Stopping components

(ns darkleaf.di.tutorial.c-stopping-components-test
  (:require
   [clojure.test :as t]
   [darkleaf.di.core :as di]))

By default, a component is built once at start and discarded on stop — DI does nothing else with it. When a component owns a resource (a connection, a thread pool, a file handle), attach a stop function via metadata. DI calls it with the built value when the system shuts down.

Declaring a stop function

::di/stop is just another function attached as metadata. It receives the built value and its return value is ignored. Below, the component returns an atom, and the stop function flips it to true — the test asserts the flip happened.

Note that root below has no {::di/kind :component} metadata. A function with ::di/stop metadata is a component by convention — only components have a lifecycle, so there is no need to state the kind explicitly.

(defn root
  {::di/stop #(reset! % true)}
  [{::keys [*stopped?]}]
  *stopped?)

(t/deftest stop-test
  (let [*stopped? (atom false)]
    (with-open [root (di/start `root {::*stopped? *stopped?})]
      (t/is (= false @@root)))
    (t/is @*stopped?)))

Stopping Java objects

In real systems, a stateful component is often a Java object — a connection pool, a server, a queue — with a close or shutdown method. Use a qualified method value (Clojure 1.12+) or memfn to call it without reflection:

(defn connection-pool
  {::di/stop ConnectionPool/.close}
  [{max-conn :env.long/MAX_CONN}]
  (ConnectionPool. max-conn))

(defn connection-pool
  {::di/stop (memfn ^ConnectionPool close)}
  [{max-conn :env.long/MAX_CONN}]
  (ConnectionPool. max-conn))

The next chapter covers the REPL workflow — redefining functions on a running system without restarting.