Spring Boot

# application.properties (Spring Boot 3.2+ on Java 21+)
spring.threads.virtual.enabled=true

public record UserRequest(
    @NotBlank(message = "Name required") String name,
    @Email(message = "Invalid email") String email
) {}

@RestControllerAdvice
public class ValidationHandler {
    @ExceptionHandler(MethodArgumentNotValidException.class)
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    public Map<String, String> handleErrors(MethodArgumentNotValidException ex) {
        Map<String, String> errors = new HashMap<>();
        ex.getBindingResult().getFieldErrors().forEach(e -> errors.put(e.getField(), e.getDefaultMessage()));
        return errors;
    }
}

@SpringBootTest
@Testcontainers
class UserIntegrationTest {
    @Container
    @ServiceConnection // Boot 3.1+ auto-wires PostgreSQL connection settings
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:15-alpine");

    @Test
    void testDbOperation() { /* Tests run against real isolated PostgreSQL Docker instance */ }
}

@Configuration
@EnableScheduling
public class SchedulerConfig {}

@Component
public class ReportJob {
    // Runs at 2:00 AM every day
    @Scheduled(cron = "0 0 2 * * ?")
    public void generateDailyReport() {
        System.out.println("Daily report generated.");
    }
}

public interface ProductRepository extends JpaRepository<Product, Long> {
    // Inherits findAll(Pageable), flush(), and saveAll()
}

# Build native executable using Native Build Tools plugin:
# ./mvnw native:compile -Pnative
# Run compiled native binary:
# ./target/myapp

# application.properties
management.tracing.sampling.probability=1.0
management.zipkin.tracing.endpoint=http://zipkin:9411/api/v2/spans

// 1. Auto-configuration class
@AutoConfiguration
@EnableConfigurationProperties(MyProps.class)
public class MyAutoConfig {
    @Bean @ConditionalOnMissingBean
    public MyClient myClient(MyProps props) { return new MyClient(props.getApiKey()); }
}
// 2. Registered in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports

# application.properties
server.shutdown=graceful
spring.lifecycle.timeout-per-shutdown-phase=30s

# Command to build production container image directly:
# mvn spring-boot:build-image -Dspring-boot.build-image.imageName=myapp:v1

# Manifest.MF entry inside Fat JAR
# Main-Class: org.springframework.boot.loader.launch.JarLauncher
# Start-Class: com.example.Application

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <scope>runtime</scope>
    <optional>true</optional>
</dependency>

@RestController
@RequestMapping("/stream")
public class StreamController {
    @GetMapping(produces = MediaType.TEXT_EVENT_STREAM_VALUE)
    public Flux<String> streamEvents() {
        return Flux.interval(Duration.ofSeconds(1))
                   .map(seq -> "Event #" + seq);
    }
}

@SpringBootTest
class OrderIntegrationTest {
    @Autowired private OrderService orderService;
    @MockBean private PaymentGateway paymentGateway; // Replaces real external gateway

    @Test
    void testCheckout() {
        Mockito.when(paymentGateway.charge(100.0)).thenReturn(true);
        assertTrue(orderService.checkout(100.0));
    }
}

@WebMvcTest(UserController.class)
class UserControllerTest {
    @Autowired private MockMvc mockMvc;
    @MockBean private UserService userService;

    @Test
    void testGetUser() throws Exception {
        Mockito.when(userService.findById(1L)).thenReturn(new User(1L, "Alice"));
        mockMvc.perform(get("/users/1")).andExpect(status().isOk());
    }
}

<!-- pom.xml -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </exclusion>
    </exclusions>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-undertow</artifactId>
</dependency>

# Disable Hibernate schema creation, let Flyway manage DDL:
spring.jpa.hibernate.ddl-auto=validate
spring.flyway.enabled=true
# Scripts placed in: src/main/resources/db/migration/V1__create_tables.sql

# application.properties
spring.datasource.hikari.maximum-pool-size=20
spring.datasource.hikari.minimum-idle=5
spring.datasource.hikari.idle-timeout=30000
spring.datasource.hikari.connection-timeout=20000

public interface OrderRepository extends JpaRepository<Order, Long> {
    // Solution 1: JPQL JOIN FETCH
    @Query("SELECT o FROM Order o JOIN FETCH o.items WHERE o.status = :status")
    List<Order> findWithItems(@Param("status") String status);

    // Solution 2: @EntityGraph
    @EntityGraph(attributePaths = {"items"})
    List<Order> findAll();
}

public interface UserRepository extends JpaRepository<User, Long> {
    // Derived query method automatically generated:
    Optional<User> findByEmail(String email);
    List<User> findByAgeGreaterThan(int age, Pageable pageable);
}

// Overriding via CLI parameter at startup:
// java -jar app.jar --server.port=9090 --spring.profiles.active=prod

@Service
@Profile("dev")
public class MockPaymentService implements PaymentService {}

@Service
@Profile("prod")
public class ProductionPaymentService implements PaymentService {}

@ConfigurationProperties(prefix = "app.mail")
@Validated
public record MailProperties(
    @NotBlank String host,
    @Min(1) int port,
    List<String> defaultRecipients
) {}

@Autowired
private ApplicationEventPublisher eventPublisher;

public void pauseTraffic() {
    // Marks container unready without killing it
    AvailabilityChangeEvent.publish(eventPublisher, this, ReadinessState.REFUSING_TRAFFIC);
}

# application.properties
management.endpoints.web.exposure.include=health,metrics,info,loggers
management.endpoint.health.show-details=always

<!-- Web Starter includes Spring MVC, Jackson, Tomcat, and Validation -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

@Configuration
public class PaymentConfig {
    @Bean
    @ConditionalOnProperty(name = "payment.gateway", havingValue = "stripe", matchIfMissing = true)
    public PaymentGateway stripeGateway() { return new StripeGateway(); }
}

@AutoConfiguration
@ConditionalOnClass(DataSource.class)
@ConditionalOnMissingBean(DataSource.class)
public class DataSourceAutoConfiguration {
    @Bean
    public DataSource defaultDataSource() { return new HikariDataSource(); }
}

// Equivalent to @SpringBootApplication:
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(basePackages = "com.example.app")
public class CustomApplicationConfig {}

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}