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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);
  }
}

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,
      Type[] types) {
    return new AutoValue_Foo.GsonTypeAdapter<>(gson, types);
  }
}

Note that the types is an array of the Type representations of the given type's generics. If Foo is parameterized as Foo<String, Integer, Boolean>, then the Type array passed in should be an array of {String.class, Integer.class, Boolean.class}.

Transient types

To ignore certain properties from serialization, you can use the @AutoTransient annotation. This comes from a shared transience annotations library and is an api dependency of the runtime artifact. You can annotate a property and it will be treated as transient for both serialization and deserialization. Note that this should only be applied to nullable properties.

Builder Support

If your @AutoValue class has a builder, auto-value-gson will use the builder to instantiate the class. If the @AutoValue class has a static no-argument factory method for its builder, it will be used. If there are multiple factory methods, the one annotated @AutoValueGsonBuilder will be used. This can be useful for setting default values.

@AutoValue public abstract class Foo {
  abstract int bar();
  abstract String quux();

  public static Builder builder() {
    return new AutoValue_Foo.Builder();
  }

  @AutoValueGsonBuilder
  public static Builder builderWithDefaults() {
    return new builder().quux("QUUX");
  }
}

Field name policy

If you want the generated adapter classes to use the input Gson instance's field name policy, you can enable this via autovaluegson.useFieldNamePolicy processor option. This acts as a flag (any value is ignored) and can be set like any other annotation processor option.

In Gradle, this could look like this:

tasks.withType(JavaCompile) {
    options.compilerArgs += "-Aautovaluegson.useFieldNamePolicy"
}

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();

@GenerateTypeAdapter

There is an annotation in the auto-value-gson-runtime 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. The extension that runs during annotation processing will automatically generate custom .pro rules for Proguard/R8 for this, so it should require no extra configuration.

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). There is no need to declare a static TypeAdapter<...> typeAdapter() method anymore for this case, though you can optionally define one if you still want to use the @GsonTypeAdapterFactory generator for them.

@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 {
  // ...
}

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

// Or with generics
@GenerateTypeAdapter
@AutoValue
public class Foo<T> {
  // ...
}

// 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()
    .registerTypeAdapterFactory(GenerateTypeAdapter.FACTORY)
    .create()
    .toJson(myFooInstance);

Download

Add a Gradle dependency to the annotationProcessor/kapt and implementation/api configuration.

annotationProcessor("com.ryanharter.auto.value:auto-value-gson-extension:1.3.1")
implementation("com.ryanharter.auto.value:auto-value-gson-runtime:1.3.1")

// Optional @GsonTypeAdapterFactory support
annotationProcessor("com.ryanharter.auto.value:auto-value-gson-factory:1.3.1")

// Legacy generic artifact that includes both -extension and -factory above. This exists to not
// break existing users, but shouldn't be used because it includes both the -factory artifact as
// well as the -extension artifact. This can have a negative impact on build times if you don't
// actually use the factory support, as it is an aggregating incremental processor thus slower
// compared to just using the isolating incremental behavior of the extension.
annotationProcessor("com.ryanharter.auto.value:auto-value-gson:1.3.1")

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

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.