Schema Generator Hooks Provider

GraphQL Kotlin plugins can generate GraphQL schema as your build artifact directly from your source code. Plugins will scan your classpath for classes implementing graphql-kotlin-server marker Query, Mutation and Subscription interfaces and then generate corresponding GraphQL schema using graphql-kotlin-schema-generator. By default, plugins will generate the schema using NoopSchemaGeneratorHooks. If your project uses custom hooks or needs to generate the federated GraphQL schema, you will need to provide an instance of SchemaGeneratorHooksProvider that will be used to create an instance of your custom hooks.

SchemaGeneratorHooksProvider is a service provider interface that exposes a single hooks method to generate an instance of SchemaGeneratorHooks that will be used to generate your schema. By utilizing Java ServiceLoader we can dynamically load your custom provider from the classpath. Service provider can be provided as part of your project sources, included inside of one of your project dependencies or through explicitly provided artifact. Since we need to be able to deterministically choose a single hooks provider, generation of schema will fail if there are multiple providers on the classpath.

Federated Hooks Provider

If no custom hook providers are specified, graphql-kotlin plugins will default to use NoopSchemaGeneratorHooks. If you want to generate Federated schemas, you need to explicitly specify a dependency on graphql-kotlin-federated-hooks-provider.

Gradle

// build.gradle.kts
graphqlSDL("com.expediagroup", "graphql-kotlin-federated-hooks-provider", latestVersion)

Maven

<dependency>
    <groupId>com.expediagroup</groupId>
    <artifactId>graphql-kotlin-federated-hooks-provider</artifactId>
    <version>${latestVersion}</version>
</dependency>

Creating Custom Hooks Service Provider

Add dependency on graphql-kotlin-hooks-provider

SchemaGeneratorHooksProvider interface is defined in graphql-kotlin-hooks-provider module.

Gradle

// build.gradle.kts
implementation("com.expediagroup", "graphql-kotlin-hooks-provider", latestVersion)

Maven

<dependency>
    <groupId>com.expediagroup</groupId>
    <artifactId>graphql-kotlin-hooks-provider</artifactId>
    <version>${latestVersion}</version>
</dependency>

Create new SchemaGeneratorHooksProvider implementation

Service provider implementation has to implement SchemaGeneratorHooksProvider interface that provides a way to instantiate schema generator hooks that will be used to generate the GraphQL schema.

package com.example

class MyCustomSchemaGeneratorHooksProvider : SchemaGeneratorHooksProvider {
    override fun hooks(): SchemaGeneratorHooks = MyCustomHooks()
}

Create provider configuration file

Service loader provider configuration file should be created under JAR /META-INF/services directory (e.g. src/main/resources/META-INF/services in default project structure). Name of the provider configuration should be fully qualified service provider interface name, i.e. com.expediagroup.graphql.plugin.schema.hooks.SchemaGeneratorHooksProvider and contain single entry - a fully qualified name of the service provider implementation.

Using the example service provider implementation from the above, our project structure should look like

my-project
|- src
  |- main
    |- kotlin
      |- com
        |- example
          |- MyCustomSchemaGeneratorHooksProvider.kt
    |- resources
      |- META-INF
        |- services
          |- com.expediagroup.graphql.plugin.schema.hooks.SchemaGeneratorHooksProvider

Our service provider configuration file should have following content

com.example.MyCustomSchemaGeneratorHooksProvider

Limitations

We don't support Java 9 module mechanism for declaring ServiceLoader implementations. As a workaround, you have to define your service providers in the provider configuration file under META-INF/services.