Here's a refactoring of the JSON Schema code to enhance clarity and maintainability.

src/main/kotlin/wu/seal/jsontokotlin/model/ConfigManager.kt

@@ -2,13 +2,26 @@ package wu.seal.jsontokotlin.model
 
 import com.intellij.ide.util.PropertiesComponent
 import wu.seal.jsontokotlin.test.TestConfig
+import java.math.BigDecimal
+import java.time.LocalDate
+import java.time.LocalTime
+import java.time.OffsetDateTime
 
 /**
  * Config Manager
  * Created by Seal.Wu on 2018/2/7.
  */
 object ConfigManager : IConfigManager {
 
+    //https://json-schema.org/understanding-json-schema/reference/string.html#format
+    val JSON_SCHEMA_FORMAT_MAPPINGS = mapOf(
+        "date-time" to OffsetDateTime::class.java.canonicalName,
+        "date" to LocalDate::class.java.canonicalName,
+        "time" to LocalTime::class.java.canonicalName,
+        "decimal" to BigDecimal::class.java.canonicalName
+        //here can be another formats
+    )
+
     private const val INDENT_KEY = "json-to-kotlin-class-indent-space-number"
     private const val ENABLE_MAP_TYP_KEY = "json-to-kotlin-class-enable-map-type"
     private const val ENABLE_MINIMAL_ANNOTATION = "json-to-kotlin-class-enable-minimal-annotation"

src/main/kotlin/wu/seal/jsontokotlin/model/jsonschema/JsonObjectDef.kt

@@ -2,46 +2,121 @@ package wu.seal.jsontokotlin.model.jsonschema
 
 import com.google.gson.annotations.SerializedName
 
+/**
+ * Represents a base JSON object definition in a JSON Schema.
+ * This class includes common properties found in JSON Schema objects, such as `id`, `\$ref`,
+ * `title`, `description`, `type`, and validation keywords like `properties`, `oneOf`, etc.
+ *
+ * See JSON Schema Specification: [https://json-schema.org/understanding-json-schema/](https://json-schema.org/understanding-json-schema/)
+ */
 open class JsonObjectDef(
-    //See: https://json-schema.org/understanding-json-schema/structuring.html
+    /**
+     * The `\$id` keyword defines a URI for the schema, and the base URI that other URI references within the schema are resolved against.
+     * See: [https://json-schema.org/understanding-json-schema/structuring.html#the-id-property](https://json-schema.org/understanding-json-schema/structuring.html#the-id-property)
+     */
     @SerializedName("\$id")
     val id: String? = null,
+
+    /**
+     * The `\$ref` keyword is used to reference another schema.
+     * This allows for reusing parts of schemas or creating complex recursive schemas.
+     * See: [https://json-schema.org/understanding-json-schema/structuring.html#ref](https://json-schema.org/understanding-json-schema/structuring.html#ref)
+     */
     @SerializedName("\$ref")
     val ref: String? = null,
 
+    /**
+     * The `title` keyword provides a short, human-readable summary of the schema's purpose.
+     */
     val title: String? = null,
+
+    /**
+     * The `description` keyword provides a more detailed explanation of the schema's purpose.
+     */
     val description: String? = null,
 
-    /** type may contains a string or an array of string (ArrayList),
-     * where usually the first entry is "null" (property isTypeNullable)
-     * and the second entry is the type string (property typeString)
-     * */
-    protected val type: Any? = null,
+    /**
+     * The `type` keyword defines the data type for a schema.
+     * It can be a string (e.g., "object", "string", "number") or a list of strings (e.g., ["string", "null"]).
+     * This property stores the raw value from JSON, which can be a String or a List.
+     * Use [typeString] to get the actual type name and [isTypeNullable] to check for nullability.
+     * See: [https://json-schema.org/understanding-json-schema/reference/type.html](https://json-schema.org/understanding-json-schema/reference/type.html)
+     */
+    protected val type: Any? = null, /* String or List<String> */
 
+    /**
+     * The `properties` keyword defines a map of property names to their schema definitions for an object type.
+     * See: [https://json-schema.org/understanding-json-schema/reference/object.html#properties](https://json-schema.org/understanding-json-schema/reference/object.html#properties)
+     */
     val properties: Map<String, PropertyDef>? = null,
-    val additionalProperties: Any? = null,
+
+    /**
+     * The `additionalProperties` keyword controls whether additional properties are allowed in an object,
+     * and can also define a schema for those additional properties.
+     * It can be a boolean (true/false) or a schema object.
+     * See: [https://json-schema.org/understanding-json-schema/reference/object.html#additionalproperties](https://json-schema.org/understanding-json-schema/reference/object.html#additionalproperties)
+     */
+    val additionalProperties: Any? = null, // Boolean or PropertyDef
+
+    /**
+     * The `required` keyword specifies an array of property names that must be present in an object.
+     * See: [https://json-schema.org/understanding-json-schema/reference/object.html#required](https://json-schema.org/understanding-json-schema/reference/object.html#required)
+     */
     val required: Array<String>? = null,
 
-    /** See: https://json-schema.org/understanding-json-schema/reference/combining.html */
+    /**
+     * The `oneOf` keyword specifies that an instance must be valid against exactly one of the subschemas in the array.
+     * See: [https://json-schema.org/understanding-json-schema/reference/combining.html#oneof](https://json-schema.org/understanding-json-schema/reference/combining.html#oneof)
+     */
     val oneOf: Array<PropertyDef>? = null,
+
+    /**
+     * The `allOf` keyword specifies that an instance must be valid against all of the subschemas in the array.
+     * See: [https://json-schema.org/understanding-json-schema/reference/combining.html#allof](https://json-schema.org/understanding-json-schema/reference/combining.html#allof)
+     */
     val allOf: Array<PropertyDef>? = null,
+
+    /**
+     * The `anyOf` keyword specifies that an instance must be valid against at least one of the subschemas in the array.
+     * See: [https://json-schema.org/understanding-json-schema/reference/combining.html#anyof](https://json-schema.org/understanding-json-schema/reference/combining.html#anyof)
+     */
     val anyOf: Array<PropertyDef>? = null,
+
+    /**
+     * The `not` keyword specifies that an instance must not be valid against the given subschema.
+     * See: [https://json-schema.org/understanding-json-schema/reference/combining.html#not](https://json-schema.org/understanding-json-schema/reference/combining.html#not)
+     */
     val not: Array<PropertyDef>? = null,
 
+    /**
+     * Custom extension property `x-abstract`. If true, suggests this schema definition is intended
+     * as an abstract base and might not be instantiated directly.
+     */
     @SerializedName("x-abstract")
     val x_abstract: Boolean? = null
 
 ) {
 
-    /** returns correct JsonSchema type as string */
+    /**
+     * Gets the primary JSON Schema type as a string.
+     * If the `type` property is an array (e.g., `["null", "string"]`), this returns the first non-"null" type.
+     * If `type` is a single string, it returns that string.
+     * Returns `null` if the type cannot be determined or is explicitly "null" without other types.
+     */
     val typeString: String?
-        get() = if (type is ArrayList<*>) type.first { it != "null" } as String else type as? String
+        get() = if (type is ArrayList<*>) type.firstOrNull { it != "null" } as? String else type as? String
 
-    /** returns true if the object can be null */
+    /**
+     * Checks if the schema definition allows for a "null" type.
+     * This can be true if:
+     * - The `type` property is an array containing "null" (or actual `null`).
+     * - Any subschema under `oneOf` allows for a "null" type.
+     * - The [typeString] itself is "null" or `null`.
+     */
     val isTypeNullable: Boolean
         get() = when {
             type is ArrayList<*> -> type.any { it == null || it == "null" }
-            oneOf?.any { it.type == null || it.type == "null" } == true -> true
+            oneOf?.any { it.type == null || (it.type as? String) == "null" || (it.type is ArrayList<*> && it.type.any { subType -> subType == null || subType == "null"}) } == true -> true
             else -> typeString == null || typeString == "null"
         }
 

src/main/kotlin/wu/seal/jsontokotlin/model/jsonschema/JsonSchema.kt

@@ -2,7 +2,19 @@ package wu.seal.jsontokotlin.model.jsonschema
 
 import com.google.gson.annotations.SerializedName
 
-// See specification: https://json-schema.org/understanding-json-schema/reference/object.html
+/**
+ * Represents a JSON schema document.
+ *
+ * This class models the structure of a JSON schema, allowing for parsing and resolving
+ * references (`$ref`) within the schema. It supports standard schema keywords like
+ * `definitions`, `$defs`, and `properties`.
+ *
+ * See JSON Schema Specification: [https://json-schema.org/understanding-json-schema/](https://json-schema.org/understanding-json-schema/)
+ *
+ * @property schema The value of the `\$schema` keyword, indicating the schema dialect.
+ * @property definitions A map of definitions, primarily used in older JSON schema versions.
+ * @property defs A map of definitions, introduced in Draft 2019-09 as a replacement for `definitions`.
+ */
 class JsonSchema(
     @SerializedName("\$schema")
     val schema: String? = null,
@@ -14,47 +26,106 @@ class JsonSchema(
   // Get a combined map of both definitions and $defs
   // This allows backward compatibility with older schema versions
   private val allDefinitions: Map<String, PropertyDef>
-    get() {
-      val combinedMap = definitions.toMutableMap()
-      defs?.let { combinedMap.putAll(it) }
-      return combinedMap
-    }
-
-  //See: https://json-schema.org/understanding-json-schema/structuring.html
+    get() = definitions + (defs ?: emptyMap())
+
+  /**
+   * Resolves a JSON Pointer reference (`$ref`) to a [PropertyDef] within this schema.
+   *
+   * Currently, only local references (starting with `#`) are supported.
+   * Examples of supported $ref formats:
+   * - `#/definitions/MyType`
+   * - `#/\$defs/AnotherType`
+   * - `#/properties/user/properties/address`
+   * - `#MyObject` (if `MyObject` is an id of a definition at the root)
+   *
+   * See JSON Schema structuring: [https://json-schema.org/understanding-json-schema/structuring.html](https://json-schema.org/understanding-json-schema/structuring.html)
+   *
+   * @param ref The JSON Pointer reference string (e.g., `#/definitions/User`).
+   * @return The resolved [PropertyDef].
+   * @throws IllegalArgumentException if the `ref` string is malformed (e.g., too short).
+   * @throws NotImplementedError if the `ref` points to a non-local definition (doesn't start with '#').
+   * @throws ClassNotFoundException if the definition pointed to by `ref` cannot be found.
+   */
   fun resolveDefinition(ref: String): PropertyDef {
     if (ref.length < 2) throw IllegalArgumentException("Bad ref: $ref")
     if (!ref.startsWith("#")) throw NotImplementedError("Not local definitions are not supported (ref: $ref)")
 
+    return resolveLocalDefinition(ref)
+  }
+
+  /**
+   * Resolves a local definition path (starting with '#').
+   * @param ref The definition reference string.
+   * @return The resolved [PropertyDef].
+   * @throws ClassNotFoundException if the definition is not found.
+   * @throws NotImplementedError if the path structure is not supported.
+   * @throws IllegalArgumentException if the path contains unknown components.
+   */
+  private fun resolveLocalDefinition(ref: String): PropertyDef {
     val path = ref.split('/')
     return when {
-      path.count() == 1 -> allDefinitions.values.firstOrNull { it.id == path[0] }
-          ?: throw ClassNotFoundException("Definition $ref not found")
-      path[1] == "definitions" -> definitions[path[2]] 
-          ?: throw ClassNotFoundException("Definition $ref not found")
-      path[1] == "\$defs" -> defs?.get(path[2]) 
+      path.count() == 1 -> allDefinitions.values.firstOrNull { it.id == path[0] } // TODO: This could be path[0].substring(1) if # is always present
           ?: throw ClassNotFoundException("Definition $ref not found")
+      path[1] == "definitions" -> findDefinitionInMaps(path[2], definitions, ref, "definitions")
+      path[1] == "\$defs" -> findDefinitionInMaps(path[2], defs, ref, "\$defs")
       path[1] == "properties" -> {
-        var property: PropertyDef = properties?.get(path[2])
-            ?: throw ClassNotFoundException("Definition $ref not found")
-        val iterator = path.subList(3, path.count()).iterator()
-        do {
-          val next = iterator.next()
-          property = when (next) {
-            "properties" -> {
-              val propName = iterator.next()
-              property.properties?.get(propName)
-                  ?: throw ClassNotFoundException("Definition $propName not found at path $ref")
-            }
-            "items" -> property.items ?: throw ClassNotFoundException("Definition $next not found at path $ref")
-            else -> throw IllegalArgumentException("Unknown json-object property $next not found at path $ref")
-          }
-        } while (iterator.hasNext())
-
-        property
+        val initialProperty = properties?.get(path[2])
+            ?: throw ClassNotFoundException("Definition '${path[2]}' not found in properties at path $ref")
+        findPropertyRecursive(initialProperty, path.subList(3, path.count()).iterator(), ref)
       }
       else -> throw NotImplementedError("Cannot resolve ref path: $ref")
     }
   }
 
+  /**
+   * Finds a definition in the provided map.
+   * @param defName The name of the definition to find.
+   * @param map The map to search in (can be nullable, e.g. `defs`).
+   * @param originalRef The original reference string (for error reporting).
+   * @param mapName The name of the map being searched (for error reporting).
+   * @return The resolved [PropertyDef].
+   * @throws ClassNotFoundException if the definition is not found in the map.
+   */
+  private fun findDefinitionInMaps(
+      defName: String,
+      map: Map<String, PropertyDef>?,
+      originalRef: String,
+      mapName: String
+  ): PropertyDef {
+    return map?.get(defName)
+        ?: throw ClassNotFoundException("Definition '$defName' not found in '$mapName' at path $originalRef")
+  }
+
+  /**
+   * Recursively traverses properties based on the path segments.
+   * @param currentProperty The current [PropertyDef] being inspected.
+   * @param pathIterator An iterator for the remaining path segments.
+   * @param originalRef The original reference string (for error reporting).
+   * @return The resolved [PropertyDef].
+   * @throws ClassNotFoundException if a segment in the path is not found.
+   * @throws IllegalArgumentException if an unknown path segment is encountered.
+   */
+  private fun findPropertyRecursive(
+      currentProperty: PropertyDef,
+      pathIterator: Iterator<String>,
+      originalRef: String
+  ): PropertyDef {
+    var property = currentProperty
+    while (pathIterator.hasNext()) {
+      val segment = pathIterator.next()
+      property = when (segment) {
+        "properties" -> {
+          if (!pathIterator.hasNext()) throw IllegalArgumentException("Missing property name after 'properties' in $originalRef")
+          val propName = pathIterator.next()
+          property.properties?.get(propName)
+            ?: throw ClassNotFoundException("Property '$propName' not found under '${property.id ?: "unknown"}' at path $originalRef")
+        }
+        "items" -> property.items
+            ?: throw ClassNotFoundException("Property 'items' not found under '${property.id ?: "unknown"}' at path $originalRef")
+        else -> throw IllegalArgumentException("Unknown json-object property '$segment' in path $originalRef")
+      }
+    }
+    return property
+  }
 }