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Spring Boot Basics

Spring Boot is the most widely used Java framework for building web services. It provides sensible defaults, an embedded server and dependency injection.

Create a project​

Generate a project at start.spring.io with Spring Web, Validation and Spring Data JPA, or use the template for your IDE. The entry point:

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

A REST controller​

import jakarta.validation.Valid;
import jakarta.validation.constraints.*;
import org.springframework.http.*;
import org.springframework.web.bind.annotation.*;
import java.util.*;

record NoteRequest(@NotBlank @Size(max = 500) String text) {}
record Note(long id, String text) {}

@RestController
@RequestMapping("/api/notes")
class NoteController {

    private final NoteService service;

    NoteController(NoteService service) {      // constructor injection
        this.service = service;
    }

    @GetMapping
    List<Note> list() {
        return service.findAll();
    }

    @PostMapping
    ResponseEntity<Note> create(@Valid @RequestBody NoteRequest req) {
        Note created = service.create(req.text());
        return ResponseEntity.status(HttpStatus.CREATED).body(created);
    }

    @GetMapping("/{id}")
    Note get(@PathVariable long id) {
        return service.find(id).orElseThrow(
            () -> new ResponseStatusException(HttpStatus.NOT_FOUND));
    }
}

Dependency injection​

Spring creates and wires objects (beans) for you.

import org.springframework.stereotype.Service;

@Service
class NoteService {
    private final NoteRepository repo;
    NoteService(NoteRepository repo) { this.repo = repo; }
    // business logic here
}
AnnotationRole
@RestController, @ControllerWeb layer
@ServiceBusiness logic
@RepositoryData access
@Configuration, @BeanExplicit bean definitions
@Value, @ConfigurationPropertiesRead configuration

Configuration​

server:
  port: 8080
spring:
  datasource:
    url: ${DATABASE_URL}
    username: ${DB_USER}
    password: ${DB_PASSWORD}
  jpa:
    hibernate:
      ddl-auto: validate

Keep secrets in environment variables, never in the repository.

Testing​

@WebMvcTest(NoteController.class)
class NoteControllerTest {
    @Autowired MockMvc mvc;
    @MockBean NoteService service;

    @Test
    void rejectsBlankText() throws Exception {
        mvc.perform(post("/api/notes")
                .contentType("application/json")
                .content("{\"text\":\"\"}"))
           .andExpect(status().isBadRequest());
    }
}

Production readiness​

  • Add spring-boot-starter-actuator for /actuator/health and metrics.
  • Handle errors with @ControllerAdvice.
  • Secure endpoints with Spring Security; hash passwords with bcrypt.
  • See Authentication and web security for concepts.
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