Category: Java Programming

What is a REST API?

REST stands for Representational State Transfer which simply is a set of rules for software to interact with each other over the network. There are many other protocols for software to communicate with each other such as SOAP (Simple Object Access Protocol), GraphQL (An API query language), gRPC (gRPC Remote Procedure Call) etc. REST is one among them. REST is widely popular due to being simple, interoperable, lightweight and scalable. It is suited well for web services and applications with CRUD operations.

Key Features of REST APIs

Understanding Resources and URIs are key to understanding REST APIs. Resources are data objects that can be accessed via REST APIs. They are identified with a Uniform Resource Identifier (URI). HTTP methods such as GET, POST, PUT and DELETE are used to perform operations on the resources. The resources are represented in formats such as JSON, XML etc.

REST APIs have the following key features.

Client Server Architecture

REST operates on a Client-Server model where a client sends a request to the server and the server sends back an appropriate response. This is done using the URIs mentioned above. This type of model also allows for the client and server to be modified independently offering a separation of concerns.

Statelessness

REST is stateless meaning the client should have the all the information that the server needs to process a request, contained within the request. The server won’t store any client state. Each request is handled independently by the server.

Layered Architecture

REST has a layered architecture where each layer is independent and interacts with the layers interfacing with it. This enables scalability and security in the system. For example, layers such as load balancing, security etc can be added to the system and the client will not know which layers it is talking to.

REST API Methods

REST APIs use standard HTTP methods making them simpler to use.

• GET – To retrieve data
• HEAD – Transfer only the status line and header section
• POST – To create a new resource
• PUT – Update an existing resource
• DELETE – Delete a resource
• PATCH – Modify an existing resource partially

We will go over examples for some of these in the section below.

Build a REST API using Java Spring Boot

Let us build a simple User REST API using Java Spring Boot. Go to https://start.spring.io/ and generate a new project called restApi with the dependencies – Spring Data JPA (For database operations) and Spring Web (for REST APIs).

You can find the full working code on my Github link here.

Next, we will create a simple User REST API. There are 5 main components to the REST API.

• User class – This defines the structure of a User Entity.
• User Repository – handles data operations.
• User Service – contains basic methods to manipulate User objects.
• User REST API – Contains the GET, POST etc methods to perform http operations.
• application.properties – To define the database connection for the REST API.

Design a User Entity

User has three defining class variables – user id, user name and user email. We will also add getters and setters for these variables.

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String userName;
    private String userEmail;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getUserEmail() {
        return userEmail;
    }

    public void setUserEmail(String userEmail) {
        this.userEmail = userEmail;
    }
}

Define a User Repository

The repository will handle data operations. JpaRepository is a JPA (Java Persistence API) extension of Repository with API for basic CRUD operations and also API for pagination and sorting.

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

Define a User Service

This service interacts with the User Repository and will be called by the REST API to perform operations.

@Service
public class UserService {

    @Autowired
    private UserRepository userRepository;

    public List<User> getAllUsers() {
        return userRepository.findAll();
    }

    public User getUserById(Long id) {
        return userRepository.findById(id).orElse(null);
    }

    public User createUser(User user) {
        return userRepository.save(user);
    }

    public void deleteUser(Long id) {
        userRepository.deleteById(id);
    }
}

Define the User REST APIs

This is the final User REST API. We need to specify @RestController for Spring Application to recognize our REST API.

We will use @GetMapping and @PostMapping annotations to mark the functionality of different endpoints of the REST API.

@RestController
@RequestMapping("/users")
public class UserController {

    @Autowired
    private UserService userService;

    @GetMapping
    public List<User> getAllUsers() {
        return userService.getAllUsers();
    }

    @GetMapping("/{id}")
    public User getUserById(@PathVariable Long id) {
        return userService.getUserById(id);
    }

    @PostMapping
    public User createUser(@RequestBody User user) {
        return userService.createUser(user);
    }

    @DeleteMapping("/{id}")
    public void deleteUser(@PathVariable Long id) {
        userService.deleteUser(id);
    }
}

Add a properties file for the Database connection

For the connection to work, you need to install MySQL connector and MySQL workbench, set up username and password and create a database called restApi. The MySQL server should be running when you run the Spring Boot Application.

spring.application.name=restApi
spring.datasource.url=jdbc:mysql://localhost:3306/restApi
spring.datasource.username=
spring.datasource.password=
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
spring.jpa.hibernate.ddl-auto = update

For the full code, refer to my Github project here. Run the main application and your app should now be running at localhost:8080

Using the REST API

We initially do not have any users in our database. Let’s add some users.

Add users

curl -X POST http://localhost:8080/users \
     -H "Content-Type: application/json" \
     -d '{
           "userName": "John Doe",
           "userEmail": "johndoe@example.com"
         }'

curl -X POST http://localhost:8080/users \
     -H "Content-Type: application/json" \
     -d '{
           "userName": "Alex",
           "userEmail": "alex@example.com"
         }'

curl -X POST http://localhost:8080/users \
     -H "Content-Type: application/json" \
     -d '{
           "userName": "Tom",
           "userEmail": "tom@example.com"
         }'

List Users

Delete User

curl -X DELETE http://localhost:8080/users/1

Now that we know how to build a simple REST API, let’s look at some best practices to keep in mind when designing REST APIs.

REST API Best Practices

Here are some best practices to adhere to when designing REST APIs.

• Use the right HTTP methods for different operations.
• Use nouns for resource names such as id etc. and plural forms for collections such as users, items etc.
• Be consistent with the naming conventions.
• Use the right HTTP Status codes.
• Handle errors on the server side and offer meaningful debug information.
• Add rate limiting for your application.
• Implement the right authentication to prevent unauthorized access to the application or data.
• Document your REST APIs for easy usage.

When to Use REST Vs Other Protocols?

There are various other protocols out there such as SOAP, gRPC, GraphQL, WebsSockets etc.

REST works well in most cases for general purpose APIs, they are simple to implement and interface with and are compatible with most web technologies.

Other protocols such as SOAP are used in more enterprise environments, gRPC is better in use cases which need high performance and low latencies, GraphQL allows more flexibility with fetching data, Web Sockets work well for real time communication APIs.

REST API Observability and Monitoring

It is important to ensure that we log meaningful information and track metrics of our system for observability and performance monitoring.

Define the key metrics for your application and track metrics such as Latency, Throughput, Error rates, CPU usage, memory usage etc. There are many tools these days such as Prometheus for metrics collection and Grafana for visualizing metrics data.

Tracking logs such as Error Logs, Access Logs is also important when designing and maintaining APIs.

Once the metrics and logs are tracked, you can set up alerting on these to take action based on various scenarios.

Summary

I hope you enjoyed reading this ultimate guide to REST APIs where we started from the basics of REST APIs, went on build a simple REST API from scratch using Java SpringBoot and went over REST API design best practices as well Observability and Monitoring to-dos for a system.

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