Deine erste App: start.spring.io

Enough theory - let's create a running application. This is the birth of BookVault, the community-library API you'll grow across the whole Spring path.

Generate the project

The fastest way to start any Spring project is start.spring.io (the "Spring Initializr"). You pick your options and it hands you a ready-to-run zip. For BookVault:

• Project: Maven · Language: Java · Spring Boot: 4.x
• Group: com.bookvault · Artifact: bookvault · Java: 21
• Dependencies: Spring Web

What it generated

Unzip it and you'll find a complete, runnable project. The build file declares the one dependency you chose (plus test), and the Boot parent that manages versions:

<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>4.0.0</version>
</parent>

<properties>
<java.version>21</java.version>
</properties>

<dependencies>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test</artifactId>
  <scope>test</scope>
</dependency>
</dependencies>

The rest of the tree is small and predictable:

src/main/java/com/bookvault/BookVaultApplication.java   # the @SpringBootApplication main class
src/main/resources/application.yml                       # configuration
src/test/java/com/bookvault/BookVaultApplicationTests.java

Run it

./mvnw spring-boot:run          # Maven
./gradlew bootRun               # Gradle

Boot starts an embedded Tomcat server - there's no separate server to install or deploy a WAR to. The app is the server:

  ::  Spring Boot  ::                (v4.0.0)

INFO  Starting BookVaultApplication using Java 21
INFO  Tomcat initialized with port 8080 (http)
INFO  Tomcat started on port 8080 (http) with context path '/'
INFO  Started BookVaultApplication in 1.24 seconds

Add the first endpoint

A single controller makes BookVault answer HTTP. For now it returns a hard-coded catalog - we'll make it real in the Web and Data modules:

@RestController
@RequestMapping("/api/books")
class BookController {

    record Book(String isbn, String title, String author) {}

    @GetMapping
    List<Book> all() {
        return List.of(
            new Book("978-0134685991", "Effective Java", "Joshua Bloch"),
            new Book("978-1617294945", "Spring in Action", "Craig Walls")
        );
    }
}

Restart, then:

curl http://localhost:8080/api/books

[{"isbn":"978-0134685991","title":"Effective Java","author":"Joshua Bloch"},
{"isbn":"978-1617294945","title":"Spring in Action","author":"Craig Walls"}]

You wrote a class and a method; Boot supplied the server, the HTTP handling, the JSON serialization, and the routing. That's auto-configuration earning its keep.