Why FastJson Crashes with Kotlin Data Classes and How to Fix It

Introduction

Since Google released Kotlin in 2017, Android development has shifted from Java to Kotlin. Many legacy projects still use Java data classes and JSON parsers such as FastJson, Gson, etc. When migrating to Kotlin data classes, developers encounter parsing errors, especially after enabling code obfuscation.

Common JSON Parsing Frameworks

FastJson : Alibaba's JSON library, no longer maintained (last version 1.2.83, 2022). Supports Kotlin partially.

FastJson2 : Successor released in 2022, supports Android4, but still relies on reflection.

Gson : Google’s JSON library, uses reflection.

Kotlinx-serialization : Official Kotlin serialization library, generates code at compile time, no reflection.

Problem Scenario

Parsing a JSON string into a Kotlin data class fails with default constructor not found because Kotlin data classes generate only a primary constructor and do not provide a no‑arg constructor or setter methods for val properties. When ProGuard removes the default constructor, FastJson cannot instantiate the class.

FastJson Solution

Modify the data class to use var with default values, add a no‑arg constructor, and keep the class from being obfuscated:

data class ComplexEntity(
    var id: Int = 0,
    var name: String = "",
    var score: Float = 0f,
    var userInfo: UserInfo = UserInfo()
)

Add ProGuard rules:

-keep class kotlin.reflect.jvm.** {*;}
-keep class * implements java.io.Serializable {*;}

FastJson2

FastJson2 behaves the same; it also requires kotlin-reflect for Kotlin data classes. Without it, parsing fails when a JSON field is null for a non‑nullable Kotlin property.

Gson

Gson can parse Kotlin data classes but will assign null to non‑nullable properties, causing runtime exceptions.

Kotlinx-serialization

Kotlinx-serialization does not use reflection. Annotate data classes with @Serializable and add the Kotlin serialization plugin. Example:

@kotlinx.serialization.Serializable
data class ComplexEntity(
    val id: Int,
    val name: String,
    val score: Float,
    val userInfo: UserInfo,
    val default: Int = 1223
)

Configure the JSON parser:

val json = Json {
    ignoreUnknownKeys = true
    coerceInputValues = true
}

Serialize and deserialize:

val jsonString = json.encodeToString(complexEntity)
val obj = json.decodeFromString<ComplexEntity>(jsonString)

Utility Wrapper

A Kotlin utility object provides safe APIs for parsing JSON strings to objects, converting objects to JsonElement, and extracting primitive values:

inline fun <reified T> parseObject(jsonStr: String?): T? = jsonStr?.let {
    try { json.decodeFromString<T>(it) } catch (e: Exception) { null }
}

Additional extension functions for JsonElement such as containsKey, getString, getInt, and getJsonArray simplify data extraction.

Conclusion

When working with Kotlin data classes in Android, FastJson and Gson require reflection and careful ProGuard configuration, while Kotlinx-serialization offers a lightweight, reflection‑free alternative. Refactoring data classes to provide default values or using var properties resolves most parsing crashes.