We can use the environment variable SPRING_APPLICATION_JSON with a JSON value as configuration source for our Grails 3 application. The JSON value is parsed and merged with the configuration. Instead of the environment variable we can also use the Java system property spring.application.json.

Let's create a simple controller that reads the configuration property app.message:

// File: grails-app/controllers/mrhaki/grails/config/SampleController.groovy
package mrhaki.grails.config

import org.springframework.beans.factory.annotation.Value

class MessageController {

    @Value('${app.message}')
    String message

    def index() { 
        render message
    }
}

Next we start Grails and set the environment variable SPRING_APPLICATION_JSON with a value for app.message:

$ SPRING_APPLICATION_JSON='{"app":{"message":"Grails 3 is Spring Boot on steroids"}}' grails run-app
| Running application...
Grails application running at http://localhost:8080 in environment: development

When we request the sample endpoint we see the value of app.message:

http -b :8080/message
Grails 3 is Spring Boot on steroids
$

If we want to use the Java system property spring.application.json with the Grails command we must first configure the bootRun task so all system properties are passed along:

// File: build.gradle
...
bootRun {
    systemProperties System.properties
}
...

With the following command we pass the configuration as inline JSON:

$ grails -Dspring.application.json='{"app":{"message":"Grails 3 is Spring Boot on steroids"}}' run-app
| Running application...
Grails application running at http://localhost:8080 in environment: development

Written with Grails 3.1.8.