Moshi is a modern JSON library for Android and Java created by Square (makers of Retrofit). It's designed for Kotlin with built-in null safety support and is faster and smaller than Gson.

Should I use Moshi or Gson for Android?

Use Moshi for new Android projects. Moshi respects Kotlin null safety, has better performance, and integrates seamlessly with Retrofit. Gson uses unsafe reflection that can cause runtime crashes with Kotlin.

What is @JsonClass in Moshi?

@JsonClass(generateAdapter = true) tells Moshi to generate a compile-time adapter for your class. This is faster than reflection and works with ProGuard/R8 without extra rules.

How to handle null values in Moshi?

Declare nullable fields with '?' in Kotlin: val name: String?. Moshi respects Kotlin's null safety - it will throw an error if a non-null field receives null from JSON.

Moshi JSON to POJO: The Android Way

Q: How to use Moshi with Retrofit?

Add moshi-kotlin dependency and use MoshiConverterFactory: Retrofit.Builder().addConverterFactory(MoshiConverterFactory.create(moshi)).build(). Moshi will automatically parse JSON responses to your data classes.

Q: Moshi vs kotlinx.serialization?

Both are excellent for Kotlin. Moshi is better for Android apps using Retrofit. kotlinx.serialization is official JetBrains library, better for Kotlin Multiplatform projects.

Square's modern JSON library for Android and Kotlin. Faster than Gson, safer for Kotlin, and built for Retrofit.

Generate Data Classes Full Android Guide

What is Moshi?

Moshi is a modern JSON library created by Square (the same team behind Retrofit, OkHttp, and Picasso). It's designed specifically for Kotlin and Android.

Faster

Compile-time adapters

Kotlin-Safe

Respects null safety

Retrofit Ready

Native integration

Basic Moshi Usage

// 1. Define your data class with @JsonClass
@JsonClass(generateAdapter = true)
data class User(
  @Json(name = "user_name") val name: String,
  val age: Int,
  val email: String? // nullable
)

// 2. Create Moshi instance
val moshi = Moshi.Builder()
  .add(KotlinJsonAdapterFactory())
  .build()

// 3. Parse JSON
val adapter = moshi.adapter(User::class.java)
val user = adapter.fromJson(jsonString)

Moshi vs Gson Comparison

Feature	Moshi	Gson
Kotlin Null Safety		✗ Unsafe
Code Generation		✗ Reflection only
Retrofit Native
ProGuard/R8	No rules needed	Needs keep rules
APK Size	~120 KB	~250 KB

Moshi with Retrofit

Moshi integrates seamlessly with Retrofit for API calls:

// build.gradle.kts dependencies
implementation("com.squareup.moshi:moshi-kotlin:1.15.0")
implementation("com.squareup.retrofit2:converter-moshi:2.9.0")
kapt("com.squareup.moshi:moshi-kotlin-codegen:1.15.0")

// Create Retrofit with Moshi
val moshi = Moshi.Builder()
  .add(KotlinJsonAdapterFactory())
  .build()

val retrofit = Retrofit.Builder()
  .baseUrl("https://api.example.com/")
  .addConverterFactory(MoshiConverterFactory.create(moshi))
  .build()

Custom Type Adapters

Handle custom types like Date, BigDecimal, or enums with custom adapters:

// Custom Date adapter
class DateAdapter {
  @ToJson
  fun toJson(date: Date): String {
    return SimpleDateFormat("yyyy-MM-dd").format(date)
  }

  @FromJson
  fun fromJson(json: String): Date {
    return SimpleDateFormat("yyyy-MM-dd").parse(json)!!
  }
}

// Add to Moshi
val moshi = Moshi.Builder()
  .add(DateAdapter())
  .add(KotlinJsonAdapterFactory())
  .build()

Common Questions

When to use @JsonClass?

Always use @JsonClass(generateAdapter = true) for compile-time code generation. It's faster and works with ProGuard without extra rules.

Moshi vs kotlinx.serialization?

Use Moshi for Android apps with Retrofit. Use kotlinx.serialization for Kotlin Multiplatform.

Generate Moshi-Ready Data Classes

Our converter outputs Kotlin data classes compatible with Moshi annotations.

Convert JSON to Kotlin