Spring Boot Overview

• Starter dependencies: curated dependency descriptors — they contain no library code themselves but pull in a set of compatible transitive dependencies for a given Spring Boot version.
• Autoconfiguration: Spring Boot guesses and configures components automatically based on classpath entries, environment variables, and other factors.
• Actuator: provides runtime insight into the application — metrics, thread dumps, health status, and environment properties.
• Flexible environment properties: supports configuration via application.properties, application.yml, environment variables, and command-line arguments.
• Spring Boot CLI: allows writing a Spring application as a collection of Groovy scripts and running it from the command line.

Starter Dependencies

• Named with the pattern spring-boot-starter-[dependency] — e.g. spring-boot-starter-web, spring-boot-starter-data-jpa.
• Starter dependencies contain no library code — they are pure dependency aggregators that pull in a set of libraries known to work together for a given Spring Boot version.
• Individual library versions do not need to be managed explicitly — the parent POM (spring-boot-starter-parent) manages all compatible versions.

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

Spring Boot Maven Plugin

• Declared as spring-boot-maven-plugin in the <build> section of pom.xml.
• Enables running the application directly via Maven: mvn spring-boot:run.
• Packages the application as an executable fat JAR — all dependency libraries are bundled inside and available on the runtime classpath.
• Adds a manifest file to the JAR that designates the bootstrap class as the entry point.

DevTools

• Added via the spring-boot-devtools dependency — it is not an IDE plugin.
• Automatically disabled in production environments.
• Enables automatic application restart when source files change.

How Automatic Restart Works

• DevTools loads the application into two separate classloaders in the JVM:
  • One classloader for application code (src/main).
  • One classloader for dependency libraries.
• When application code changes, only the application classloader is reloaded and the Spring application context is restarted — the dependency classloader is reused, making the restart fast.
• If a dependency library itself changes, a full restart of the JVM is required.

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-devtools</artifactId>
  <optional>true</optional> <!-- prevents transitively pulling devtools into downstream projects -->
</dependency>

Spring Initializr

• Generates a skeleton Spring Boot project structure with the selected dependencies, build tool, Java version, and packaging.
• Available at https://start.spring.io/ — both a browser-based UI and a REST API.
• Accessible via: web browser, Spring Tool Suite, Spring Boot CLI, or curl.

Spring Tool Suite (STS)

• Spring-specific IDE extensions available for Eclipse, VS Code, and IntelliJ — download at https://spring.io/tools.
• Spring Boot Dashboard: central hub to manage, run, and debug Spring Boot applications from within the IDE.
• Smart code completion: context-aware suggestions for Spring-specific elements — bean names, configuration property keys, etc.
• Live metadata integration: surfaces live Actuator data directly in the editor while the application is running.
• Spring Initializr wizard: built-in project generation — Eclipse: File → New → Spring Starter Project.