed in the IDE of their favorite choice. If someone needs to go through the Lombok installation on IntelliJ IDE, please watch this video. For installation on Eclipse IDE, please watch this video. 2.2 Tools Used and Project Structure We are using Eclipse Kepler SR2, JDK 8, and Maven. In case you’re confused about where you should create the corresponding files or folder, let us review the project structure of the spring boot application.

Fig. 1: Project structure

Let us start building the application! 3. Creating a Spring Boot application Below are the steps involved in developing the application. 3.1 Maven Dependency Here, we specify the dependency for the Spring Boot, WebFlux, Thymeleaf, Faker, and Lombok. Maven will automatically resolve the other dependencies. The updated file will have the following code. pom.xml

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62

4.0.0 com.springboot.server-events SpringbootServerEvents 0.0.1-SNAPSHOT Springboot Server Events tutorial A tutorial on springboot and server events 1.8 org.springframework.boot spring-boot-starter-parent 2.3.4.RELEASE org.springframework.boot spring-boot-starter-webflux org.springframework.boot spring-boot-starter-thymeleaf com.github.javafaker javafaker 1.0.2 org.projectlombok lombok provided SpringbootServerEvents org.springframework.boot spring-boot-maven-plugin

3.2 Application Properties Create a new properties file at the location: SpringbootServerEvents/src/main/resources/ and add the following code to it. application.properties

1 2

server.port=10093 spring.application.name=springboot-and-server-events

3.3 Java Classes Let us write all the java classes involved in this application. 3.3.1 Implementation/Main class Add the following code to the main class to bootstrap the application from the main method. Always remember, the entry point of the spring boot application is the class containing @SpringBootApplication annotation and the static main method. SpringbootServerEvents.java

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20

package com.springboot.serverevents; import lombok.extern.slf4j.Slf4j; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; // Lombok annotation // Causes Lombok to generate a logger field. @Slf4j // Spring framework annotation // Main implementation class which serves two purposes in a spring boot application: Configuration and bootstrapping. @SpringBootApplication public class SpringbootServerEvents { // Main program to start up the spring boot application. public static void main(String[] args) { SpringApplication.run(SpringbootServerEvents.class, args); log.info("Spring-boot server events application started successfully."); } }

3.3.2 Model class Add the following code to the Book model class. GameOfThrone.java

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

package com.springboot.serverevents.model; import lombok.AllArgsConstructor; import lombok.Builder; import lombok.Data; import lombok.NoArgsConstructor; import org.springframework.stereotype.Component; // Lombok annotations // Causes lombok to generate toString(), equals(), hashCode(), getter() & setter(), and Required arguments constructor in one go. @Data // Causes Lombok to implement the Builder design pattern for the Pojo class. // Usage can be seen in GameOfThroneReactiveRepository.java -> get() method. @Builder // Causes Lombok to generate a constructor with no parameters. @NoArgsConstructor // Causes Lombok to generate a constructor with 1 parameter for each field in your class. @AllArgsConstructor // Spring framework annotation @Component public class GameOfThrone { String house; String character; String dragon; String city; }

3.3.3 Configuration class Add the following code to the bean class that will return the bean object for the faker object. The usage of this object can be seen in the GameOfThroneReactiveRepository.java class. BeanConfiguration.java

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16

package com.springboot.serverevents.config; import com.github.javafaker.Faker; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import java.util.Locale; @Configuration public class BeanConfiguration { @Bean public Faker faker() { return new Faker(new Locale("en-US")); } }

3.3.4 Data-Access-Object class Add the following code to the Dao class that returns the Flux events. Currently, we are returning the dummy events for the simplest implementation and understanding. GameOfThroneReactiveRepository.java

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

package com.springboot.serverevents.repo; import com.github.javafaker.Faker; import com.springboot.serverevents.model.GameOfThrone; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Repository; import reactor.core.publisher.Flux; import java.time.Duration; import java.util.Collections; import java.util.List; @Repository public class GameOfThroneReactiveRepository { @Autowired private Faker faker; public Flux findAll() { // Simulate the data streaming every 2 seconds. return Flux.interval(Duration.ofSeconds(2)) .onBackpressureDrop() .map(interval -> get()) .flatMapIterable(v -> v); } private List get() { final GameOfThrone gameOfThrone = GameOfThrone.builder() .house(faker.gameOfThrones().house()) .character(faker.gameOfThrones().character()) .dragon(faker.gameOfThrones().dragon()) .city(faker.gameOfThrones().city()).build(); return Collections.singletonList(gameOfThrone); } }

3.3.5 Service class Add the following code to the service class where we will call the methods of the Dao class to fetch the data. GameOfThroneService.java

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

package com.springboot.serverevents.service; import com.springboot.serverevents.model.GameOfThrone; import com.springboot.serverevents.repo.GameOfThroneReactiveRepository; import lombok.extern.slf4j.Slf4j; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Service; import reactor.core.publisher.Flux; // Lombok annotation // Causes Lombok to generate a logger field. @Slf4j // Spring framework annotation @Service public class GameOfThroneService { @Autowired GameOfThroneReactiveRepository gameOfThroneReactiveRepository; public Flux getAllThronesFlux() { log.info("Fetching the game of throne data from the persistent storage and sending back via the flux mechanism."); return gameOfThroneReactiveRepository.findAll(); } }

3.3.6 Index Controller class Add the following code to the controller class designed to handle the incoming requests. The class is annotated with the @Controller annotation were the HTTP GET method would return the index page of the application. IndexController.java

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22

package com.springboot.serverevents.controller; import lombok.extern.slf4j.Slf4j; import org.springframework.stereotype.Controller; import org.springframework.web.bind.annotation.GetMapping; // Lombok annotation // Causes Lombok to generate a logger field. @Slf4j // Spring framework annotation @Controller public class IndexController { private static final String INDEX_PAGE = "index"; // URL - http://localhost:10093/ @GetMapping(value = "/") public String index() { log.info("Returning the application's welcome page to the user."); return INDEX_PAGE; } }

3.3.7 Rest Controller class Add the following code to the controller class designed to handle the incoming requests. The class is annotated with the @RestController annotation were the HTTP GET method would return the flux events. GameOfThroneRestController.java

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

package com.springboot.serverevents.controller; import com.springboot.serverevents.model.GameOfThrone; import com.springboot.serverevents.service.GameOfThroneService; import lombok.extern.slf4j.Slf4j; 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.RequestMapping; import org.springframework.web.bind.annotation.RestController; import reactor.core.publisher.Flux; // Lombok annotation // Causes Lombok to generate a logger field. @Slf4j // Spring framework annotation @RestController @RequestMapping(value = "/api") public class GameOfThroneRestController { @Autowired GameOfThroneService gameOfThroneService; // URL - http://localhost:10093/api/gameOfThroneStream @GetMapping(path = "/gameOfThroneStream", produces = MediaType.TEXT_EVENT_STREAM_VALUE) public Flux gameOfThroneStream() { log.info("Returning the game of throne streaming events via flux mechanism."); return gameOfThroneService.getAllThronesFlux(); } }

4. Thymeleaf Changes We will create a simple HTML page that will display the reactive events on the browser. Create a new HTML file at the location: SpringbootServerEvents/src/main/resources/templates/ and add the following code to it. index.html

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Index .bold-font { font-weight: bold; } th, td { text-align: center; } Spring Boot and Server Events Tutorial Using Webflux House Character Dragon City [[${gameOfThrone.house}]] [[${gameOfThrone.character}]] [[${gameOfThrone.dragon}]] [[${gameOfThrone.city}]]

5. JavaScript Changes To listen to the message events we will use the JavaScript EventSource object and update the streaming data into the HTML table. Create a new JS file at the location: SpringbootServerEvents/src/main/resources/static/js/ and add the following code to it. main.js

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

6. Run the Application To execute the application, right-click on the SpringbootServerEvents.java class, Run As -> Java Application.

Fig. 2: Run the Application

7. Project Demo Open the browser of your choice and hit the following URL. The stream of data having the game of throne characters data will be displayed every 2 seconds.

1

http://localhost:10093/

Fig. 3: Index page

That is all for this tutorial and I hope the article served you whatever you were looking for. Happy Learning and do not forget to share! 8. Server-Sent Events using Webflux - Summary In this Spring Boot Server-Sent Events tutorial using WebFlux, you learned:

Spring Boot, Lombok and it features, and SSE (Server-Sent Events) Flux implementation in Spring Boot and displaying the streaming data on the browser

You can download the sample application as an Eclipse project in the Downloads section. 9. Download the Eclipse Project This was an example of Server-Sent Events (SSE) in Spring Boot using WebFlux. DownloadYou can download the full source code of this example here: Spring Boot Server-Sent Events Tutorial Using WebFlux

Spring Boot Server-Sent Events Tutorial Using WebFlux

Thank you!

Yatin

Thank you!

Related Articles