Java Spring Boot

# Java Spring Boot Skill

Build production-ready Spring Boot applications with modern best practices.

## Overview

This skill covers Spring Boot development including REST APIs, security configuration, data access, actuator monitoring, and cloud integration. Follows Spring Boot 3.x patterns with emphasis on production readiness.

## When to Use This Skill

Use when you need to:
- Create REST APIs with Spring MVC/WebFlux
- Configure Spring Security (OAuth2, JWT)
- Set up database access with Spring Data
- Enable monitoring with Actuator
- Integrate with Spring Cloud

## Topics Covered

### Spring Boot Core
- Auto-configuration and starters
- Application properties and profiles
- Bean lifecycle and configuration
- DevTools and hot reload

### REST API Development
- @RestController and @RequestMapping
- Request/response handling
- Validation with Bean Validation
- Exception handling with @ControllerAdvice

### Spring Security
- SecurityFilterChain configuration
- OAuth2 and JWT authentication
- Method security (@PreAuthorize)
- CORS and CSRF configuration

### Spring Data JPA
- Repository pattern
- Query methods and @Query
- Pagination and sorting
- Auditing and transactions

### Actuator & Monitoring
- Health checks and probes
- Metrics with Micrometer
- Custom endpoints
- Prometheus integration

## Quick Reference

```java
// REST Controller
@RestController
@RequestMapping("/api/users")
@Validated
public class UserController {

    @GetMapping("/{id}")
    public ResponseEntity<User> getUser(@PathVariable Long id) {
        return userService.findById(id)
            .map(ResponseEntity::ok)
            .orElse(ResponseEntity.notFound().build());
    }

    @PostMapping
    public ResponseEntity<User> createUser(@Valid @RequestBody UserRequest request) {
        User user = userService.create(request);
        URI location = URI.create("/api/users/" + user.getId());
        return ResponseEntity.created(location).body(user);
    }
}

// Security Configuration
@Configuration
@EnableWebSecurity
public class SecurityConfig {

    @Bean
    public SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
        return http
            .csrf(csrf -> csrf.disable())
            .sessionManagement(s -> s.sessionCreationPolicy(STATELESS))
            .authorizeHttpRequests(auth -> auth
                .requestMatchers("/actuator/health/**").permitAll()
                .requestMatchers("/api/public/**").permitAll()
                .anyRequest().authenticated())
            .oauth2ResourceServer(oauth2 -> oauth2.jwt(Customizer.withDefaults()))
            .build();
    }
}

// Exception Handler
@RestControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(EntityNotFoundException.class)
    public ProblemDetail handleNotFound(EntityNotFoundException ex) {
        return ProblemDetail.forStatusAndDetail(NOT_FOUND, ex.getMessage());
    }
}
```

## Configuration Templates

```yaml
# application.yml
spring:
  application:
    name: ${APP_NAME:my-service}
  profiles:
    active: ${SPRING_PROFILES_ACTIVE:local}
  jpa:
    open-in-view: false
    properties:
      hibernate:
        jdbc.batch_size: 50

management:
  endpoints:
    web:
      exposure:
        include: health,info,metrics,prometheus
  endpoint:
    health:
      probes:
        enabled: true

server:
  error:
    include-stacktrace: never
```

## Common Patterns

### Layer Architecture
```
Controller → Service → Repository → Database
     ↓