Lesson 35 • Advanced

REST APIs with Java

REST APIs are how modern apps communicate. Whether it's a mobile app talking to a backend, or microservices talking to each other, Spring Boot makes building production-quality REST APIs surprisingly straightforward. This is one of the most marketable Java skills you can have.

Before You Start

You should know Annotations (Spring is annotation-driven) and JDBC (data layer). Understanding HTTP methods (GET, POST, PUT, DELETE) and JSON is essential.

What You'll Learn

✅ REST principles: resources, HTTP methods, status codes
✅ Spring Boot @RestController and @RequestMapping
✅ Path variables, query parameters, request bodies
✅ Response entities and proper HTTP status codes
✅ Input validation with @Valid and Bean Validation
✅ Global exception handling with @ControllerAdvice

1️⃣ REST Design Principles

Analogy: Think of a REST API like a restaurant menu. Each endpoint is a menu item (resource). You use different HTTP methods to interact: GET (read the menu), POST (place an order), PUT (change your order), DELETE (cancel). The waiter (server) responds with a status code — 200 (here's your food), 404 (we don't serve that), 500 (kitchen exploded).

Method	Endpoint	Action	Status
GET	/api/users	List all users	200 OK
GET	/api/users/42	Get user #42	200 / 404
POST	/api/users	Create user	201 Created
PUT	/api/users/42	Replace user	200 OK
DELETE	/api/users/42	Delete user	204 No Content

Try It: REST API Request Router

Try it Yourself »

JavaScript

// 💡 Try modifying this code and see what happens!
// Build a mini REST API router (simulating Spring Boot)

console.log("=== REST API Request Router ===\n");

// In-memory database
let users = [
  { id: 1, name: "Alice", email: "alice@test.com" },
  { id: 2, name: "Bob", email: "bob@test.com" },
];
let nextId = 3;

// REST Controller (simulating @RestController)
function handleRequest(method, path, body) {
  let match;
  
  // GET /api/users
  if (method === "GET" && path === "/api/users") {
 
...

2️⃣ Input Validation & Error Handling

Never trust client input. Spring Boot's @Valid annotation with Bean Validation annotations automatically rejects malformed requests with a 400 status code. Combine with @ControllerAdvice for consistent error responses across your entire API.

@NotBlank — field must not be null or empty

@Email — must be a valid email format

@Min(0) @Max(150) — numeric range validation

@Size(min=2, max=50) — string length constraints

Try It: Validation & Error Handling

Try it Yourself »

JavaScript

// 💡 Try modifying this code and see what happens!
// Input validation and global error handling simulation

console.log("=== Validation & Error Handling ===\n");

// Validation rules (simulating Bean Validation annotations)
const validationRules = {
  name: { required: true, minLength: 2, maxLength: 50 },
  email: { required: true, pattern: /^[^@]+@[^@]+\.[^@]+$/ },
  age: { min: 0, max: 150 },
};

function validate(data) {
  let errors = [];
  
  // @NotBlank name
  if (!data.name || data.nam
...

Common Mistakes

⚠️ Using verbs in URLs: /api/getUser is wrong — use GET /api/users/42
⚠️ Always returning 200: Use proper status codes — 201 for creation, 204 for deletion, 400 for bad input
⚠️ Exposing entities directly: Use DTOs to control what data reaches the client
⚠️ No input validation: Always use @Valid — never trust client input

Pro Tips

💡 Use @ControllerAdvice for global exception handling — one place for all error responses
💡 ResponseEntity<T> gives you full control over headers and status codes
💡 Use pagination for list endpoints: ?page=0&size=20&sort=name,asc
💡 Version your API: /api/v1/users — don't break existing clients

Try It: Full REST API with Pagination

Try it Yourself »

JavaScript

// 💡 Try modifying this code and see what happens!
// Full REST API with pagination and filtering

console.log("=== REST API with Pagination ===\n");

// Generate sample data
let products = [];
for (let i = 1; i <= 50; i++) {
  products.push({
    id: i,
    name: "Product " + i,
    category: ["Electronics", "Books", "Clothing"][i % 3],
    price: Math.round(Math.random() * 200 + 10),
  });
}
console.log("Database: " + products.length + " products\n");

// Paginated endpoint: GET /api/products
...

📋 Quick Reference

Annotation	Purpose	Example
@RestController	REST controller class	Returns JSON by default
@GetMapping	Handle GET requests	@GetMapping("/{id}")
@RequestBody	Deserialize JSON body	POST/PUT payloads
@PathVariable	URL path parameter	/users/{id}
@Valid	Trigger validation	Bean Validation checks

🎉 Lesson Complete!

You can now build production-quality REST APIs with Java and Spring Boot! Next: JSON & XML — serialization and data interchange formats.

REST APIs with Java

Before You Start

What You'll Learn

1️⃣ REST Design Principles

Try It: REST API Request Router

2️⃣ Input Validation & Error Handling

Try It: Validation & Error Handling

Common Mistakes

Pro Tips

Try It: Full REST API with Pagination

📋 Quick Reference

🎉 Lesson Complete!

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