Spring Boot

Spring Boot is the framework that makes Spring actually usable. It takes the notoriously complex Spring Framework and gives you sensible defaults, auto-configuration, and embedded servers so you can go from zero to running API in minutes.

Used by a huge chunk of enterprise Java. You get dependency injection, web servers, database access, security, messaging, and basically every integration you can think of, all through starter dependencies that just work. The ecosystem is unmatched in the Java world.

Completely free. Apache 2.0 license. VMware (now Broadcom) maintains it but the framework itself costs nothing. Solo developers: if you're choosing Java, Spring Boot is the default choice. Small to large teams: same thing. The hiring pool is enormous because everyone knows Spring Boot.

The catch: it's Java. Startup times and memory usage are higher than Go or Node. And Spring's "magic" (auto-configuration, annotation processing) means when something goes wrong, debugging can feel like archaeology. The abstraction layers run deep.