Serialization strategies

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Serialization Strategies Deep Dive

Overview

Serialization converts Kotlin objects to/from JSON. Performance and flexibility vary across libraries.

Core Concepts

Serialization Libraries

Gson (Google): - Reflection-based - Flexible, lenient parsing - No compile-time checks - mature, widely used

Moshi (Square): - Reflection + type adapters - Customizable parsing - Better error messages - faster than Gson

Kotlin Serialization (JetBrains): - Compiler plugin generates code - No reflection needed - Compile-time safety - Newer, growing adoption

Internal Implementation

Gson vs Moshi vs Kotlin Serialization Comparison

Aspect	Gson	Moshi	Kotlin Serialization
Speed	250μs	150μs	50μs
Reflection	Yes	Partial	No
Type Safety	Runtime	Runtime	Compile-time
Error Messages	Good	Excellent	Good
Setup	Easy	Medium	Medium

Request/Response Flow

1. OkHttp receives response bytes
2. Retrofit converter deserializes
3. Object passed to collecting flow/callback

Code Examples

Custom Gson Serializer

data class User(val id: Int, val createdAt: Date)

class DateSerializer : JsonSerializer<Date>,
    JsonDeserializer<Date> {
    private val fmt = SimpleDateFormat("yyyy-MM-dd")

    override fun serialize(src: Date?, typeOfSrc: Type?,
        context: JsonSerializationContext?) = JsonPrimitive(fmt.format(src))

    override fun deserialize(json: JsonElement?, typeOfT: Type?,
        context: JsonDeserializationContext?) = 
        fmt.parse(json?.asString ?: "")
}

Common Interview Questions

Q: Why is Kotlin Serialization faster? A: Compiler generates code instead of using reflection at runtime.

Q: Should I migrate from Gson? A: Only if performance is critical. Gson ecosystem is larger.

Production Considerations

Backwards Compatibility

When API adds fields:

@Serializable
data class User(
    val id: Int,
    val name: String,
    @Transient  // skip serialization
    val newFeature: String = "default"
)

Performance Insights

Caching Serializers

Create serializers once:

val gson = GsonBuilder()
    .registerTypeAdapter(...)
    .build()

// reuse
val user = gson.fromJson(json, User::class.java)

Senior-Level Insights

Version Stability

Track API schema versions:

// v1 field removed, v2 added
if (apiVersion >= 2) {
    useNewField()
}