To simplify searches and improve your documentation experience, we're splitting the 1st gen and 2nd gen documentation into separate sets.

JVM Languages

It's possible to write your function in using different JVM languages (such as Kotlin, Groovy, or Scala) as long as they conform to the following rules:

The function is a public class that implements one of the function interfaces (HttpFunction, BackgroundFunction, or RawBackgroundFunction) and has a public no-argument constructor.
If you are deploying from source:
- It can be built from Maven.
- The build file contains all the plugins to produce compiled classes.
If you are deploying from a pre-built JAR:
- You can use any build tools to produce this JAR.
- The pre-built JAR must be a Fat JAR with all of its dependency classes, or its manifest must contain a Class-Path entry with the relative locations of jars containing those dependency classes.

HTTP function examples

You use HTTP functions when you want to invoke your function via an HTTP(S) request. The following examples output the message "Hello World!"

Kotlin

import com.google.cloud.functions.HttpFunction
import com.google.cloud.functions.HttpRequest
import com.google.cloud.functions.HttpResponse
import java.io.IOException
import java.util.logging.Logger

class HelloWorld : HttpFunction {
    // Simple function to return "Hello World"
    @Throws(IOException::class)
    override fun service(request: HttpRequest, response: HttpResponse) {
        response.writer.write("Hello World!")
    }
}

Groovy

import com.google.cloud.functions.HttpFunction
import com.google.cloud.functions.HttpRequest
import com.google.cloud.functions.HttpResponse

class GroovyHelloWorld implements HttpFunction {
    @Override
    void service(HttpRequest request, HttpResponse response) {
        response.writer.write("Hello World!")
    }
}

Scala

class ScalaHelloWorld extends HttpFunction {
  override def service(httpRequest: HttpRequest, httpResponse: HttpResponse): Unit = {
    httpResponse.getWriter.write("Hello World!")
  }
}

`pom.xml` file for HTTP examples

Here are the pom.xml files for the above samples:

Kotlin

<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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.example.functions</groupId>
  <artifactId>functions-kotlin-hello-world</artifactId>


  <properties>
    <maven.compiler.target>11</maven.compiler.target>
    <maven.compiler.source>11</maven.compiler.source>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <kotlin.version>1.9.22</kotlin.version>
  </properties>

  <dependencies>
    <!-- Required for Function primitives -->
    <dependency>
      <groupId>com.google.cloud.functions</groupId>
      <artifactId>functions-framework-api</artifactId>
      <version>1.1.0</version>
      <scope>provided</scope>
    </dependency>

    <dependency>
      <groupId>org.jetbrains.kotlin</groupId>
      <artifactId>kotlin-stdlib-jdk8</artifactId>
      <version>${kotlin.version}</version>
    </dependency>
    <dependency>
      <groupId>org.jetbrains.kotlin</groupId>
      <artifactId>kotlin-test</artifactId>
      <version>${kotlin.version}</version>
      <scope>test</scope>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <!--
          Google Cloud Functions Framework Maven plugin

          This plugin allows you to run Cloud Functions Java code
          locally. Use the following terminal command to run a
          given function locally:

          mvn function:run -Drun.functionTarget=your.package.yourFunction
        -->
        <groupId>com.google.cloud.functions</groupId>
        <artifactId>function-maven-plugin</artifactId>
        <version>0.11.0</version>
        <configuration>
          <functionTarget>functions.HelloWorld</functionTarget>
        </configuration>
      </plugin>
      <plugin>
        <groupId>org.jetbrains.kotlin</groupId>
        <artifactId>kotlin-maven-plugin</artifactId>
        <version>${kotlin.version}</version>
        <executions>
          <execution>
            <id>compile</id>
            <phase>compile</phase>
            <goals>
              <goal>compile</goal>
            </goals>
            <configuration>
              <sourceDirs>
                <source>src/main/kotlin</source>
              </sourceDirs>
            </configuration>
          </execution>
          <execution>
            <id>test-compile</id>
            <phase>test-compile</phase>
            <goals>
              <goal>test-compile</goal>
            </goals>
          </execution>
        </executions>
        <configuration>
          <jvmTarget>1.8</jvmTarget>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

Groovy

<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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.example.functions</groupId>
  <artifactId>functions-groovy-hello-world</artifactId>

  <properties>
    <maven.compiler.target>11</maven.compiler.target>
    <maven.compiler.source>11</maven.compiler.source>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <groovy.version>3.0.20</groovy.version>
  </properties>

  <dependencyManagement>
    <dependencies>
      <dependency>
        <artifactId>libraries-bom</artifactId>
        <groupId>com.google.cloud</groupId>
        <scope>import</scope>
        <type>pom</type>
        <version>26.32.0</version>
      </dependency>
    </dependencies>
  </dependencyManagement>

  <dependencies>
    <!-- Required for Function primitives -->
    <dependency>
      <groupId>com.google.cloud.functions</groupId>
      <artifactId>functions-framework-api</artifactId>
      <version>1.1.0</version>
      <scope>provided</scope>
    </dependency>

    <!-- Required for groovy samples -->
    <dependency>
      <groupId>org.codehaus.groovy</groupId>
      <artifactId>groovy-all</artifactId>
      <version>${groovy.version}</version>
      <type>pom</type>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <!--
          Google Cloud Functions Framework Maven plugin

          This plugin allows you to run Cloud Functions Java code
          locally. Use the following terminal command to run a
          given function locally:

          mvn function:run -Drun.functionTarget=your.package.yourFunction
        -->
        <groupId>com.google.cloud.functions</groupId>
        <artifactId>function-maven-plugin</artifactId>
        <version>0.11.0</version>
        <configuration>
          <functionTarget>functions.GroovyHelloWorld</functionTarget>
        </configuration>
      </plugin>
      <plugin>
        <groupId>org.codehaus.gmavenplus</groupId>
        <artifactId>gmavenplus-plugin</artifactId>
        <version>3.0.2</version>
        <executions>
          <execution>
            <id>groovy-compile</id>
            <phase>process-resources</phase>
            <goals>
              <goal>addSources</goal>
              <goal>compile</goal>
            </goals>
          </execution>
        </executions>
        <dependencies>
          <dependency>
            <groupId>org.codehaus.groovy</groupId>
            <artifactId>groovy-all</artifactId>
            <!-- any version of Groovy \>= 1.5.0 should work here -->
            <version>${groovy.version}</version>
            <scope>runtime</scope>
            <type>pom</type>
          </dependency>
        </dependencies>
      </plugin>
    </plugins>
  </build>
</project>

Scala

<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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.example.functions</groupId>
  <artifactId>functions-scala-hello-world</artifactId>

  <properties>
    <maven.compiler.target>11</maven.compiler.target>
    <maven.compiler.source>11</maven.compiler.source>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencyManagement>
    <dependencies>
      <dependency>
        <artifactId>libraries-bom</artifactId>
        <groupId>com.google.cloud</groupId>
        <scope>import</scope>
        <type>pom</type>
        <version>26.32.0</version>
      </dependency>
    </dependencies>
  </dependencyManagement>

  <dependencies>
    <!-- Required for scala samples -->
    <dependency>
      <groupId>org.scala-lang</groupId>
      <artifactId>scala-library</artifactId>
      <version>2.13.5</version>
    </dependency>

    <!-- Required for Function primitives -->
    <dependency>
      <groupId>com.google.cloud.functions</groupId>
      <artifactId>functions-framework-api</artifactId>
      <version>1.1.0</version>
      <scope>provided</scope>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <!--
          Google Cloud Functions Framework Maven plugin

          This plugin allows you to run Cloud Functions Java code
          locally. Use the following terminal command to run a
          given function locally:

          mvn function:run -Drun.functionTarget=your.package.yourFunction
        -->
        <groupId>com.google.cloud.functions</groupId>
        <artifactId>function-maven-plugin</artifactId>
        <version>0.11.0</version>
        <configuration>
          <functionTarget>functions.ScalaHelloWorld</functionTarget>
        </configuration>
      </plugin>
      <plugin>
        <groupId>net.alchim31.maven</groupId>
        <artifactId>scala-maven-plugin</artifactId>
        <version>4.4.1</version>
        <executions>
          <execution>
            <id>scala-compile</id>
            <phase>process-resources</phase>
            <goals>
              <goal>add-source</goal>
              <goal>compile</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
</project>

Deploying the HTTP functions

Kotlin

gcloud functions deploy kotlin-helloworld --entry-point functions.KotlinHelloWorld --runtime java17 --trigger-http --allow-unauthenticated --memory 512MB

Groovy

gcloud functions deploy groovy-helloworld --entry-point functions.GroovyHelloWorld --runtime java17 --trigger-http --allow-unauthenticated --memory 512MB

Scala

gcloud functions deploy scala-helloworld --entry-point functions.ScalaHelloWorld --runtime java17 --trigger-http --allow-unauthenticated --memory 512MB

Event-driven function examples

You use event-driven functions when you want to have your Cloud Function invoked indirectly in response to an asynchronous event, such as a message on a Pub/Sub topic, a change in a Cloud Storage bucket, or a Firebase event.

There are two types of event-driven functions: background functions and CloudEvent functions. The JVM languages only support background functions.

Kotlin

import com.google.cloud.functions.BackgroundFunction
import com.google.cloud.functions.Context
import functions.eventpojos.PubsubMessage
import java.nio.charset.StandardCharsets
import java.util.Base64
import java.util.logging.Logger


class KotlinHelloPubSub : BackgroundFunction<PubsubMessage> {
    override fun accept(message: PubsubMessage, context: Context) {
        // name's default value is "world"
        var name = "world"
        if (message?.data != null) {
            name = String(
                    Base64.getDecoder().decode(message.data!!.toByteArray(StandardCharsets.UTF_8)),
                    StandardCharsets.UTF_8)
        }
        LOGGER.info(String.format("Hello %s!", name))
        return;
    }

    companion object {
        private val LOGGER = Logger.getLogger(KotlinHelloPubSub::class.java.name)
    }
}

When developing background functions, you define classes for the events triggering your functions. However, GSON marshalling may not work out of the box for Kotlin, if your event class doesn't follow certain guidelines.

In your Kotlin event class, properties must conform to these guidelines:

They are able to be set to null.
They do not have a default value assigned to them.
They are not delegate properties.

Another approach is to create your event classes in Java, and use them from your Kotlin function class.

Groovy

import com.google.cloud.functions.BackgroundFunction
import com.google.cloud.functions.Context
import functions.eventpojos.PubsubMessage
import java.nio.charset.StandardCharsets
import java.util.logging.Logger

class GroovyHelloPubSub implements BackgroundFunction<PubsubMessage> {
    private static final Logger LOGGER = Logger.getLogger(GroovyHelloPubSub.class.name)

    @Override
    void accept(PubsubMessage message, Context context) {
        // name's default value is "world"
        String name = "world"

        if (message?.data) {
             name = new String(Base64.decoder.decode(message.data), StandardCharsets.UTF_8)
        }

        LOGGER.info("Hello ${name}!")
        return
    }
}

Scala

import java.nio.charset.StandardCharsets
import java.util.Base64
import java.util.logging.Logger

import com.google.cloud.functions.{BackgroundFunction, Context}
import functions.eventpojos.PubsubMessage

class ScalaHelloPubSub extends BackgroundFunction[PubsubMessage] {

  val LOGGER = Logger.getLogger(this.getClass.getName)

  override def accept(message: PubsubMessage, context: Context): Unit = {
    // name's default value is "world"
    var name = "world"
    if (message != null && message.getData != null) {
      name = new String(Base64.getDecoder.decode(
        message.getData.getBytes(StandardCharsets.UTF_8)), StandardCharsets.UTF_8)
    }
    LOGGER.info(String.format("Hello %s!", name))
  }
}

Deploying the background functions

Kotlin

gcloud functions deploy kotlin-hello-pubsub --entry-point functions.KotlinHelloPubSub --runtime java17 --trigger-topic my-topic --allow-unauthenticated --memory 512MB

Groovy

gcloud functions deploy groovy-hello-pubsub --entry-point functions.GroovyHelloPubSub --runtime java17 --trigger-topic my-topic --allow-unauthenticated --memory 512MB

Scala

gcloud functions deploy scala-hello-pubsub --entry-point functions.ScalaHelloPubSub --runtime java17 --trigger-topic my-topic --allow-unauthenticated --memory 512MB

Test the background examples

You can test the background examples as follows:

Publish a message to your Pub/Sub Topic to trigger your function:
```
gcloud pubsub topics publish my-topic --message Flurry
```
Look at the logs:
```
gcloud functions logs read --limit 10
```

You should see something like this, with a message that includes the name you published to the Pub/Sub topic:

D      my-function  ...  Function execution started
I      my-function  ...  Hello Flurry!
D      my-function  ...  Function execution took 39 ms, finished with status: 'ok'

JVM Languages

HTTP function examples

Kotlin

Groovy

Scala

pom.xml file for HTTP examples

Kotlin

Groovy

Scala

Deploying the HTTP functions

Kotlin

Groovy

Scala

Event-driven function examples

Kotlin

Groovy

Scala

Deploying the background functions

Kotlin

Groovy

Scala

Test the background examples

`pom.xml` file for HTTP examples