AutoValue: Gson Extension

An extension for Google's AutoValue that creates a simple Gson TypeAdapterFactory for each AutoValue annotated object.

Usage

Simply include auto-value-gson in your project and add a public static method to your @AutoValue annotated class returning a TypeAdapter. You can also annotate your properties using @SerializedName to define an alternate name for de/serialization.

@AutoValue public abstract class Foo {
  abstract String bar();
  @SerializedName("Baz") abstract String baz();
  abstract int quux();
  abstract String with_underscores();

  // The public static method returning a TypeAdapter<Foo> is what
  // tells auto-value-gson to create a TypeAdapter for Foo.
  public static TypeAdapter<Foo> typeAdapter(Gson gson) {
    return new AutoValue_Foo.GsonTypeAdapter(gson);
  }
}

Additionally, you can set custom default values. This is disabled by default. See Compiler options on how to enable this.

@AutoValue public abstract class Foo {
  abstract String bar();
  @SerializedName("Baz") abstract String baz();
  abstract int quux();
  abstract String with_underscores();

  public static TypeAdapter<Foo> typeAdapter(Gson gson) {
    return new AutoValue_Foo.GsonTypeAdapter(gson)
      // You can set custom default values
      .setDefaultQuux(4711)
      .setDefaultWith_underscores("");
  }
}

Now build your project and de/serialize your Foo.

The TypeAdapter

To trigger TypeAdapter generation, you need include a non-private static factory method that accepts a Gson parameter and returns a TypeAdapter for your AutoValue type. From within this method you can instantiate a new GsonTypeAdapter which will have been generated as an inner class of your AutoValue generated implementation.

@AutoValue public abstract class Foo {
  // properties...
  
  public static TypeAdapter<Foo> typeAdapter(Gson gson) {
    return new AutoValue_Foo.GsonTypeAdapter(gson);
  }
}

Generics support

If your annotated class uses generics, you'll have to modify your static method a little so AutoValue will know how to generate an appropriate adapter. Simply add a TypeToken parameter and pass it to the generated GsonTypeAdapter class.

To have support for fields with generic parameters (eg. List<B>) you need to upgrade your Gson dependency to at least 2.8.0, which introduces the helper TypeToken.getParameterized() see Gson Changelog.

@AutoValue public abstract class Foo<A, B, C> {

  abstract A data();
  abstract List<B> dataList();
  abstract Map<String, List<C>> dataMap();

  public static <A, B, C> TypeAdapter<Foo<A, B, C>> typeAdapter(Gson gson,
      TypeToken<? extends Foo<A, B, C>> typeToken) {
    return new AutoValue_Foo.GsonTypeAdapter(gson, typeToken);
  }
}

Factory

Optionally, auto-value-gson can create a single TypeAdapterFactory so that you don't have to add each generated TypeAdapter to your Gson instance manually.

To generate a TypeAdapterFactory for all of your auto-value-gson classes, simply create an abstract class that implements TypeAdapterFactory and annotate it with @GsonTypeAdapterFactory, and auto-value-gson will create an implementation for you. You simply need to provide a static factory method, just like your AutoValue classes, and you can use the generated TypeAdapterFactory to help Gson de/serialize your types.

@GsonTypeAdapterFactory
public abstract class MyAdapterFactory implements TypeAdapterFactory {

  // Static factory method to access the package
  // private generated implementation
  public static TypeAdapterFactory create() {
    return new AutoValueGson_MyAdapterFactory();
  }
  
}

Then you simply need to register the Factory with Gson.

Gson gson = new GsonBuilder()
    .registerTypeAdapterFactory(MyAdapterFactory.create())
    .create();

Compiler options

autovaluegson.defaultCollectionsToEmpty - If specified, maps/collections will default to their empty types (e.g. List -> Collections.emptyList()). Value is true or false. By default this is false.
autovaluegson.mutableAdaptersWithDefaultSetters - Indicates that the generated TypeAdapter should be mutable with setters for default values. Value is true or false. By default this is false.

android {
  // ...
  defaultConfig {
    // ...
    javaCompileOptions {
      annotationProcessorOptions {
        arguments = ['autovaluegson.defaultCollectionsToEmpty': 'true']
      }
    }
  }
}

@GenerateTypeAdapter

There is an annotation in the auto-value-gson-annotations artifact called @GenerateTypeAdapter. This annotation can be set on types to indicate to the extension that you want the generated adapter to be a top level class in the same package. The name of this class will be the AutoValue class's name plus _GsonTypeAdapter suffix.

Types annotated with this can also be (de)serialized dynamically at runtime with a provided runtime TypeAdapterFactory implementation in the annotation called FACTORY. The type name and generated typeadapter class's name must not be obfuscated for this to work.

When this annotation is used, there will be no intermediate AutoValue class generated (as opposed to the default logic, which generates an intermediate class and generates the TypeAdapter as a static inner class of it).

@GenerateTypeAdapter is compatible with the factory approach above, just make your static method's implementation point to it. It can also be an alternative to it if you use the runtime factory, particularly if you have a multimodule project and are willing to accept a small amount of (heavily cached) reflection.

The generated class will have the same parameters as if it were the inner class. If it's generic, its constructor accepts a Gson instance and TypeToken of the generics. If it's not generic, it's just a Gson instance.

Example usage:

@GenerateTypeAdapter
@AutoValue
public class Foo {
  // ...
  public static TypeAdapter<Foo> typeAdapter(Gson gson) {
    return new Foo_GsonTypeAdapter(gson);
  }
}

// Generates
public final class Foo_GsonTypeAdapter extends TypeAdapter<Foo> {
  public Foo_GsonTypeAdapter(Gson gson) {
    //...
  }
}

// Or with generics
@GenerateTypeAdapter
@AutoValue
public class Foo<T> {
  // ...
  public static TypeAdapter<Foo> typeAdapter(Gson gson, TypeToken<? extends Foo<T>> typeToken) {
    return new Foo_GsonTypeAdapter(gson, typeToken);
  }
}

// Generates
public final class Foo_GsonTypeAdapter extends TypeAdapter<Foo> {
  public Foo_GsonTypeAdapter(Gson gson, TypeToken<? extends Foo<T>> typeToken) {
    //...
  }
}

// Using the runtime FACTORY
new GsonBuilder()
    .addTypeAdapterFactory(GenerateTypeAdapter.FACTORY)
    .build()
    .toJson(myFooInstance);

Proguard rules for obfuscation:

# Retain generated classes that end in the suffix
-keepnames class **_GsonTypeAdapter

# Prevent obfuscation of types which use @GenerateTypeAdapter since the simple name
# is used to reflectively look up the generated adapter.
-keepnames @com.ryanharter.auto.value.gson.GenerateTypeAdapter class *

Download

Add a Gradle dependency to the apt and provided configuration.

annotationProcessor 'com.ryanharter.auto.value:auto-value-gson:0.7.0'
compile 'com.ryanharter.auto.value:auto-value-gson-annotations:0.7.0'

Snapshots of the latest development version are available in Sonatype's snapshots repository.

You will also need a normal runtime dependency for gson itself.

compile 'com.google.code.gson:gson:2.8.0'

License

Copyright 2015 Ryan Harter.

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.

seanfchan/auto-value-gson