Basic Dependencies

• Projects created with Spring Initializr include essential starter dependencies by default
• Starters enable Spring Boot’s core goals: simplicity and auto-configuration

spring-boot-starter

• Core starter with essential Spring Boot components
  • Provides basic Bean registration, logging, and Spring Context setup
  • Serves as the foundation for all Spring Boot projects
  • Included transitively by other starters
• Examples of Included Libraries:
  • spring-core
  • spring-context
  • spring-boot-autoconfigure
  • SLF4J + Logback (default logging system)

spring-boot-starter-test

• A comprehensive set of integrated testing tools for Spring-based tests.
• Includes libraries for unit testing, integration testing, mocking, and more.

Library	Purpose
JUnit 5 (Jupiter)	The primary testing framework.
Mockito	For creating mock objects.
Spring Test	Provides annotations like @WebMvcTest, @DataJpaTest, etc.
AssertJ	For fluent-style test assertions.

• Since the test environment also runs on a Spring context, it’s possible to write test code with a structure similar to the actual service.

@SpringBootTest
class MyServiceTest {
  
	  @Autowired MyService myService;
	
	  @Test
	  void testLogic() {
		    assertThat(myService.doSomething()).isEqualTo("expected");
	  }
}

Executable JARs

Spring Boot projects are, by default, built as standalone “fat” JARs.

• Fat JAR
  • a single, executable file that packages your project’s code, all its dependency libraries, configuration files, resources, and an embedded web server into one JAR file
  • can be executed using the java -jar command

$ ./gradlew bootJar # Or ./mvnw spring-boot:repackage for Maven
$ java -jar build/libs/myapp-0.0.1-SNAPSHOT.jar

(Reference) Internal Structure:

myapp.jar
├─ BOOT-INF/
│   ├─ classes/       <- Application classes
│   └─ lib/           <- Dependency libraries
├─ META-INF/
└─ org.springframework.boot.loader.Launcher

• spring-boot-loader
  • • Entry point that separates and loads app classes and libraries at runtime
  • Comes directly from the Spring Boot project itself.
  • Not something you manually add as a dependency like spring-boot-starter-web ⇒ - it’s automatically integrated into your executable JAR when you build your Spring Boot application using the official Spring Boot Maven Plugin or Spring Boot Gradle Plugin