Chapter 4: Professional 3-Layer Architecture

Introduction

This chapter synthesizes everything you have learned in the previous chapters to create a professional, robust, and maintainable architecture.

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Architecture Overview

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Responsibilities by Layer

Layer	Role	Technologies	Annotations
Controller	HTTP Management, Validation, Conversion	Spring MVC	@RestController, @GetMapping
Service	Business Logic, Orchestration, Transactions	Spring Core	@Service, @Transactional
Repository	Data Access, DB Queries	Spring Data JPA	@Repository, JpaRepository
Exception Handler	Error Handling, Response Formatting	Spring MVC	@ControllerAdvice

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Complete Project: Product Management

Package Structure

src/main/java/com/example/demo/
├── DemoApplication.java
├── controller/
│   └── ProductController.java
├── service/
│   └── ProductService.java
├── repository/
│   └── ProductRepository.java
├── model/
│   └── Product.java
└── exception/
    ├── ProductNotFoundException.java
    ├── InvalidProductException.java
    ├── ErrorResponse.java
    └── GlobalExceptionHandler.java

1. Entity (Model)

package com.example.demo.model;

import jakarta.persistence.*;
import jakarta.validation.constraints.*;

@Entity
@Table(name = "products")
public class Product {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @NotBlank(message = "Name cannot be empty")
    @Size(min = 3, max = 50, message = "Name must be between 3 and 50 characters")
    private String name;
    
    @NotNull(message = "Price is required")
    @Min(value = 0, message = "Price cannot be negative")
    private Double price;
    
    @NotNull(message = "Quantity is required")
    @Min(value = 0, message = "Quantity cannot be negative")
    private Integer quantity;
    
    // Constructors
    public Product() {}
    
    public Product(String name, Double price, Integer quantity) {
        this.name = name;
        this.price = price;
        this.quantity = quantity;
    }
    
    // Getters and Setters
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    
    public Double getPrice() { return price; }
    public void setPrice(Double price) { this.price = price; }
    
    public Integer getQuantity() { return quantity; }
    public void setQuantity(Integer quantity) { this.quantity = quantity; }
}

2. Repository

package com.example.demo.repository;

import com.example.demo.model.Product;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

import java.util.List;

@Repository
public interface ProductRepository extends JpaRepository<Product, Long> {
    // Inherited methods: save, findById, findAll, deleteById
    
    // Custom methods
    List<Product> findByName(String name);
    List<Product> findByPriceLessThan(Double price);
}

3. Service

package com.example.demo.service;

import com.example.demo.exception.ProductNotFoundException;
import com.example.demo.model.Product;
import com.example.demo.repository.ProductRepository;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

import java.util.List;

@Service
@Transactional
public class ProductService {

    private final ProductRepository productRepository;
    
    public ProductService(ProductRepository productRepository) {
        this.productRepository = productRepository;
    }
    
    // CREATE
    public Product createProduct(Product product) {
        // Additional business validation if necessary
        return productRepository.save(product);
    }
    
    // READ - All
    public List<Product> getAllProducts() {
        return productRepository.findAll();
    }
    
    // READ - By ID
    public Product getProductById(Long id) {
        return productRepository.findById(id)
            .orElseThrow(() -> new ProductNotFoundException(id));
    }
    
    // UPDATE
    public Product updateProduct(Long id, Product productDetails) {
        Product product = getProductById(id);
        
        product.setName(productDetails.getName());
        product.setPrice(productDetails.getPrice());
        product.setQuantity(productDetails.getQuantity());
        
        return productRepository.save(product);
    }
    
    // DELETE
    public void deleteProduct(Long id) {
        Product product = getProductById(id);
        productRepository.delete(product);
    }
}

4. Controller

package com.example.demo.controller;

import com.example.demo.model.Product;
import com.example.demo.service.ProductService;
import jakarta.validation.Valid;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/api/products")
public class ProductController {

    private final ProductService productService;
    
    public ProductController(ProductService productService) {
        this.productService = productService;
    }
    
    // CREATE
    @PostMapping
    public ResponseEntity<Product> createProduct(@Valid @RequestBody Product product) {
        Product created = productService.createProduct(product);
        return ResponseEntity.status(HttpStatus.CREATED).body(created);
    }
    
    // READ - All
    @GetMapping
    public ResponseEntity<List<Product>> getAllProducts() {
        List<Product> products = productService.getAllProducts();
        return ResponseEntity.ok(products);
    }
    
    // READ - By ID
    @GetMapping("/{id}")
    public ResponseEntity<Product> getProductById(@PathVariable Long id) {
        Product product = productService.getProductById(id);
        return ResponseEntity.ok(product);
    }
    
    // UPDATE
    @PutMapping("/{id}")
    public ResponseEntity<Product> updateProduct(@PathVariable Long id, 
                                                  @Valid @RequestBody Product product) {
        Product updated = productService.updateProduct(id, product);
        return ResponseEntity.ok(updated);
    }
    
    // DELETE
    @DeleteMapping("/{id}")
    public ResponseEntity<Void> deleteProduct(@PathVariable Long id) {
        productService.deleteProduct(id);
        return ResponseEntity.noContent().build();
    }
}

5. Exceptions

package com.example.demo.exception;

public class ProductNotFoundException extends RuntimeException {
    public ProductNotFoundException(Long id) {
        super("Product not found with id: " + id);
    }
}

package com.example.demo.exception;

import java.time.LocalDateTime;

public class ErrorResponse {
    private LocalDateTime timestamp;
    private int status;
    private String error;
    private String message;
    private String path;
    
    public ErrorResponse(int status, String error, String message, String path) {
        this.timestamp = LocalDateTime.now();
        this.status = status;
        this.error = error;
        this.message = message;
        this.path = path;
    }
    
    // Getters and Setters (generated)
    public LocalDateTime getTimestamp() { return timestamp; }
    public void setTimestamp(LocalDateTime timestamp) { this.timestamp = timestamp; }
    
    public int getStatus() { return status; }
    public void setStatus(int status) { this.status = status; }
    
    public String getError() { return error; }
    public void setError(String error) { this.error = error; }
    
    public String getMessage() { return message; }
    public void setMessage(String message) { this.message = message; }
    
    public String getPath() { return path; }
    public void setPath(String path) { this.path = path; }
}

6. Exception Handler

package com.example.demo.exception;

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.validation.FieldError;
import org.springframework.web.bind.MethodArgumentNotValidException;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.context.request.WebRequest;

import java.util.HashMap;
import java.util.Map;

@ControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(ProductNotFoundException.class)
    public ResponseEntity<ErrorResponse> handleProductNotFound(
            ProductNotFoundException ex, WebRequest request) {
        
        ErrorResponse error = new ErrorResponse(
            HttpStatus.NOT_FOUND.value(),
            "Not Found",
            ex.getMessage(),
            request.getDescription(false).replace("uri=", "")
        );
        
        return ResponseEntity.status(HttpStatus.NOT_FOUND).body(error);
    }
    
    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<Map<String, String>> handleValidationExceptions(
            MethodArgumentNotValidException ex) {
        
        Map<String, String> errors = new HashMap<>();
        ex.getBindingResult().getAllErrors().forEach((error) -> {
            String fieldName = ((FieldError) error).getField();
            String errorMessage = error.getDefaultMessage();
            errors.put(fieldName, errorMessage);
        });
        
        return ResponseEntity.status(HttpStatus.BAD_REQUEST).body(errors);
    }
    
    @ExceptionHandler(Exception.class)
    public ResponseEntity<ErrorResponse> handleGlobalException(
            Exception ex, WebRequest request) {
        
        ErrorResponse error = new ErrorResponse(
            HttpStatus.INTERNAL_SERVER_ERROR.value(),
            "Internal Server Error",
            ex.getMessage(),
            request.getDescription(false).replace("uri=", "")
        );
        
        return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(error);
    }
}

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Configuration application.yml

spring:
  datasource:
    url: jdbc:h2:mem:testdb
    driver-class-name: org.h2.Driver
    username: sa
    password:
  
  h2:
    console:
      enabled: true
      path: /h2-console
  
  jpa:
    hibernate:
      ddl-auto: create-drop
    show-sql: true
    properties:
      hibernate:
        format_sql: true

server:
  port: 8080

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Testing with Postman

1. CREATE - POST /api/products

Request:

POST http://localhost:8080/api/products
Content-Type: application/json

{
  "name": "Laptop",
  "price": 999.99,
  "quantity": 10
}

Response: 201 Created

{
  "id": 1,
  "name": "Laptop",
  "price": 999.99,
  "quantity": 10
}

2. GET ALL - GET /api/products

Response: 200 OK

[
  {
    "id": 1,
    "name": "Laptop",
    "price": 999.99,
    "quantity": 10
  }
]

3. GET BY ID - GET /api/products/1

Response: 200 OK

{
  "id": 1,
  "name": "Laptop",
  "price": 999.99,
  "quantity": 10
}

4. UPDATE - PUT /api/products/1

Request:

{
  "name": "Laptop Pro",
  "price": 1299.99,
  "quantity": 5
}

Response: 200 OK

{
  "id": 1,
  "name": "Laptop Pro",
  "price": 1299.99,
  "quantity": 5
}

5. DELETE - DELETE /api/products/1

Response: 204 No Content

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Naming Conventions

Packages

com.example.demo.controller
com.example.demo.service
com.example.demo.repository
com.example.demo.model
com.example.demo.exception

Classes

• Controller: ProductController, UserController
• Service: ProductService, UserService
• Repository: ProductRepository, UserRepository
• Model: Product, User
• Exception: ProductNotFoundException, InvalidProductException

Methods

• CREATE: create...() or add...()
• READ: get...(), find...(), getAll...()
• UPDATE: update...()
• DELETE: delete...()

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Chapter Summary

Professional Architecture

✅ Controller: Only HTTP management
✅ Service: Business logic and orchestration
✅ Repository: Data access
✅ Exception Handler: Global error handling

Key Points

• Clear separation of responsibilities
• Constructor injection
• Two-level validation (syntactic + business)
• Professional error handling
• Appropriate HTTP codes
• Standardized project structure

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Proceed to TP3.1: Complete CRUD Architecture

Back to Module 3