Send Emails with SendGrid from the Micronaut Framework

Learn how to send emails with SendGrid from a Micronaut application

Authors: Sergio del Amo

Micronaut Version: 3.9.2

1. Getting Started

In this guide, we will create a Micronaut application written in Kotlin.

2. What you will need

To complete this guide, you will need the following:

Some time on your hands
A decent text editor or IDE
JDK 1.8 or greater installed with JAVA_HOME configured appropriately

3. Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

Download and unzip the source

4. Writing the Application

Create an application using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app example.micronaut.micronautguide \
    --features=graalvm,email-sendgrid,reactor \
    --build=gradle
    --lang=kotlin

If you don’t specify the --build argument, Gradle is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.

The previous command creates a Micronaut application with the default package example.micronaut in a directory named micronautguide.

If you use Micronaut Launch, select Micronaut Application as application type and add graalvm, email-sendgrid, and reactor features.

If you have an existing Micronaut application and want to add the functionality described here, you can view the dependency and configuration changes from the specified features and apply those changes to your application.

Setup IntelliJ IDEA to develop Micronaut Applications.

4.1. Controller

Create a MailController class. This class uses a collaborator, emailSender, to send an email.

You can send emails asynchronously using the AysnEmailSender API or synchronously using the EmailSender API.

src/main/kotlin/example/micronaut/MailController.kt

package example.micronaut

import com.sendgrid.Request
import com.sendgrid.Response
import io.micronaut.email.AsyncEmailSender
import io.micronaut.email.BodyType.HTML
import io.micronaut.email.Email
import io.micronaut.http.HttpResponse
import io.micronaut.http.annotation.Body
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Post
import org.reactivestreams.Publisher
import org.slf4j.LoggerFactory
import reactor.core.publisher.Mono

@Controller("/mail") (1)
class MailController(private val emailSender: AsyncEmailSender<Request, Response>) { (2)

    @Post("/send") (3)
    fun send(@Body("to") to: String): Publisher<HttpResponse<*>> { (4)
        return Mono.from(emailSender.sendAsync(Email.builder()
                .to(to)
                .subject("Sending email with Twilio Sendgrid is Fun")
                .body("and <em>easy</em> to do anywhere with <strong>Micronaut Email</strong>", HTML)))
                .doOnNext { rsp: Response ->
                    LOG.info("response status {}\nresponse body {}\nresponse headers {}",
                            rsp.statusCode, rsp.body, rsp.headers)
                }.map { rsp: Response ->
                    if (rsp.statusCode >= 400) HttpResponse.unprocessableEntity() else HttpResponse.accepted<Any>()
                } (5)
    }

    companion object {
        private val LOG = LoggerFactory.getLogger(MailController::class.java)
    }
}

1	The class is defined as a controller with the @Controller annotation mapped to the path `/mail/send`.
2	Use constructor injection to inject a bean of type `AsyncEmailSender`.
3	The @Post annotation maps the `send` method to an HTTP POST request on `/mail/send`.
4	You can use a qualifier within the HTTP request body. For example, you can use a reference to a nested JSON attribute.
5	Return 202 ACCEPTED as the result if the email delivery succeeds

4.2. Configuration

If you want to send every email with the same address, you can set it via configuration:

src/main/resources/application.yml

---
micronaut:
  email:
    from:
      email: 'john@micronaut.example'

This is possible thanks to Email Decorators.

4.3. SendGrid

SendGrid is a transactional email service.

SendGrid is responsible for sending billions of emails for some of the best and brightest companies in the world.

4.3.1. SendGrid API Key

To use the Micronaut Sendgrid integration, you need an API Key.

You need to configure the sendgrid.api-key property. However, don’t put it in application.yml. It’s a password. Expose it via the SENDGRID_API_KEY environment variable, using a secret manager or create an environment specific configuration file that won’t be included in source version control.

4.3.2. Dependency

Because we added the email-sendgrid feature, the application contains the following dependency:

build.gradle

implementation("com.micronaut.email:micronaut-email-sendgrid")

4.4. Test

Create a test bean that replaces the bean of type AsyncTransactionalEmailSender.

src/test/kotlin/example/micronaut/EmailSenderReplacement.kt

package example.micronaut

import com.sendgrid.Request
import com.sendgrid.Response
import io.micronaut.context.annotation.Replaces
import io.micronaut.context.annotation.Requires
import io.micronaut.email.AsyncEmailSender
import io.micronaut.email.AsyncTransactionalEmailSender
import io.micronaut.email.Email
import io.micronaut.email.EmailException
import io.micronaut.http.HttpStatus.ACCEPTED
import jakarta.inject.Named
import jakarta.inject.Singleton
import org.reactivestreams.Publisher
import reactor.core.publisher.Mono
import java.util.function.Consumer
import javax.validation.Valid

@Requires(property = "spec.name", value = "MailControllerTest") (1)
@Singleton
@Replaces(AsyncEmailSender::class)
@Named(EmailSenderReplacement.NAME)
open class EmailSenderReplacement : AsyncTransactionalEmailSender<Request, Response> {

    val emails = mutableListOf<Email>()

    override fun getName(): String = NAME

    @Throws(EmailException::class)
    override fun sendAsync(@Valid email: Email,
                           emailRequest: Consumer<Request>): Publisher<Response> {
        emails.add(email)
        val response = Response()
        response.statusCode = ACCEPTED.code
        return Mono.just(response)
    }

    companion object {
        const val NAME = "sendgrid"
    }
}

1	Combine `@Requires` and properties (either via the `@Property` annotation or by passing properties when starting the context) to avoid bean pollution.

Write a test that uses EmailSenderReplacement to verify that the contents of the email match expectations.

src/test/kotlin/example/micronaut/MailControllerTest.kt

package example.micronaut

import io.micronaut.context.BeanContext
import io.micronaut.context.annotation.Property
import io.micronaut.core.util.CollectionUtils
import io.micronaut.email.AsyncTransactionalEmailSender
import io.micronaut.email.BodyType.HTML
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpResponse
import io.micronaut.http.HttpStatus.ACCEPTED
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.test.extensions.junit5.annotation.MicronautTest
import jakarta.inject.Inject
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Assertions.assertNotNull
import org.junit.jupiter.api.Assertions.assertTrue
import org.junit.jupiter.api.Test

@Property(name = "spec.name", value = "MailControllerTest") (1)
@MicronautTest (2)
class MailControllerTest(@Client("/") val httpClient: HttpClient) { (3)

    @Inject
    lateinit var beanContext: BeanContext

    @Test
    fun mailSendEndpointSendsAnEmail() {

        val response: HttpResponse<*> = httpClient.toBlocking().exchange<Map<String, String>, Any>(
                HttpRequest.POST("/mail/send", mapOf("to" to "johnsnow@micronaut.example")))
        assertEquals(ACCEPTED, response.status())

        val sender = beanContext.getBean(AsyncTransactionalEmailSender::class.java)
        assertTrue(sender is EmailSenderReplacement)

        val sendgridSender = sender as EmailSenderReplacement
        assertTrue(CollectionUtils.isNotEmpty(sendgridSender.emails as Collection<*>?))
        assertEquals(1, sendgridSender.emails.size)

        val email = sendgridSender.emails[0]
        assertEquals(email.from.email, "john@micronaut.example")
        assertNotNull(email.to)
        assertTrue(email.to!!.stream().findFirst().isPresent)
        assertEquals(email.to!!.stream().findFirst().get().email, "johnsnow@micronaut.example")
        assertEquals(email.subject, "Sending email with Twilio Sendgrid is Fun")
        assertNotNull(email.body)
        assertTrue(email.body!![HTML].isPresent)
        assertEquals(email.body!![HTML].get(), "and <em>easy</em> to do anywhere with <strong>Micronaut Email</strong>")
    }
}

1	Combine `@Requires` and properties (either via the `@Property` annotation or by passing properties when starting the context) to avoid bean pollution.
2	Annotate the class with `@MicronautTest` so the Micronaut framework will initialize the application context and the embedded server. More info.
3	Inject the `HttpClient` bean and point it to the embedded server.

5. Testing the Application

To run the tests:

./gradlew test

Then open build/reports/tests/test/index.html in a browser to see the results.

6. Running the Application

6.1. API Key

Set the SendGrid API Key as an environment variable before you run the application:

export SENDGRID_API_KEY=xxx

6.2. From email address

Change the property micronaut.email.from.email to match your SendGrid configuration.

6.3. Run

To run the application, use the ./gradlew run command, which starts the application on port 8080.

6.4. Invoke

curl -d '{"to":"john@micronaut.example"}' \
     -H "Content-Type: application/json" \
     -X POST http://localhost:8080/mail/send

7. Next steps

Explore more features with Micronaut Guides.

Learn more about Micronaut Email integration.

8. Help with the Micronaut Framework

The Micronaut Foundation sponsored the creation of this Guide. A variety of consulting and support services are available.

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