Mastering Spring Boot: Your Guide to Creating Powerful RESTful APIs

Java developers know that building an application in plain Java can be challenging, and several frameworks have been developed to make their work easier. The most popular is the Spring Framework.

The Spring Framework provides valuable features like dependency injection, enabling an easier way to develop microservices and distributed network applications. It also has built-in support for exception handling, validation, events, resource handling, etc.

Later, an advanced framework was developed, the Spring Boot framework. Due to its ease of use, java developers quickly adopted spring boot on a broad scale. Developing REST-based microservice web applications in Java with Spring Boot is undoubtedly one of the fastest ways, which offers greater flexibility and an easier way to develop Java applications.

Spring Boot has three core features that make it so powerful:

1. Autoconfiguration - offers features like health checks and metrics.
2. Embedded servers - enables developers to integrate servers like Jetty, Tomcat, or Undertow directly.
3. Creating standalone - meaning your application can run anywhere without needing internet or other installations.

Spring Boot offers a production-grade application setup with minimal setup and configurations, thanks to an opinionated approach to configuration and starter dependencies.

To build a Spring Boot project, you will need the following:

• Java Development Kit (JDK)
• An IDE (E.g., Eclipse or IntelliJ)
• Maven

Setting up a SpringBoot project

Setting up a project in spring boot is very simple; this can be done with Spring Initializr; if your IDE does not have the integration of Spring Initializr, you can configure your setup by following the steps below in a browser;

1. Go to the Spring Initializr website at? https://start.spring.io
2. Choose the project's metadata, such as the name, description, package name, and other relevant information. It is good to use Java naming conventions and your company naming convention if they have any.
3. Select the dependencies you need for your project; you may want to select Spring Web, Spring Data JPA, MySQL, etc., depending on the requirements of your project. You can always add more later.
4. Click the "Generate" button to generate the project.
5. Once the project is generated, you can download it as a zip file and extract it to a folder on your computer where you want to keep your project.

To import the project into your IDE, you'll need to follow these steps:

• Open your IDE and select "Import Project" or "New Project from Existing Source. "You can just browse your menu.
• Browse to the folder where you extracted the project and select it.
• Choose the import settings that match your project's configuration.
• Wait for the IDE to import the project and set up the necessary dependencies.
• If you're using IntelliJ, you can open the project by choosing "Open Project" from the welcome screen and selecting the project's root directory. IntelliJ will automatically detect the project's configuration and set up any necessary dependencies.

To build a project, we will need the following

1. An entity
2. A service
3. A repository
4. A controller

Defining an entity in Spring boot?

Entities are the actual objects in your application. Defining entities can be done in a package with classes to hold the various entities in your application.

1. Define the entity class: Define a plain old Java object (POJO) class for your entity. The class should have private fields for each column in the MySQL table and public getter and setter methods for accessing the fields. You can annotate the class with the @Entity annotation to mark it as a JPA entity. When using Lombok, you will not need to generate the getters and setters for your project manually. Use the @Data annotation, and Lombok will handle the rest.
2. Define the primary key: Use the @Id annotation to mark one of the fields as the primary key for the entity.
3. Define relationships: If your entity has relationships with other entities, you can define them using JPA annotations such as @OneToMany or @ManyToOne.
4. Define additional properties: You can use JPA annotations to define additional properties for your entity, such as the table name, column names, and data types.

Here is an example of a simple entity class for a "Clients" table in MySQL:

package com.example.domain
import...
@Data
@Table(name = "tbl_clients")
@Entity
public class Client {
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    private Long id;
    private String clientNumber;
    private String fullName;
    private String firstName;
    private String lastName;
    @Column(unique = true)
    private String msisdn;
    // Relations
    @ManyToOne
    @JoinColumn(name = "organization_id")
    private Organization organization;
}        

Once you have defined your entities, you can use Spring Data JPA to perform the database's CRUD (create, read, update, delete) operations. This is handled in the service class, where all the business logic is handled.

Defining a service

In Spring Boot, a service class encapsulates business logic and provides an abstraction layer between the API controller and the data access layer. Here are the steps to write a service class :

1. Define the service interface: Define a Java interface for your service. The interface should declare methods for the business logic that you want to encapsulate. You can annotate the interface with the @Service annotation to mark it as a Spring service.
2. Implement the service interface: Create a Java class that implements the service interface. In the implementation class, you can write the business logic for each method declared in the interface. You can annotate the implementation class with the @Service annotation to mark it as a Spring service.
3. Inject the service into the controller: In your API controller class, you can inject an instance of the service class using Spring's dependency injection.
4. Use the service in the controller: In your controller methods, you can call methods on the service to perform business logic. The service can also interact with the data access layer (e.g., the repository classes) to retrieve and manipulate data.

@Service
public class ClientServiceImpl implements ClientService {
    @Autowired
    private ClientRepository clientRepository;

    public List<Clients> getAllClients() {
        return clientRepository.findAll();
    }

    public Client getClientById(Long id) {
        return clientRepository.findById(id).orElse(null);
    }

    public void createClient(Client client) {
        clientRepository.save(client);
    }
}        

Defining the repository

The JpaRepository interface is part of the Spring Data JPA library, and it provides a set of pre-defined methods that can be used to perform CRUD operations on a specific entity. These methods include:

• findById(): finds an entity by its ID.
• findAll(): finds all entities of a certain type.
• Save (): saves a new entity or updates an existing one.
• deleteById(): deletes an entity by its ID.

Extending JpaRepository in your repository interface automatically inherits all of these methods. This makes it easy to perform common database operations without writing SQL code.

In addition to the basic CRUD methods, you can define custom repository methods by following Spring Data JPA naming conventions or using the @Query annotation to write custom SQL queries. This allows you to define more complex database operations not covered by the default JpaRepository methods.

Here's an example of a repository interface for a "Client" entity that defines a custom method using Spring Data JPA naming conventions:

@Repository
public interface ClientRepository extends JpaRepository<Client, Client> {
? ? List<Client> findById(Long id);
}        

Connecting to the database

Configure MySQL database connection in application.properties. This will utilize the dependencies you set up when initializing the project?

Next, you can configure your MySQL database connection in the application.properties file. This can be done by adding the following code:

spring.datasource.url=jdbc:mysql://localhost:3306/mydatabase
spring.datasource.username=root
spring.datasource.password=password
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver        

?Use can also use the YAML file to set the configurations for your database

pring:
  datasource:

    url: jdbc:mysql://localhost:3306/name?allowPublicKeyRetrieval=true&useUnicode=true&characterEncoding=utf8&useSSL=false&autoReconnect=true
    username: root
    password: pass

jpa
  hibernate:
    ddl-auto: update
  properties:
    hibernate:
      dialect: org.hibernate.dialect.MySQL8Dialect
      format_sql: true
  show-sql: true:        

Defining endpoints for your API

Endpoints are URLs that your clients use to interact with your RESTful API. Each endpoint corresponds to a specific resource in your application and defines the operations that can be performed on that specific resource.

In a RESTful API, endpoints are organized around resources and use HTTP methods to define the operation performed on that resource. Endpoints can be placed in a specific package, called controllers or resources. In the controller package, you can generate different classes corresponding to the various entities you work with within your RESTful APIs.

The four most common HTTP methods used in RESTful APIs are:

GET: Used to retrieve a resource or a collection of resources.

POST: Used to create a new resource.

PUT: Used to update an existing resource.

DELETE: Used to delete a resource.

To define endpoints in a Spring Boot application, you can use the @RestController annotation. This annotation indicates that the class should be treated as a controller that handles incoming requests and returns responses.

@RestController
@RequestMapping("/api/clients")
public class ClientController {

? ? @GetMapping
? ? public List<Client> getClients() {
? ? 
? ? }
}        

Handling requests and responses

In a RESTful API, the HTTP protocol sends requests and responses between the client and the server. When a client requests an endpoint in the API, the server receives the request, processes it, and sends back a response. The response typically includes a status code like 200- for successful requests, 400 for client-side errors, or 500 for server-side errors,?headers, and a body containing the requested data or error messages.

To handle incoming requests in a Spring Boot application, several annotations help you manage how requests and responses are handled. You can use the @RequestMapping annotation. This annotation maps an incoming request to a specific method in your controller based on the URL and HTTP method of the request.

If you want to handle form data, you can use the @RequestParam annotation to extract individual parameters from the request:

Handling requests and responses in a Spring Boot application involves mapping incoming requests to specific methods in your controller and using annotations like @RequestBody and @RequestParam to extract and manipulate the data in those requests.

@PostMapping("/api/clients"
public Client createClient(@RequestParam("username") String username, @RequestParam("password") String password) {
? ? 
}

)        

Validating input and output

Validating input and output is important in a RESTful API to ensure that the data sent and received is accurate, consistent, and secure. Input validation helps prevent errors and malicious attacks by ensuring that the data sent to the API is in the expected format and within acceptable limits. Output validation ensures that the data sent back to the client is also in the expected format and contains no sensitive or unnecessary information.?

We use the Java validation API to add validations in our APIs and return the appropriate response to the user; we use the @Valid annotation, a Javax validation API in spring-boot-starter-web. We can? also use the @NotNull annotation to ensure that a parameter or argument is not null or the @Size annotation to check the size of a string?

Here's an example of how to use validation annotations in Spring Boot:

@PostMapping("/api/clients"
public User createClient(@Valid @RequestBody Client client) {
? ? // code to create a new client from the request data
}

)        

Adding security to your API

Security is critical to any RESTful API, as it helps protect sensitive data and ensure that only authorized users can access certain endpoints. There are several types of security threats that RESTful APIs may face, such as SQL injection, cross-site scripting (XSS), cross-site request forgery (CSRF), and unauthorized access.

To add security to a Spring Boot application, developers can use Spring Security, a powerful and flexible security framework that provides a wide range of authentication and authorization features. Spring Security can protect web and non-web applications and integrates seamlessly with other frameworks.

To secure a RESTful API using Spring Security, developers can use the @Secured annotation to add security constraints to individual endpoints or controllers. The @Secured annotation allows developers to specify which roles or authorities are required to access a particular endpoint.

In addition to role-based security, Spring Security supports various authentication mechanisms, such as basic and token-based authentication. Basic authentication involves sending the username and password in the header of each request, while token-based authentication involves using a token or JWT (JSON Web Token) to authenticate requests.

Token-based authentication is becoming increasingly popular in RESTful APIs as it provides a more secure and scalable approach to authentication. With token-based authentication, a client logs in once and receives a token that can be used to authenticate subsequent requests. The token is typically stored in the client's browser or mobile app and sent in the header of each request to the API.

This article explored the basics of building a RESTful API using Spring Boot. It discussed setting up a new Spring Boot project using the Spring Initializr, defining endpoints using the @RestController annotation, and handling requests and responses in a RESTful API.

The article also covered the importance of input and output validation and adding security to a RESTful API using Spring Security and different authentication mechanisms.

Using Spring Boot, developers can quickly and easily create a production-grade, standalone Spring application without requiring manual configuration or setup.?