Custom config parser for more complex config structure (corda#3513)

* custom config parser for more complex config structure

* address PR issues

node-api/src/main/kotlin/net/corda/nodeapi/internal/config/ConfigUtilities.kt

@@ -25,13 +25,26 @@ import java.util.*
 import kotlin.reflect.KClass
 import kotlin.reflect.KProperty
 import kotlin.reflect.KType
+import kotlin.reflect.full.createInstance
+import kotlin.reflect.full.findAnnotation
 import kotlin.reflect.full.memberProperties
 import kotlin.reflect.full.primaryConstructor
 import kotlin.reflect.jvm.jvmErasure
 
 @Target(AnnotationTarget.PROPERTY)
 annotation class OldConfig(val value: String)
 
+/**
+ * This annotation can be used to provide ConfigParser for the class,
+ * the [parseAs] method will use the provided parser instead of data class constructs to parse the object.
+ */
+@Target(AnnotationTarget.CLASS)
+annotation class CustomConfigParser(val parser:  KClass<out ConfigParser<*>>)
+
+interface ConfigParser<T> {
+    fun parse(config: Config): T
+}
+
 const val CUSTOM_NODE_PROPERTIES_ROOT = "custom"
 
 // TODO Move other config parsing to use parseAs and remove this
@@ -40,7 +53,10 @@ operator fun <T : Any> Config.getValue(receiver: Any, metadata: KProperty<*>): T
 }
 
 fun <T : Any> Config.parseAs(clazz: KClass<T>, onUnknownKeys: ((Set<String>, logger: Logger) -> Unit) = UnknownConfigKeysPolicy.FAIL::handle, nestedPath: String? = null): T {
-    require(clazz.isData) { "Only Kotlin data classes can be parsed. Offending: ${clazz.qualifiedName}" }
+    // Use custom parser if provided, instead of treating the object as data class.
+    clazz.findAnnotation<CustomConfigParser>()?.let { return uncheckedCast(it.parser.createInstance().parse(this)) }
+
+    require(clazz.isData) { "Only Kotlin data classes or class annotated with CustomConfigParser can be parsed. Offending: ${clazz.qualifiedName}" }
     val constructor = clazz.primaryConstructor!!
     val parameters = constructor.parameters
     val parameterNames = parameters.flatMap { param ->

node-api/src/test/kotlin/net/corda/nodeapi/internal/config/ConfigParsingTest.kt

@@ -227,6 +227,36 @@ class ConfigParsingTest {
         }
     }
 
+    @Test
+    fun `parse with provided parser`() {
+        val type1Config = mapOf("type" to "1", "value" to "type 1 value")
+        val type2Config = mapOf("type" to "2", "value" to "type 2 value")
+
+        val configuration = config("values" to listOf(type1Config, type2Config))
+        val objects = configuration.parseAs<TestObjects>()
+
+        assertThat(objects.values).containsExactly(TestObject.Type1("type 1 value"), TestObject.Type2("type 2 value"))
+    }
+
+    class TestParser : ConfigParser<TestObject> {
+        override fun parse(config: Config): TestObject {
+            val type = config.getInt("type")
+            return when (type) {
+                1 -> config.parseAs<TestObject.Type1>(onUnknownKeys = UnknownConfigKeysPolicy.IGNORE::handle)
+                2 -> config.parseAs<TestObject.Type2>(onUnknownKeys = UnknownConfigKeysPolicy.IGNORE::handle)
+                else -> throw IllegalArgumentException("Unsupported Object type : '$type'")
+            }
+        }
+    }
+
+    data class TestObjects(val values: List<TestObject>)
+
+    @CustomConfigParser(TestParser::class)
+    sealed class TestObject {
+        data class Type1(val value: String) : TestObject()
+        data class Type2(val value: String) : TestObject()
+    }
+
     private inline fun <reified S : SingleData<V>, reified L : ListData<V>, V : Any> testPropertyType(
             value1: V,
             value2: V,
@@ -310,6 +340,7 @@ class ConfigParsingTest {
             require(positive > 0) { "$positive is not positive" }
         }
     }
+
     data class OldData(
             @OldConfig("oldValue")
             val newValue: String)