GH-220 Generate "nullable: true" for types explicitly marked as nullable (Resolve #220)

Author: dzikoysk

examples/javalin-gradle-kotlin/src/main/kotlin/io/javalin/openapi/plugin/test/KotlinEntity.kt

@@ -13,7 +13,8 @@ data class KotlinEntity(
     val name: String,
     val primitive: Int,
     val custom: Elements,
-    val oneOfResult: Result
+    val oneOfResult: Result,
+    val nullable: Any?,
 )
 
 @JsonSchema(

openapi-annotation-processor/src/test/kotlin/io/javalin/openapi/processor/ComponentAnnotationsTest.kt

@@ -67,6 +67,7 @@ internal class ComponentAnnotationsTest : OpenApiAnnotationProcessorSpecificatio
                     "properties": {
                         "testProperty": {
                             "type": "number",
+                            "nullable": true,
                             "format": "double"
                         }
                     }

openapi-specification/src/main/kotlin/io/javalin/openapi/experimental/processor/generators/TypeSchemaGenerator.kt

@@ -278,13 +278,25 @@ internal fun ClassDefinition.findAllProperties(requireNonNulls: Boolean): Collec
                 else -> requireNonNulls && isNotNull
             }
 
+            val extra = mutableMapOf<String, Any?>()
+
+            val isExplicitlyNullable = when {
+                customType?.nullability == Nullability.NULLABLE -> true
+                property.hasAnnotation("Nullable") -> true
+                else -> false
+            }
+
+            if (isExplicitlyNullable) {
+                extra["nullable"] = true
+            }
+
             properties.add(
                 Property(
                     name = name,
                     type = propertyType.toClassDefinition(),
                     composition = findCompositionInElement(context, property),
                     required = required,
-                    extra = property.findExtra(context)
+                    extra = extra + property.findExtra(context)
                 )
             )
         }