An open source FaaS (Function as a service) framework for writing portable Java functions -- brought to you by the Google Cloud Functions team.

The Functions Framework lets you write lightweight functions that run in many different environments, including:

• Google Cloud Functions
• Your local development machine
• Cloud Run and Cloud Run for Anthos
• Knative-based environments

The Functions Framework for Java uses Java and Maven (the mvn command), for building and deploying functions from source.

However, it is also possible to build your functions using Gradle, as JAR archives, that you will deploy with the gcloud command-line.

A function is typically structured as a Maven project. We recommend using an IDE that supports Maven to create the Maven project. Add this dependency in the pom.xml file of your project:

    <dependency>
      <groupId>com.google.cloud.functions</groupId>
      <artifactId>functions-framework-api</artifactId>
      <version>1.0.4</version>
      <scope>provided</scope>
    </dependency>

If you are using Gradle to build your functions, you can define the Functions Framework dependency in your build.gradle project file as follows:

    dependencies {
        implementation 'com.google.cloud.functions:functions-framework-api:1.0.4'
    }

Create a file src/main/java/com/example/HelloWorld.java with the following contents:

package com.example;

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

public class HelloWorld implements HttpFunction {
  @Override
  public void service(HttpRequest request, HttpResponse response)
      throws Exception {
    response.getWriter().write("Hello, World\n");
  }
}

There are two ways to write a Background function, which differ in how the payload of the incoming event is represented. In a "raw" background function this payload is presented as a JSON-encoded Java string. In a "typed" background function the Functions Framework deserializes the JSON payload into a Plain Old Java Object (POJO).

Create a file src/main/java/com/example/Background.java with the following contents:

package com.example;

import com.google.cloud.functions.Context;
import com.google.cloud.functions.RawBackgroundFunction;
import com.google.gson.Gson;
import com.google.gson.JsonObject;
import java.util.logging.Logger;

public class Background implements RawBackgroundFunction {
  private static final Logger logger =
      Logger.getLogger(Background.class.getName());

  @Override
  public void accept(String json, Context context) {
    Gson gson = new Gson();
    JsonObject jsonObject = gson.fromJson(json, JsonObject.class);
    logger.info("Received JSON object: " + jsonObject);
  }
}

Create a file src/main/java/com/example/PubSubBackground with the following contents:

package com.example;

import com.google.cloud.functions.BackgroundFunction;
import com.google.cloud.functions.Context;
import java.util.Map;
import java.util.logging.Logger;

// This is the Pub/Sub message format from the Pub/Sub emulator. 
class PubSubMessage {
  String data;
  Map<String, String> attributes;
  String messageId;
  String publishTime;
}

public class PubSubBackground implements BackgroundFunction<PubSubMessage> {
  private static final Logger logger =
      Logger.getLogger(PubSubBackground.class.getName());

  @Override
  public void accept(PubSubMessage pubSubMessage, Context context) {
    logger.info("Received message with id " + context.eventId());
  }
}

The Maven plugin called function-maven-plugin allows you to run functions on your development machine.

You can configure the plugin in pom.xml:

<plugin>
  <groupId>com.google.cloud.functions</groupId>
  <artifactId>function-maven-plugin</artifactId>
  <version>0.10.1</version>
  <configuration>
    <functionTarget>com.example.HelloWorld</functionTarget>
  </configuration>
</plugin>

Then run it from the command line:

mvn function:run

You can alternatively configure the plugin with properties on the command line:

  mvn com.google.cloud.functions:function-maven-plugin:0.10.1:run \
      -Drun.functionTarget=com.example.HelloWorld

You can also run a function by using the Functions Framework jar directly. Copy the Functions Framework jar to a local location like this:

mvn dependency:copy \
    -Dartifact='com.google.cloud.functions.invoker:java-function-invoker:1.1.1' \
    -DoutputDirectory=.

In this example we use the current directory . but you can specify any other directory to copy to. Then run your function:

java -jar java-function-invoker-1.1.1 \
    --classpath myfunction.jar \
    --target com.example.HelloWorld

From Gradle, similarily to running functions with the Functions Framework jar, we can invoke the Invoker class with a JavaExec task.

configurations {
    invoker
}

dependencies {
    implementation 'com.google.cloud.functions:functions-framework-api:1.0.4'
    invoker 'com.google.cloud.functions.invoker:java-function-invoker:1.1.1'
}

tasks.register("runFunction", JavaExec) {
    main = 'com.google.cloud.functions.invoker.runner.Invoker'
    classpath(configurations.invoker)
    inputs.files(configurations.runtimeClasspath, sourceSets.main.output)
    args(
            '--target', project.findProperty('run.functionTarget'),
            '--port', project.findProperty('run.port') ?: 8080
    )
    doFirst {
        args('--classpath', files(configurations.runtimeClasspath, sourceSets.main.output).asPath)
    }
}

Then in your terminal or IDE, you will be able to run the function locally with:

gradle runFunction -Prun.functionTarget=com.example.HelloWorld \
                   -Prun.port=8080

Or if you use the Gradle wrapper provided by your Gradle project build:

./gradlew runFunction -Prun.functionTarget=com.example.HelloWorld \
                      -Prun.port=8080

There are a number of options that can be used to configure the Functions Framework, whether run directly or on the command line.

A function is a Java class. You must specify the name of that class when running the Functions Framework:

--target com.example.HelloWorld
<functionTarget>com.example.HelloWorld</functionTarget>
-Drun.functionTarget=com.example.HelloWorld
-Prun.functionTarget=com.example.HelloWorld

• Invoker argument: --target com.example.HelloWorld
• Maven pom.xml: <functionTarget>com.example.HelloWorld</functionTarget>
• Maven CLI argument: -Drun.functionTarget=com.example.HelloWorld
• Gradle CLI argument: -Prun.functionTarget=com.example.HelloWorld

The Functions Framework is an HTTP server that directs incoming HTTP requests to the function code. By default this server listens on port 8080. Specify an alternative value like this:

• Invoker argument: --port 12345
• Maven pom.xml: <port>12345</port>
• Maven CLI argument: -Drun.port=12345
• Gradle CLI argument: -Prun.port=12345

Function code runs with a classpath that includes the function code itself and its dependencies. The Maven plugin automatically computes the classpath based on the dependencies expressed in pom.xml. When invoking the Functions Framework directly, you must use --classpath to indicate how to find the code and its dependencies. For example:

java -jar java-function-invoker-1.1.1 \
    --classpath 'myfunction.jar:/some/directory:/some/library/*' \
    --target com.example.HelloWorld

The --classpath option works like java -classpath. It is a list of entries separated by : (; on Windows), where each entry is:

• a directory, in which case class com.example.Foo is looked for in a file com/example/Foo.class under that directory;
• a jar file, in which case class com.example.Foo is looked for in a file com/example/Foo.class in that jar file;
• a directory followed by /* (\* on Windows), in which case each jar file in that directory (file called foo.jar) is treated the same way as if it had been named explicitly.

Specifying the right classpath can be tricky. A simpler alternative is to build the function as a "fat jar", where the function code and all its dependencies are in a single jar file. Then --classpath myfatfunction.jar is enough. An example of how this is done is the Functions Framework jar itself, as seen here.

Alternatively, you can arrange for your jar to have its own classpath, as described here.