Ktor 3.2.0 Config EAP

Ktor 3.2.0 will add support for automatically deserializing configuration files into data classes. Let's define a simple application.yaml that defines some database configuration.

database:
    driverClassName: "$DB_DRIVER_CLASS_NAME:org.postgresql.Driver"
    host: "$DB_HOST:localhost"
    port: "$DATABASE_PORT:5432"
    name: "$DB_NAME:ktor_sample"
    username: "$DB_USERNAME:ktor_user"
    password: "$DB_PASSWORD:ktor_password"
    flyway:
        locations: "$FLYWAY_LOCATIONS:classpath:db/migration"
        baselineOnMigrate: "$FLYWAY_BASELINE_ON_MIGRATE:true"

Note: Ktor HOCON/YAML supports overriding from enviroment variables using: "$ENVIROMENT_KEY:fallback-value" as shown above.

In order to deserialise into structure data, we need to define it first. So we define 2 data classes that match our application.yaml.

@Serializable
data class DatabaseConfig(
    val driverClassName: String,
    val host: String,
    val port: Int,
    val name: String,
    val username: String,
    val password: String,
    val flyway: FlywayConfig
)

@Serializable data class FlywayConfig(val locations: String, val baselineOnMigrate: Boolean)

We need to make sure that the names of the data class properties match the fields defined in our yaml. Or you'll see errors like MissingRequiredPropertyException or UnknownPropertyException depending on the kind-of missmatch.

To deserialise we need to explicitely extract the desired property from the configuration, this can be done directly within the Application DSL.

fun Application.module() {
    property<DatabaseConfig>("database")
    
    // Can also retrieve nested fields/objects
    val flyway = property<FlywayConfig>("database.flyway")
}

In the test enviroment you can continue leveraging, and overwrite certain values.

fun test() = testApplication {
   enviroment {
       config = ApplicationConfig("application.yaml")
           .mergeWith(MapApplicationConfig(
               "database.host" to "other-host",
               "database.username" to "SA",
               "database.password" to ""
           ))
   } 
}

See Ktor EAP for more details.

nomisRev/Ktor3.2EAP.md

Ktor 3.2.0 Config EAP