Spring Boot targets the fast development of production-ready Spring applications by being strongly opinionated about how to set up both core modules from the Spring Framework and third-party products, such as libraries that are used for logging or connecting to a database. Spring Boot does that by applying a number of conventions by default, minimizing the need for configuration. Whenever required, each convention can be overridden by writing some configuration, case by case. This design pattern is known as convention over configuration and minimizes the need for initial configuration.

Configuration, when required, is in my opinion written best using Java and annotations. The good old XML-based configuration files can still be used, although they are significantly smaller than before Spring Boot was introduced.

Added to the usage of convention over configuration, Spring Boot also favors a runtime model based on a standalone JAR file, also known as a fat JAR file. Before Spring Boot, the most common way to run a Spring application was to deploy it as a WAR file on a Java EE web server, such as Apache Tomcat. WAR file deployment is still supported by Spring Boot.

A fat JAR file contains not only the classes and resource files of the application itself, but also all the JAR files the application depends on. This means that the fat JAR file is the only JAR file required to run the application; that is, we only need to transfer one JAR file to an environment where we want to run the application instead of transferring the application's JAR file along with all the JAR files the application depends on.

Starting a fat JAR requires no separately installed Java EE web server, such as Apache Tomcat. Instead, it can be started with a simple command such as java -jar app.jar, making it a perfect choice for running in a Docker container! If the Spring Boot application, for example, uses HTTP to expose a REST API, it will also contain an embedded web server.