Kotshi

An annotation processor that generates Moshi adapters from Kotlin classes.

There is a reflective adapter for Kotlin but that requires the kotlin reflection library which adds a lot of methods and increases the binary size which in a constrained environment such as Android is not preferable.

This is where Kotshi comes in, it generates fast and optimized adapters for your Kotlin data classes, just as if you'd written them by hand yourself. It will automatically regenerate the adapters when you modify your class.

It's made to work with minimal setup, through there are limitations. Most of the limitations will be addressed as the support for Kotlin annotation processors improves.

You can find the generated documentation by visiting kotshi.ansman.se.

Usage

First you must annotate your types with the @JsonSerializable annotation

Annotated class

@JsonSerializable
data class Person(
    val name: String,
    val email: String?,
    val hasVerifiedAccount: Boolean,
    // This property has a different name in the Json than here so @JsonProperty must be applied.
    @JsonProperty(name = "created_at")
    val signUpDate: Date,
    // This field has a default value which will be used if the field is missing.
    val jobTitle: String? = null
)

The following types are supported:

• data object (serialized as an empty JSON object)
• data class
• enum class
• sealed class

Then create a class that will be your factory.

Factory setup

@KotshiJsonAdapterFactory
object ApplicationJsonAdapterFactory : JsonAdapter.Factory by KotshiApplicationJsonAdapterFactory

Lastly just add the factory to your Moshi instance, and you're all set.

Add to moshi

val moshi = Moshi.Builder()
    .add(ApplicationJsonAdapterFactory)
    .build()

By default adapters aren't requested for primitive types (even boxed primitive types) since it is worse for performance and most people will not have custom adapters anyway. If you need to use custom adapters you can enable it per module be passing the useAdaptersForPrimitives to @KotshiJsonAdapterFactory or on a per adapter by passing the same argument to @JsonSerializable (the default is to follow the module wide setting).

Annotations

• @JsonSerializable is the annotation used to generate JsonAdapter's. Should only be placed on data classes, enums, sealed classes and objects.
• @KotshiJsonAdapterFactory makes Kotshi generate a JsonAdapter factory. Should be placed on an object that implements JsonAdapter.Factory.
• @JsonDefaultValue can be used to annotate a fallback for enums or sealed classes when an unknown entry is encountered. The default is to thrown an exception.
• @JsonProperty can be used to customize how a property or enum entry is serialized to and from JSON.
• @Polymorphic and @PolymorphicLabel used on sealed classes and their implementations.
• @RegisterJsonAdapter registers a json adapter into the Kotshi json adapter factory.

Default Values

You can use default values just like you normally would in Kotlin.

Due to limitations in Kotlin two instances of the object will be created when a class uses default values (youtrack issue). This also means that composite default values are not supported (for example a fullName property that is "$firstName $lastName").

For enum entries and sealed classes you may annotate a single type with @JsonDefaultValue to indicate that the entry should be used when an unknown value is encountered (by default an exception is thrown).

Transient Values

Properties marked with @Transient are not serialized. All transient properties must have a default value.

Only properties declared in the constructor need to be annotated since other properties are ignored.

Custom Names

By default, the property or enum entry name is used when reading and writing JSON. To change the name used you may use the @JsonProperty annotation or the regular @Json annotation from Moshi to annotate the property or enum entry.

Json Qualifiers

Kotshi has full support for @JsonQualifier, both plain and those with arguments. Simply annotate a property with the desired qualifiers and Kotshi will pick them up.

Registered adapters

It's often required to have a few adapters that are handwritten, for example for framework classes. Handling this in a custom factory can be tedious, especially for generic types. To make this easier you may annotate any class or object that extends JsonAdapter with @RegisterJsonAdapter and Kotshi will generate the needed code in the adapter factory.

Options

kotshi.generatedAnnotation

This option tells Kotshi to add the @Generated annotation to all generated classes which is disabled by default.

For Java 9+ use javax.annotation.processing.Generated and for Java 8 and below use javax.annotation.Generated.

Examples:

KSP

ksp {
    // When using Java 9 and above
    arg("kotshi.generatedAnnotation", "javax.annotation.processing.Generated")
    // When using Java 8 and below
    arg("kotshi.generatedAnnotation", "javax.annotation.Generated")
}

KAPT

kapt {
    arguments {
      // When using Java 9 and above
      arg("kotshi.generatedAnnotation", "javax.annotation.processing.Generated")
      // When using Java 8 and below
      arg("kotshi.generatedAnnotation", "javax.annotation.Generated")
    }
}

Limitations

• Kotshi only processes files written in Kotlin, types written in Java are not supported.
• Only data classes, enums, sealed classes and data objects are supported.
  • Only constructor properties will be serialized.
  • Qualifiers whose arguments are named as a Java keyword cannot be seen by annotations processors and cannot be used.
• Due to limitation in KAPT, properties with a java keyword as a name cannot be marked as transient.
• Due to a KAPT bug/limitation you cannot add qualifiers to parameters that are inline classes (youtrack issue).

Download

Kotlin with KSP

plugins {
  id("com.google.devtools.ksp") version "<version>"
}

dependencies {
  val kotshiVersion = "3.0.0"
  implementation("se.ansman.kotshi:api:$kotshiVersion")
  ksp("se.ansman.kotshi:compiler:$kotshiVersion")
}

Kotlin with KAPT

plugins {
  kotlin("kapt")
}

dependencies {
  val kotshiVersion = "3.0.0"
  implementation("se.ansman.kotshi:api:$kotshiVersion")
  kapt("se.ansman.kotshi:compiler:$kotshiVersion")
}

Groovy with KSP

plugins {
  id "com.google.devtools.ksp" version "<version>"
}

dependencies {
  def kotshiVersion = "3.0.0"
  implementation "se.ansman.kotshi:api:$kotshiVersion"
  ksp "se.ansman.kotshi:compiler:$kotshiVersion"
}

Groovy with KAPT

plugins {
  id "org.jetbrains.kotlin.kapt"
}

dependencies {
  def kotshiVersion = "3.0.0"
  implementation "se.ansman.kotshi:api:$kotshiVersion"
  kapt "se.ansman.kotshi:compiler:$kotshiVersion"
}

Snapshots of the development version are available in the sonatype snapshots repository.

License

Copyright 2017-2024 Nicklas Ansman Giertz.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.