An API with Spring Boot

Ludovic Deneuville

Spring

Spring Framework

  • Open-source, lightweight framework
  • Simplifies Java development
  • Loosely coupled components
  • Promotes good design practices
  • Makes integration with other systems seamless

Modules

  • Spring MVC: to build web applications
  • Spring Security: authentication, authorization…
  • Spring Data: simplifies database access
  • Spring Batch: handling large-scale batch processing

Key features

  • Inversion of Control (IoC)
  • Dependency Injection (DI)
  • Aspect-Oriented Programming (AOP)

Manually creating dependency

Bread.java
public class Bread {
    private boolean isToasted;

    public void toast(){
        this.isToasted = true;
    }
}


Sandwich.java
public class Sandwich {
    private Bread bread;
    private ArrayList<String> ingredients;

    public Sandwich(Bread bread) {
        this.bread = bread;
    }
}
  1. Create a Bread object
  2. Create the sandwich object


Bread bread = new Bread();
Sandwich sandwich = new Sandwich(bread);

Dependency Injection

Bread.java
@Component
public class Bread {
    private boolean isToasted;

    public void toast(){
        this.isToasted = true;
    }
}


Sandwich.java
public class Sandwich {
    @Autowired
    private Bread bread;
    
    private ArrayList<String> ingredients;
}

What happens now?

  • Spring scans @Component classes
  • It automatically creates and injects a Bread instance into Sandwich
  • The @Autowired annotation tells Spring to resolve and inject the dependency
  • Component subclasses: @Service, @Repository, @Controller

Bean principles

  • No args constructor
  • Managed by the Spring IoC container
  • Scope (Singleton, Prototype)
  • Lifecycle (creation, usage, destruction)

Annotations

What is Annotation?

  • Preceded by an @
  • Assigns extra metadata
  • Modify or examine behavior
  • Heavily used in Spring

Lombok

Lombok Annotations

Auto-generates:

  • @Getter and @Setter
  • @AllArgsConstructor
  • @NoArgsConstructor
import lombok.AllArgsConstructor;
import lombok.Getter;
import lombok.Setter;

@AllArgsConstructor
@Getter
public class Book {
    private String title;

    @Setter
    private String author;
}

Layers

Reminder

  • Controller: API
  • Service: Business Logic
  • Repository: Data Access Object
  • Model: Entities

Zoom on Business Layer

Workflow

  • A client sends an HTTPS request
  • The request is received by the Controller
  • If needed, the Controller calls the Service Layer
  • The Service Layer executes the business logic and interacts with the Repository Layer to perform data operations
  • The Repository Layer retrieves or modifies data in the database using JPA
  • The response is sent back to the client

Sequence diagram

  • On the GUI, you click on the book entitled ‘Java’
  • The front office asks the back office for informations

sequenceDiagram
    participant Client
    participant Controller
    participant Service
    participant Repository
    participant Database

    Client->>Controller: GET /books?title=Java
    Controller->>Service: getBookByTitle("Java")
    Service->>Repository: findByTitle("Java")
    Repository->>Database: SELECT * FROM books WHERE title="Java"
    Database-->>Repository: Returns book data
    Repository-->>Service: Returns Book entity
    Service-->>Controller: Returns Book DTO
    Controller-->>Client: HTTP 200 OK (Book JSON)

HTTP status codes

HTTP Status Code Description
200 OK The request was successful.
400 Bad Request The server could not understand the request due to invalid syntax.
401 Unauthorized Authentication is required, and the client should authenticate itself.
403 Forbidden The client does not have access rights to the content.
404 Not Found The server can not find the requested resource.
500 Internal Server Error The server encountered an unexpected error.
501 Not Implemented The server does not support requested functionality.
502 Bad Gateway The server received an invalid response from the upstream server.
503 Service Unavailable The server is not ready to handle the request.

Spring Boot

Why use Spring Boot?

  • Includes all Spring features
  • Simplify configuration
  • Allows developers to focus on business logic
  • Quick development of production-ready applications
  • Optimisation of dependency management

Features

  • Auto-Configuration
  • Creat REST APIs is easy
  • Embedded Tomcat Server

Starters

Dependencies:

  • spring-boot-starter-core
  • spring-boot-starter-data-jpa
  • spring-boot-starter-test
  • spring-boot-starter-web

Create a project

Properties

application.yml

server:
  port: 9000

spring:
  application:
    name: running
  datasource:
    driver-class-name: org.h2.Driver
    url: jdbc:h2:mem:bootapp;DB_CLOSE_DELAY=-1
    username: sa
    password: password

logging:
  level:
    root: ERROR
    fr:
      ensai: INFO
    org:
      springframework:
        boot:
          web:
            embedded:
              tomcat: INFO

YAML

  • YAML Ain’t Markup Language
  • Text format
  • Indentation-based syntax
  • Easily readable by humans

YAML Strucure

  • Key-Value pairs
  • indentation: 2 spaces
  • CASE sensitive

YAML Examples

person:
  name: Charlotte         # str
  city: "Amiens"          # str
  age: 25                 # int
  height: 1.70            # float
  student: false          # bool
  birth_date: 1990-05-15  # date


hobbies: 
  - chess
  - hiking
  - landart

Data Mapping

Why is it usefull?

  • Facilitates interaction between Java objects and database tables
  • ensure accurate data transfer between layers, databases
  • Object-Relational Mapping (ORM)

Database side

CREATE TABLE user (
    id_user      SERIAL PRIMARY KEY,
    firstname    VARCHAR(255),
    birth_date   DATE,
    gender       VARCHAR(10)
);

Java class

public class User {
    private Long id;
    private String name;
    private LocalDate birthDate;
    private String gender;
  • Many similarities
  • A few differences:
    • User instead of user
    • birthDate vs birth_date
    • id vs id_user

Hibernate

  • Simplify data interactions with relational databases
  • Maps Java classes to database tables
  • Provides database independence
  • Generates SQL queries for CRUD operations

Entity Bean

@Entity
@Table(name = "user")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    @Column(name = "id_user")
    private Long id;

    private String name;

    @Column(name = "birth_date")
    private LocalDate birthDate;

    @Column(name = "gender", length = 10)
    private String gender;
}

Entity annotations

Annotation Description
@Entity Marks the class as a JPA entity.
@Table(name="tbname") Specifies the database table name.
@Id Marks a field as the primary key.
@GeneratedValue( .. ) Defines the auto-increment strategy for the PK.
@Column(name="colname", ...) Maps a field to a database column and allows configuration.
@Enumerated(EnumType.STRING) Maps an enum to a database column.
@Transient Marks a field as non-persistent (ignored by JPA).

Entity relationships

Annotation Description
@JoinColumn(name="FK_column") Specifies the foreign key column for relationships.
@ManyToOne Defines a many-to-one relationship between entities.
@OneToMany(mappedBy="fieldName") Defines a one-to-many relationship.
@ManyToMany Defines a many-to-many relationship.
@OneToOne Defines a one-to-one relationship.

ManyToOne

Considering that:

  • a user has a club
  • a club can have several users

Where is the foreign key?

User.java
@Entity
@Table(name = "user")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    @Column(name = "id_user")
    private Long id;

    @Column(name = "name")
    private String name;
    
    @ManyToOne
    @JoinColumn(name = "id_club")
    private Club club;
}
Club.java
@Entity
@Table(name = "club")
public class Club {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    @Column(name = "id_club")
    private Long id;

    @Column(name = "name", nullable = false)
    private String name;

    @OneToMany(mappedBy = "club")
    private List<User> users = new ArrayList<>();
}

ManyToMany

  • To be used if the association table is basic
  • Otherwise create an entity for the association table with ManyToOne

Repository

A basic DAO

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
}

Basic methods

Methods included automatically without having to implement them:

  • save(Entity e)
  • findById(Long id)
  • findAll()
  • count()
  • delete(Entity e)

Native SQL Query

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;
import org.springframework.data.repository.query.Param;
import org.springframework.stereotype.Repository;
import java.util.List;

@Repository
public interface UserRepository extends JpaRepository<User, Long> {

    @Query(value = """
        SELECT * 
        FROM user u 
        WHERE u.name = :name
    """, nativeQuery = true)
    List<User> findByNameNative(@Param("name") String name);
}

JPQL

  • Java Persistence Query Language
  • Method name ➡️ Auto-generate the Query
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    List<User> findByGenderOrderByNameDesc(String gender);
}

Controller

API Rest

ApiRestController.java
@RestController
@RequestMapping("/api")
public class ApiRestController {

    @Autowired
    private UserService userService;

    /**
     * Get all users
     */
    @GetMapping("/users")
    public List<User> getAllUsers() {

        return userService.findAll();
    }
}
  • @RestController
    • handles RESTful web requests
    • methods in this class will return data directly (JSON)
  • @RequestMapping("/api"): Specifies the base URL
  • @GetMapping("/users"): Maps getAllUsers() method to endpoint /api/users

Use a PATH Parameter

ApiRestController.java
    @GetMapping("/users/{id_user}")
    public ResponseEntity<User> getUserById(@PathVariable Long id_user) {
        User user = userService.findById(id_user);
        if (user == null) {
            return ResponseEntity.notFound().build();
        }
        return ResponseEntity.ok(user);
    }

ResponseEntity:

  • Represents an entire HTTP response

Create, Update, Delete

ApiRestController.java
    @PostMapping("/users")
    public ResponseEntity<User> createUser(@RequestBody User user) {
        User createdUser = userService.save(user);
        return ResponseEntity.status(HttpStatus.CREATED).body(createdUser);
    }

    @PutMapping("/users/{id}")
    public ResponseEntity<User> updateUser(@PathVariable Long id, @RequestBody User user) {
        User existingUser = userService.findById(id);
        if (existingUser == null) {
            return ResponseEntity.notFound().build();
        }
        user.setId(id);
        User updatedUser = userService.save(user);
        return ResponseEntity.ok(updatedUser);
    }

    @DeleteMapping("/users/{id}")
    public ResponseEntity<Void> deleteUser(@PathVariable Long id) {
        User existingUser = userService.findById(id);
        if (existingUser == null) {
            return ResponseEntity.notFound().build();
        }
        userService.deleteById(id);
        return ResponseEntity.noContent().build();
    }

Run application

Main

Application.java
@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}