go-uber-fx

Compose the app with fx.New(fx.Provide(...), fx.Invoke(...)) and call app.Run(), which blocks on SIGINT/SIGTERM and fires OnStop hooks. Constructors registered with fx.Provide run lazily, so at least one fx.Invoke must reference each type for its constructor to execute.

dig is a raw dependency injection container, while fx embeds dig and adds lifecycle hooks, a module system with scoped decorators, a signal-aware run loop, structured event logging, and start/stop timeouts. Choose fx for long-running services and dig for CLI tools or libraries managing their own lifecycle.

Inject fx.Lifecycle into a constructor and call lc.Append with an fx.Hook containing OnStart and OnStop callbacks. Keep OnStart non-blocking by spawning goroutines for long-running work, and respect the context passed to each hook since it is bounded by StartTimeout and StopTimeout.

Constructors registered with fx.Provide are lazy and only run when reachable from an fx.Invoke, directly or transitively. Add at least one Invoke per application that references the type, or use fx.Populate in tests to pull values from the graph.

Use fxtest.New(t, opts...) so wiring errors fail the test instead of crashing, then call app.RequireStart() and defer app.RequireStop(). Swap real dependencies with fx.Replace, extract values with fx.Populate, or use fxtest.NewLifecycle(t) to test a single constructor's hooks in isolation.

Use fx.Supply for pre-built values such as parsed config, secrets, and command-line flags instead of writing a no-op constructor that returns them. fx.Supply makes these values first-class graph members and keeps constructors reserved for actual construction logic.