type User {
  id: ID!
  name: String!
  posts: [Post!]!
}

type Post {
  id: ID!
  title: String!
  content: String
}

type Query {
  user(id: ID!): User
  allUsers: [User!]!
}

query {
  user(id: 42) {
    name
    posts {
      title
    }
  }
}

{
  "data": {
    "user": {
      "name": "Den",
      "posts": [
        { "title": "gRPC и Java" },
        { "title": "GraphQL под капотом" }
      ]
    }
  }
}

mutation {
  createUser(input: { name: "Alex" }) {
    id
    name
  }
}

subscription {
  userCreated {
    id
    name
  }
}

{
  "query": "query { user(id: 42) { name posts { title } } }"
}

GraphQL-запрос — это декларативное описание структуры данных, не код и не SQL.

GraphQLObjectType userType = newObject()
    .name("User")
    .field(field -> field
        .name("id")
        .type(Scalars.GraphQLID))
    .field(field -> field
        .name("name")
        .type(Scalars.GraphQLString))
    .field(field -> field
        .name("posts")
        .type(new GraphQLList(postType))
        .dataFetcher(env -> postService.getByUser(env.getSource())))
    .build();

{
  "data": {
    "user": {
      "name": "Den",
      "posts": [{ "title": "gRPC и Java" }]
    }
  }
}

{
  "data": { "user": null },
  "errors": [{ "message": "User not found" }]
}

@Component
public class UserResolver implements GraphQLQueryResolver {
    private final UserService userService;
    public UserResolver(UserService userService) { this.userService = userService; }

    public User user(Long id) {
        return userService.findById(id);
    }

    public List<User> allUsers() {
        return userService.findAll();
    }
}

type User {
  id: ID!
  name: String!
  posts: [Post!]!
}

Типы описывают структуру данных — аналог классов/DTO.

input CreateUserInput {
  name: String!
  age: Int
}

enum Role {
  ADMIN
  USER
  GUEST
}

interface Entity {
  id: ID!
}

type User implements Entity {
  id: ID!
  name: String!
}

type Post implements Entity {
  id: ID!
  title: String!
}

Клиент может запросить id для любого, кто реализует Entity.

union SearchResult = User | Post | Comment

schema {
  query: Query
  mutation: Mutation
}

Сущности
type User {
  id: ID!
  name: String!
  role: Role!
  posts: [Post!]!
}

type Post {
  id: ID!
  title: String!
  content: String
  author: User!
  comments: [Comment!]!
}

type Comment {
  id: ID!
  text: String!
  author: User!
}

Перечисления
enum Role {
  ADMIN
  USER
  GUEST
}

Входные данные
input CreatePostInput {
  title: String!
  content: String
}

Запросы
type Query {
  user(id: ID!): User
  users: [User!]!

  post(id: ID!): Post
  posts(limit: Int, authorId: ID): [Post!]!
}

Мутации
type Mutation {
  createPost(input: CreatePostInput!): Post!
  deletePost(id: ID!): Boolean!
}

Подписки
type Subscription {
  postCreated: Post!
}

type Query {
  user(id: ID!): User
}

@Component
public class UserResolver implements GraphQLQueryResolver {
    private final UserService service;

    public UserResolver(UserService service) {
        this.service = service;
    }

    public User user(Long id) {
        return service.findById(id);
    }
}

type User {
  id: ID!
  name: String!
  email: String # новое поле
}

type User {
  id: ID!
  name: String!

  # старое поле
  username: String @deprecated(reason: "Use 'name' instead")
}

Старое:
input CreatePostInput {
  title: String!
}

Новое:
input CreatePostInput {
  title: String!
  content: String @deprecated(reason: "Use body instead")
  body: String
}

query {
  user(id: 42) {
    name
    avatar
  }
}

{
  "data": {
    "user": {
      "name": "Den",
      "avatar": "https://example.com/den.png"
    }
  }
}

Никаких “лишних” полей.

type Query {
  posts(limit: Int, authorId: ID): [Post!]!
}

query {
  posts(limit: 5, authorId: 42) {
    id
    title
  }
}

{
  "data": {
    "posts": [
      { "id": "101", "title": "GraphQL SDL" },
      { "id": "102", "title": "gRPC vs GraphQL" }
    ]
  }
}

query GetUser($id: ID!) {
  user(id: $id) {
    name
    avatar
  }
}

{
  "id": 42
}

type Mutation {
  createPost(input: CreatePostInput!): Post!
}

mutation {
  createPost(input: { title: "GraphQL", content: "SDL explained" }) {
    id
    title
  }
}

{
  "data": {
    "createPost": {
      "id": "501",
      "title": "GraphQL"
    }
  }
}

query {
  user(id: 1) {
    name
    posts(limit: 2) {
      title
      comments {
        text
      }
    }
  }
}

{
  "data": {
    "user": {
      "name": "Den",
      "posts": [
        {
          "title": "GraphQL Basics",
          "comments": [
            { "text": "Отличная статья" }
          ]
        },
        {
          "title": "SDL Tutorial",
          "comments": []
        }
      ]
    }
  }
}

Сервер не может изменить структуру ответа — она определяется клиентом.

data

errors

query {
  user(id: 1) {
    name
    posts {
      title
      likes
    }
  }
}

{
  "data": {
    "user": {
      "name": "Den",
      "posts": null
    }
  },
  "errors": [
    {
      "message": "Database timeout",
      "path": ["user", "posts"]
    }
  ]
}

{
  "data": {
    "createPost": null
  },
  "errors": [
    {
      "message": "User not authorized",
      "path": ["createPost"]
    }
  ]
}

List<String> myList; // Объявление переменной интерфейсного типа
myList = new ArrayList<>(); // Работаем с динамическим массивом
// ... позже в коде ...
myList = new LinkedList<>(); // Теперь работаем со связным списком