Micronaut GraphQL

Learn how to use Micronaut GraphQL.

Authors: Iván López

Micronaut Version: 3.2.7

1. Getting Started

In this guide, we will create a Micronaut application written in Groovy that uses GraphQL to expose some data.

GraphQL is a query language for APIs and a runtime for fulfilling those queries with your existing data. GraphQL provides a complete and understandable description of the data in your API, gives clients the power to ask for exactly what they need and nothing more, makes it easier to evolve APIs over time, and enables powerful developer tools.

2. What you will need

To complete this guide, you will need the following:

  • Some time on your hands

  • A decent text editor or IDE

  • JDK 1.8 or greater installed with JAVA_HOME configured appropriately

3. Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

4. Writing the Application

Create an application using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app example.micronaut.micronautguide --build=gradle --lang=groovy
If you don’t specify the --build argument, Gradle is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.

The previous command creates a Micronaut application with the default package example.micronaut in a directory named micronautguide.

5. GraphQL

Add the following dependency:

build.gradle
implementation("io.micronaut.graphql:micronaut-graphql")

By default GraphQL endpoint /graphql is enabled so you don’t need to add any extra configuration.

5.1. Describe your schema

Create the file schema.graphqls in src/main/resources directory:

src/main/resources/schema.graphqls
type Query {
    bookById(id: ID): Book (1)
}

type Book { (2)
    id: ID
    name: String
    pageCount: Int
    author: Author
}

type Author { (3)
    id: ID
    firstName: String
    lastName: String
}
1 Declare a bookById query
2 Declare a Book type
3 Declare an Author type

5.2. Book and Author classes

Create Book and Author classes that will mimic the data we want to expose:

src/main/groovy/example/micronaut/Book.groovy
package example.micronaut

import io.micronaut.core.annotation.Introspected

@Introspected
class Book {

    final String id
    final String name
    final int pageCount
    final Author author

    Book(String id, String name, int pageCount, Author author) {
        this.id = id
        this.name = name
        this.pageCount = pageCount
        this.author = author
    }

}
src/main/groovy/example/micronaut/Author.groovy
package example.micronaut

import io.micronaut.core.annotation.Introspected

@Introspected
class Author {

    final String id
    final String firstName
    final String lastName

    Author(String id, String firstName, String lastName) {
        this.id = id
        this.firstName = firstName
        this.lastName = lastName
    }

}

5.3. Data repository

To keep this example simple, instead of retrieving the information from a database we will keep it in memory and just return it from there. In a real-world example you will use any external storage: relational database, SQL database, etc.

Create DbRepository

src/main/groovy/example/micronaut/DbRepository.groovy
package example.micronaut

import jakarta.inject.Singleton

@Singleton
class DbRepository {

    private static final List<Book> books = [ (1)
        new Book("book-1", "Harry Potter and the Philosopher's Stone", 223, new Author("author-1", "Joanne", "Rowling")),
        new Book("book-2", "Moby Dick", 635, new Author("author-2", "Herman", "Melville")),
        new Book("book-3", "Interview with the vampire", 371, new Author("author-3", "Anne", "Rice"))
    ]

    List<Book> findAllBooks() {
        return books
    }

    List<Author> findAllAuthors() {
        return books
            .collect { it.author }
    }
}
1 These are the only books we have in our system.

5.4. Data Fetchers

With a Data Fetcher we bind the GraphQL schema, and our domain model and execute the appropiate queries in our datastore to retrieve the requested data.

Create class GraphQLDataFetchers

src/main/groovy/example/micronaut/GraphQLDataFetchers.groovy
package example.micronaut

import graphql.schema.DataFetcher

import jakarta.inject.Singleton

@Singleton
class GraphQLDataFetchers {

    private final DbRepository dbRepository

    GraphQLDataFetchers(DbRepository dbRepository) { (1)
        this.dbRepository = dbRepository
    }

    DataFetcher<Book> getBookByIdDataFetcher() {
        return { dataFetchingEnvironment ->  (2)
                {
                    String bookId = dataFetchingEnvironment.getArgument("id") (3)
                    return dbRepository.findAllBooks() (4)
                        .stream()
                        .filter(book -> book.id == bookId)
                        .findFirst()
                        .orElse(null)
                }
        }
    }

    DataFetcher<Author> getAuthorDataFetcher() {
        return { dataFetchingEnvironment ->
                {
                    Book book = dataFetchingEnvironment.getSource() (5)
                    Author authorBook = book.getAuthor() (6)
                    return dbRepository.findAllAuthors() (7)
                        .stream()
                        .filter(author -> author.id == authorBook.id)
                        .findFirst()
                        .orElse(null)
                }
        }
    }
}
1 Constructor injection for the DbRepository bean
2 Return a GraphQL dataFetchingEnvironment with the information about the query
3 Get the id parameter from the query
4 Access the repository to find the book. Remember that this should be backed by a real datastore
5 Get the Book related to a specific author
6 Get the Author
7 Access the repository to find the Author.

5.5. Factory

Create the following factory that will bind the GraphQL schema to the code and types.

src/main/groovy/example/micronaut/GraphQLFactory.groovy
package example.micronaut

import graphql.GraphQL
import graphql.GraphQL.Builder
import graphql.schema.GraphQLSchema
import graphql.schema.idl.RuntimeWiring
import graphql.schema.idl.SchemaGenerator
import graphql.schema.idl.SchemaParser
import graphql.schema.idl.TypeDefinitionRegistry
import io.micronaut.context.annotation.Bean
import io.micronaut.context.annotation.Factory
import io.micronaut.core.io.ResourceResolver
import org.slf4j.Logger
import org.slf4j.LoggerFactory

import jakarta.inject.Singleton

import static graphql.schema.idl.TypeRuntimeWiring.newTypeWiring

@Factory (1)
class GraphQLFactory {

    private static final Logger LOG = LoggerFactory.getLogger(GraphQLFactory.class)

    @Bean
    @Singleton
    GraphQL graphQL(ResourceResolver resourceResolver, GraphQLDataFetchers graphQLDataFetchers) { (2)
        SchemaParser schemaParser = new SchemaParser()
        SchemaGenerator schemaGenerator = new SchemaGenerator()

        // Parse the schema
        TypeDefinitionRegistry typeRegistry = new TypeDefinitionRegistry()
        Optional<InputStream> graphqlSchema = resourceResolver.getResourceAsStream("classpath:schema.graphqls") (3)

        if (graphqlSchema.isPresent()) {
            typeRegistry.merge(schemaParser.parse(new BufferedReader(new InputStreamReader(graphqlSchema.get()))))  (4)

            // Create the runtime wiring
            RuntimeWiring runtimeWiring = RuntimeWiring.newRuntimeWiring() (5)
                .type(newTypeWiring("Query")
                          .dataFetcher("bookById", graphQLDataFetchers.bookByIdDataFetcher)) (6)
                .type(newTypeWiring("Book")
                          .dataFetcher("author", graphQLDataFetchers.authorDataFetcher)) (7)
                .build()

            // Create the executable schema
            GraphQLSchema graphQLSchema = schemaGenerator.makeExecutableSchema(typeRegistry, runtimeWiring) (8)

            // Return the GraphQL bean
            return GraphQL.newGraphQL(graphQLSchema).build() (9)

        } else {
            LOG.debug("No GraphQL services found, returning empty schema")
            return new Builder(GraphQLSchema.newSchema().build()).build()
        }
    }
}
1 Annotate the class with @Factory so the Micronaut framework knows that this class will create beans
2 Create a new SchemaParser
3 Get the previously created schema.graphqls file from the classpath
4 Parse the schema
5 Create the runtime wiring
6 Bind a data fetcher for the bookById query
7 Bind a data fetcher to retrieve the author related to a book
8 Create the executable schema
9 Return the GraphQL bean

6. Running the Application

To run the application, use the ./gradlew run command which starts the application on port 8080.

We want to execute a GraphQL query to retrieve a book by its id:

query {
  bookById(id:"book-1") {
    name,
    pageCount,
    author {
      firstName
      lastName
    },
  }
}

Run the following curl request:

curl -X POST 'http://localhost:8080/graphql' \
     -H 'content-type: application/json' \
     --data-binary '{"query":"{ bookById(id:\"book-1\") { name, pageCount, author { firstName, lastName} } }"}'
{"data":{"bookById":{"name":"Harry Potter and the Philosopher's Stone","pageCount":223,"author":{"firstName":"Joanne","lastName":"Rowling"}}}}

One of the nice features about GraphQL is that the client can decide the fields, and the order they want to retrieve. Now we send the following request:

curl -X POST 'http://localhost:8080/graphql' \
     -H 'content-type: application/json' \
     --data-binary '{"query":"{ bookById(id:\"book-1\") { pageCount, name, id } }"}'
{"data":{"bookById":{"pageCount":223,"name":"Harry Potter and the Philosopher's Stone","id":"book-1"}}}

Notice that now the application only responds with pageCount, name and id fields, in that order.

7. Test the application

For testing the application we will use Micronaut HTTP Client to send a POST request to the /graphql endpoint. Create the following class:

src/test/groovy/example/micronaut/GraphQLControllerSpec.groovy
package example.micronaut

import io.micronaut.core.type.Argument
import io.micronaut.http.HttpRequest
import io.micronaut.http.HttpResponse
import io.micronaut.http.HttpStatus
import io.micronaut.http.client.HttpClient
import io.micronaut.http.client.annotation.Client
import io.micronaut.test.extensions.spock.annotation.MicronautTest
import spock.lang.Specification

import jakarta.inject.Inject

@MicronautTest
class GraphQLControllerSpec extends Specification {

    @Inject
    @Client("/")
    HttpClient client

    void 'test graphQL controller'() {
        given:
        String query = '{ "query": "{ bookById(id:\\"book-1\\") { name, pageCount, author { firstName, lastName} } }" }'

        when:
        HttpRequest request = HttpRequest.POST('/graphql', query)
        HttpResponse<Map> rsp = client.toBlocking().exchange(request, Argument.of(Map))

        then:
        rsp.status() == HttpStatus.OK
        rsp.body()

        and:
        Map bookInfo = rsp.getBody(Map).get()
        bookInfo.data.bookById
        bookInfo.data.bookById.name == "Harry Potter and the Philosopher's Stone"
        bookInfo.data.bookById.pageCount == 223
        bookInfo.data.bookById.author
        bookInfo.data.bookById.author.firstName == 'Joanne'
        bookInfo.data.bookById.author.lastName == 'Rowling'
    }
}

To run the tests:

./gradlew test

Then open build/reports/tests/test/index.html in a browser to see the results.

8. GraphiQL

As an extra feature that will help you during development, you can enable GraphiQL. GraphiQL is the GraphQL integrated development environment, and it helps to execute GraphQL queries.

It should only be used for development, so it’s not enabled by default. Add the following configuration to enable it:

src/main/resources/application.yml
graphql:
  graphiql:
    enabled: true

Start the application again and open http://localhost:8080/graphiql in your browser. You can write your GraphQL queries with integrated auto-completion and execute them to get the results in an easier and nicer way:

graphiql 01

9. Next steps

Take a look at the Micronaut GraphQL documentation.

10. Help with the Micronaut Framework

Object Computing, Inc. (OCI) sponsored the creation of this Guide. A variety of consulting and support services are available.