9.1.2 FunctionBean

To write your function’s behavior, annotate your class with the @FunctionBean annotation. Your class must also implement one of the interfaces from the java.util.function package.

The following examples implement Java’s Supplier functional interface.

Example Java Function

package example;
import io.micronaut.function.FunctionBean;
import java.util.function.Supplier;
@FunctionBean("hello-world-java")
public class HelloJavaFunction implements Supplier<String> {
    @Override
    public String get() { (1)
        return "Hello world!";
    }
}

Alternatively you can also define a Factory that returns a Java lambda:

Example Java Function as a Lambda

package example;
import io.micronaut.context.annotation.*;
import io.micronaut.function.FunctionBean;
import java.util.function.Supplier;
@Factory (1)
public class MyFunctions {
    @FunctionBean("hello-world-java")
    public Supplier<String> helloWorld() { (2)
        return () -> "Hello world!";
    }
}

If you are using Kotlin then process is exactly the same:

Example Kotlin Function

package example
import io.micronaut.function.FunctionBean
import java.util.function.Supplier
@FunctionBean("hello-world-kotlin")
class HelloKotlinFunction : Supplier<String> {
    override fun get(): String { (1)
        return "Hello world!"
    }
}

The following table summarizes the supported interfaces:

In addition, functions have an input and/or an output. The input is represented by the accepted argument and represents the body consumed by the function and the output is represented by the return value of the function. The input and the output should be either a Java primitive or simple type (int, String etc.) or a POJO.

Often, you want to accept a POJO and return a POJO. Use java.util.function.Function to accept a single argument and return a single result.

import io.micronaut.function.FunctionBean;
import java.util.function.Function;
@FunctionBean("isbn-validator")
public class IsbnValidatorFunction implements Function<IsbnValidationRequest, IsbnValidationResponse> {
    @Override
    public IsbnValidationResponse apply(IsbnValidationRequest request) {
        return new IsbnValidationResponse();
    }
}

A single project can define multiple functions, however only a single function should be configured for execution by the application. This can be configured using the micronaut.function.name property in application.yml:

Configuring the Function Name to Execute

micronaut:
    function:
        name: hello-world-java

Alternatively you can specify the value when running your function (for example in the Dockerfile) either as an environment variable:

Specifying the Function to Execute as a Environment variable

$ export MICRONAUT_FUNCTION_NAME=hello-world-java
$ java -jar build/libs/hello-world-function-all.jar

Or as a system property:

Specifying the Function to Execute as a System property

$ java -Dmicronaut.function.name=hello-world-java -jar build/libs/hello-world-function-all.jar