Spring Boot

TL;DR

One-liner: Spring Boot is Java’s opinionated framework for production apps - auto-configuration and embedded servers mean zero XML, just run.

Core Strengths:

• Auto-configuration - sensible defaults, no boilerplate
• Embedded servers - Tomcat/Jetty built-in, no deployment needed
• Spring ecosystem - Security, Data, Cloud integration
• Production-ready - health checks, metrics, externalized config

Core Concepts

Concept 1: Auto-Configuration

Spring Boot configures beans automatically based on classpath:

// Add spring-boot-starter-data-jpa to pom.xml
// Spring Boot auto-configures:
// - DataSource (from application.properties)
// - EntityManagerFactory
// - TransactionManager

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    // That's it - CRUD methods are auto-generated
}

Concept 2: Annotations-Driven

Controllers and services use simple annotations:

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

    @Autowired
    private UserService userService;

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

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

    @PostMapping
    public User create(@RequestBody @Valid User user) {
        return userService.save(user);
    }
}

Concept 3: Externalized Configuration

application.properties (or .yml) drives everything:

# Server
server.port=8080

# Database
spring.datasource.url=jdbc:postgresql://localhost:5432/mydb
spring.datasource.username=${DB_USER}
spring.datasource.password=${DB_PASS}

# JPA
spring.jpa.hibernate.ddl-auto=validate
spring.jpa.show-sql=true

Quick Start

Create Project

Go to start.spring.io:

• Project: Maven
• Language: Java
• Dependencies: Spring Web
• Generate and unzip

Create Controller

// src/main/java/com/example/demo/HelloController.java
package com.example.demo;

import org.springframework.web.bind.annotation.*;

@RestController
public class HelloController {
    @GetMapping("/")
    public String hello() {
        return "Hello Spring Boot!";
    }
}

Run

./mvnw spring-boot:run
# Open http://localhost:8080

Gotchas

Constructor injection over @Autowired

// ❌ Field injection - harder to test
@Service
public class UserService {
    @Autowired
    private UserRepository repo;
}

// ✅ Constructor injection - clean and testable
@Service
public class UserService {
    private final UserRepository repo;

    public UserService(UserRepository repo) {
        this.repo = repo;
    }
}

Validation requires @Valid

// ❌ Validation won't run
@PostMapping
public User create(@RequestBody User user) { ... }

// ✅ @Valid triggers validation
@PostMapping
public User create(@RequestBody @Valid User user) { ... }

// In your DTO:
public class User {
    @NotBlank
    private String name;

    @Email
    private String email;
}

Profile-specific config

# application.properties (default)
spring.profiles.active=dev

# application-dev.properties
spring.datasource.url=jdbc:h2:mem:testdb

# application-prod.properties
spring.datasource.url=jdbc:postgresql://prod-db:5432/app

When to Use

Best for:

• Enterprise applications
• Microservices architecture
• Teams with Java experience
• Projects needing Spring ecosystem

Not ideal for:

• Simple scripts (too heavy)
• Serverless (cold start time)
• Teams new to Java

Comparison:

Feature	Spring Boot	Quarkus	Micronaut
Startup	Slow	Fast	Fast
Memory	Higher	Lower	Lower
Ecosystem	Huge	Growing	Growing
Native	GraalVM	GraalVM	GraalVM

Next Steps

• Spring Boot Documentation
• Spring Initializr
• Spring Guides
• Baeldung

Cheatsheet

Pattern	Code
REST controller	@RestController
GET route	@GetMapping("/path")
POST route	@PostMapping("/path")
Path variable	@PathVariable Long id
Query param	@RequestParam String name
Request body	@RequestBody User user
Validation	@Valid @RequestBody User user
Service bean	@Service
Repository	@Repository
Run	./mvnw spring-boot:run