Day 1: Spring Boot Basics · Learn Spring Boot 4

Why Spring Boot?

Mon, 01 Jan 0001 00:00:00 +0000

The problem it solves Plain Java gives you a language, not an application. A service needs an HTTP server, JSON mapping, a connection pool, transactions, config per environment, health checks, metrics and tests. Spring Boot wires all of that from a dependency list and sensible defaults. @RestController class Api { @GetMapping("/ping") String ping() { return "pong"; } } One dependency and four lines is a running HTTP service.

Beans & Dependency Injection

Mon, 01 Jan 0001 00:00:00 +0000

What a bean is A bean is an object the container creates, configures and hands out. You never call new on it yourself. @Service public class GreetingService { public String greet(String name) { return "Hello, " + name + "!"; } } Stereotype Meaning @Component any bean @Service business logic @Repository data access (adds exception translation) @RestController web endpoint returning data @Configuration class that declares @Bean methods All four of the first are @Component underneath — the name documents intent.

Configuration & Profiles

Mon, 01 Jan 0001 00:00:00 +0000

Where values come from Spring reads many sources and lets later ones win: command line — --server.port=9000 environment variables — SERVER_PORT=9000 application-{profile}.properties application.properties defaults in code java -jar app.jar --server.port=9000 SERVER_PORT=9000 java -jar app.jar Relaxed binding maps server.port, SERVER_PORT and server-port to the same property. Reading single values @Component class Mailer { private final String from; Mailer(@Value("${app.mail.from:noreply@example.com}") String from) { this.from = from; } } The part after : is the default.

REST Controllers

Mon, 01 Jan 0001 00:00:00 +0000

Mapping requests @RestController @RequestMapping("/api/books") public class BookController { @GetMapping List all() { … } @GetMapping("/{id}") Book one(@PathVariable long id) { … } @PostMapping Book create(@RequestBody Book b) { … } @PutMapping("/{id}") Book replace(@PathVariable long id, @RequestBody Book b) { … } @DeleteMapping("/{id}") void delete(@PathVariable long id) { … } } @RestController = @Controller + @ResponseBody: return values are serialized, not resolved as view names.

Validation & Error Handling

Mon, 01 Jan 0001 00:00:00 +0000

Bean validation Add spring-boot-starter-validation, annotate the DTO, and mark the parameter @Valid. public record BookRequest( @NotBlank String title, @NotBlank @Size(max = 100) String author, @Min(1450) @Max(2100) int year, @Email String contact) {} @PostMapping BookResponse create(@Valid @RequestBody BookRequest req) { … } A violation throws MethodArgumentNotValidException → 400 before your method runs.

Data Access with JPA

Mon, 01 Jan 0001 00:00:00 +0000

Setup org.springframework.boot spring-boot-starter-data-jpa com.h2database h2 runtime With H2 on the classpath and no spring.datasource.url, Boot configures an in-memory database. For PostgreSQL: spring.datasource.url=jdbc:postgresql://localhost:5432/demo spring.datasource.username=demo spring.datasource.password=${DB_PASSWORD} spring.jpa.hibernate.ddl-auto=validate An entity @Entity public class Book { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; @Column(nullable = false) private String title; private String author; private int year; protected Book() {} // required by JPA public Book(String title, String author, int year) { … } // getters, and setters only where they make sense } Warning Entities are mutable objects tied to a persistence context — not DTOs. Records cannot be entities. Map to a record before returning data from a controller.

Calling Other Services

Mon, 01 Jan 0001 00:00:00 +0000

RestClient The synchronous client since Spring 6.1 — fluent, blocking, the default choice in MVC apps. @Configuration class ClientConfig { @Bean RestClient booksClient(RestClient.Builder builder) { return builder.baseUrl("https://api.example.com").build(); } } Book book = client.get() .uri("/books/{id}", id) .retrieve() .body(Book.class); List all = client.get() .uri("/books?author={a}", author) .retrieve() .body(new ParameterizedTypeReference<>() {}); Book created = client.post() .uri("/books") .contentType(MediaType.APPLICATION_JSON) .body(new BookRequest("Dune", "Herbert", 1965)) .retrieve() .body(Book.class); Handling failures:

Testing

Mon, 01 Jan 0001 00:00:00 +0000

The starter spring-boot-starter-test is in every generated project and brings JUnit 5, AssertJ, Mockito, JSONassert and the Spring test support. ./mvnw test Plain unit tests Constructor injection means most classes need no Spring at all. class GreetingServiceTest { private final GreetingService service = new GreetingService(); @Test void greetsByName() { assertThat(service.greet("Ann")).isEqualTo("Hello, Ann!"); } } Fast, and where the bulk of your tests should live.