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Spring Boot Server Events with SseEmitter Tutorial

Welcome, in this tutorial, we will explain the server events with SseEmitter in a spring boot application. In this approach, we will use the SseEmitter where the service returns a SseEmitter events to the client.

1. Introduction

Before going further in this tutorial, 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 into the application
  • It helps in removing the boilerplate code, extra annotations, and XML configurations
  • It provides 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

FeatureDetails
valLocal variables are declared as final
varMutable local variables
@Slf4JCreates an SLF4J logger
@CleanupWill call close() on the resource in the finally block
@GetterCreates getter methods for all properties
@SetterCreates 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)

SSE is a web technology where a browser receives updates from a server via the HTTP connection and offers a better approach than 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

2. Spring Boot Server Events with SseEmitter Tutorial

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.

spring boot sseemitter - Project structure
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 (Web and JPA), WebFlux, Faker, Embedded H2 database, and Lombok. Maven will automatically resolve the other dependencies. The updated file will have the following code.

pom.xml

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    <modelVersion>4.0.0</modelVersion>
 
    <groupId>com.springboot.sseemitter</groupId>
    <artifactId>SpringbootSseEmitter</artifactId>
    <version>0.0.1-SNAPSHOT</version>
     
    <name>Springboot sse emitter tutorial</name>
    <description>A tutorial on springboot and sse emitter tutorial</description>
 
    <properties>
        <!-- java8 dependency -->
        <java.version>1.8</java.version>
    </properties>
 
    <!-- spring boot starter parent dependency -->
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.4.RELEASE</version>
    </parent>
 
    <dependencies>
        <!-- springboot starter web dependency -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <!-- springboot starter jpa dependency -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <!-- springboot starter webflux dependency -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-webflux</artifactId>
        </dependency>
        <!-- springboot project reactor dependency -->
        <dependency>
            <groupId>io.projectreactor</groupId>
            <artifactId>reactor-test</artifactId>
        </dependency>
        <!-- embedded h2 database -->
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
            <scope>runtime</scope>
        </dependency>
        <!-- project lombok dependency -->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <scope>provided</scope>
        </dependency>
        <!-- java faker dependency -->
        <dependency>
            <groupId>com.github.javafaker</groupId>
            <artifactId>javafaker</artifactId>
            <version>1.0.2</version>
        </dependency>
    </dependencies>
     
    <build>
        <finalName>SpringbootSseEmitter</finalName>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

3.2 Application Properties

Create a new properties file at the location: SpringbootSseEmitter/src/main/resources/ and add the following code to it.

application.properties

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# application port number
server.port=10093
# application name
spring.application.name=springboot-sseemitter-application
# h2 database settings
spring.datasource.username=sa
spring.datasource.password=
spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
# logging
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
spring.jpa.hibernate.ddl-auto=create-drop
spring.jpa.properties.hibernate.show_sql=true
# browser url for h2 console - http://localhost:10093/h2-console
spring.h2.console.enabled=true
spring.h2.console.path=/h2-console
# property to override the default async behavior such as thread pool and timeout
# time in milliseconds
# here in this example we are setting it to 50 minutes
spring.mvc.async.request-timeout=3000000

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.

SpringbootSseEmitter.java

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package com.springboot.sseemitter;
 
import lombok.extern.slf4j.Slf4j;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
 
//Causes Lombok to generate a logger field.
@Slf4j
//Serves two purposes i.e. configuration and bootstrapping.
@SpringBootApplication
public class SpringbootSseEmitter {
 
    public static void main(String[] args) {
        SpringApplication.run(SpringbootSseEmitter.class, args);
        log.info("Springboot sseemitter application is started successfully.");
    }
}

3.3.2 Model class

Add the following code to the Emp model class where we will define the basic employee attributes.

Emp.java

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package com.springboot.sseemitter.model;
 
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;
import org.springframework.stereotype.Component;
 
import javax.persistence.*;
 
@Entity
@Table(name = "employees")
// 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 DefaultEmployeesLoader.java -> createNewEmployee() 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
@Component
public class Emp {
 
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    long id;
    @Column(name = "first_name", nullable = false)
    String firstName;
    @Column(name = "last_name", nullable = false)
    String lastName;
    @Column(name = "gender")
    String gender;
    @Column(name = "email", nullable = false)
    String email;
    @Column(name = "phone_number", unique = true)
    String phoneNumber;
    @Column(name = "home_address")
    String homeAddress;
}

3.3.3 Configuration class

Add the following code to the bean class that will return the bean objects for the faker, sseEmitter, and executorService. The usage of this object can be seen in the DefaultEmpLoader.java class.

BeanConfiguration.java

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package com.springboot.sseemitter.model.config;
 
import com.github.javafaker.Faker;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.mvc.method.annotation.SseEmitter;
 
import java.util.Locale;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
 
@Configuration
public class BeanConfiguration {
 
    // Creating an object for Faker.
    @Bean
    public Faker faker() {
        return new Faker(new Locale("en-US"));
    }
 
    // Creating an object for SseEmitter.
    @Bean
    public SseEmitter sseEmitter() {
        return new SseEmitter();
    }
 
    // Creating an object for ExecutorService.
    @Bean
    public ExecutorService executorService() {
        return Executors.newSingleThreadExecutor();
    }
}

3.3.4 Data-Access-Object interface

Add the following code to the interface that extends the JpaRepository interface.

EmpRepo.java

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package com.springboot.sseemitter.model.repository;
 
import com.springboot.sseemitter.model.Emp;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;
 
@Repository
public interface EmpRepo extends JpaRepository<Emp, Long> {
 
}

3.3.5 Service class

Add the following code to the service class where we will call the DAO interface methods to save the data into the database and also fetch the data from the database.

EmpServ.java

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package com.springboot.sseemitter.model.service;
 
import com.springboot.sseemitter.model.Emp;
import com.springboot.sseemitter.model.repository.EmpRepo;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
 
import java.util.List;
 
//Causes Lombok to generate a logger field.
@Slf4j
@Service
public class EmpServ {
 
    @Autowired
    private EmpRepo repository;
 
    // Save a new employee into the database.
    public void save(final Emp employee) {
        repository.save(employee);
    }
 
    // Get the total count of employees from the database.
    public long getTotalEmployees() {
        log.info("Finding the total count of employees from the database.");
        return repository.count();
    }
 
    // Get all employees from the database.
    public List<Emp> findAll() {
        log.info("Fetching all employees from the database.");
        return repository.findAll();
    }
}

3.3.6 Bootstrap class

Add the following code to the bootstrap class to save the dummy data into the database on the application startup. This data will be saved in the H2 database.

DefaultEmpLoader.java

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package com.springboot.sseemitter.model.bootstrap;
 
import com.github.javafaker.Faker;
import com.springboot.sseemitter.model.Emp;
import com.springboot.sseemitter.model.service.EmpServ;
import lombok.RequiredArgsConstructor;
import lombok.extern.slf4j.Slf4j;
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;
 
import java.util.Random;
 
// Causes Lombok to generate a logger field.
@Slf4j
// Causes Lombok to generate a constructor with 1 parameter for each field that requires special handling.
@RequiredArgsConstructor
@Component
public class DefaultEmpLoader implements CommandLineRunner {
 
    private static final String[] GENDER = {"Male", "Female", "Transgender", "Not to specify"};
    private static final Random RANDOM = new Random();
 
    private final EmpServ service;
    private final Faker faker;
 
    @Override
    public void run(String... args) throws Exception {
        loadEmployees();
    }
 
    // Saving the default employees data in the database.
    private void loadEmployees() {
        int count = 0;
        if (service.getTotalEmployees() == 0) {
            for (int x = 0; x < 100; x++) {
                count = count + 1;
                service.save(createNewEmployee());
            }
            log.info("Total {} employees are saved in the database.", count);
        } else {
            log.info("Default employees are already present in the database.");
        }
    }
 
    private Emp createNewEmployee() {
        final String firstName = faker.name().firstName();
        final String lastName = faker.name().lastName();
        final String gender = GENDER[RANDOM.nextInt(GENDER.length)];
        final String emailAddress = firstName.toLowerCase() + "." + lastName.toLowerCase() + "@somecompany.com";
 
        return Emp.builder()
                .firstName(firstName)
                .lastName(lastName)
                .gender(gender)
                .email(emailAddress)
                .phoneNumber(faker.phoneNumber().cellPhone())
                .homeAddress(faker.address().fullAddress())
                .build();
    }
}

3.3.7 Async 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 emit the events from the server to the client.

EmpCtrl.java

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package com.springboot.sseemitter.controller;
 
import com.springboot.sseemitter.model.Emp;
import com.springboot.sseemitter.model.service.EmpServ;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.servlet.mvc.method.annotation.SseEmitter;
 
import java.io.IOException;
import java.util.List;
import java.util.concurrent.ExecutorService;
 
//Causes Lombok to generate a logger field.
@Slf4j
@RestController
public class EmpCtrl {
 
    @Autowired
    private EmpServ service;
    @Autowired
    private SseEmitter sseEmitter;
    @Autowired
    private ExecutorService executorService;
 
    @GetMapping("/emit-data-items")
    public SseEmitter fetchData() {
        log.info("Emitting the employee data sets.");
        executorService.execute(() -> {
            // Fetching the employees from the dB.
            final List<Emp> employees = service.findAll();
            try {
                for (final Emp employee : employees) {
                    randomDelay();
                    // Sending the employee details in the emitter event.
                    sseEmitter.send(employee);
                }
                // The request processing is completed so that the responsible thread can complete
                // the request and be freed up for the next response to handle.
                sseEmitter.complete();
            } catch (final IOException e) {
                log.info("IO exception occurred while emitting the employee data sets.");
                // In case of any error while processing the request, complete the process and pass the
                // exception through the normal exception handling of Spring MVC framework and after that
                // response is completed.
                sseEmitter.completeWithError(e);
            }
        });
        executorService.shutdown();
        return sseEmitter;
    }
 
    // Private method to put a random delay in between the records.
    private void randomDelay() {
        try {
            Thread.sleep(1000);
        } catch (final InterruptedException e) {
            log.info("Thread is interrupted.");
            Thread.currentThread().interrupt();
        }
    }
}

4. Run the Application

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

spring boot sseemitter - Run the Application
Fig. 2: Run the Application

5. Project Demo

Open the browser of your choice and hit the following URL. The stream of data having employee data will be displayed on the browser every 1 second.

spring boot sseemitter
Fig. 3: Displaying the server-side events using SseEmitter

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!

6. Summary

In this section, you learned,

  • Spring Boot, Lombok and it features, and SSE (Service-Sent Events)
  • SseEmitter 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.

7. Download the Eclipse Project

This was a tutorial on Spring Boot Server Events with SseEmitter.

Download
You can download the full source code of this example here: Spring Boot Server Events with SseEmitter Tutorial

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