Add Spring.allow_reloading_disabled opt-in#755
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By default Spring refuses to boot when `config.cache_classes = true`
(a.k.a. `config.enable_reloading = false`) via the
`:ensure_reloading_is_enabled` initializer, with the message "Spring
reloads, and therefore needs the application to have reloading
enabled."
That guard predates the recognition that Spring's reload mechanism is
mold-restart-on-file-change via its own watcher (loaded application
features + initializers + config paths) — which works regardless of
whether Rails' in-process reloader is enabled. For projects that want
to disable Rails' reloader (e.g. to shed the per-serve
`reloaders.any?(&:updated?)` stat() pass over thousands of watched
files in large monorepos), the guard is unnecessarily strict.
This commit adds `Spring.allow_reloading_disabled` as an explicit
opt-in. Default remains `false` (the existing strict check). When set
to `true` the initializer is not registered and Spring boots normally
with reloading disabled.
The docstring on `Spring.allow_reloading_disabled` spells out what the
user gives up:
1. Rails' `app.reloader.reload!` becomes a no-op. Anything that
depends on it won't fire between Spring serves.
2. With Zeitwerk + reloading disabled, autoloaded constants are
non-reloadable. Tests that rely on the reloader resetting class
state between runs may break.
3. The user is responsible for asserting that lazy resources (routes,
I18n, view paths) stay unmaterialized in the Spring mold so that
each fork loads them fresh.
Acceptance test added covering the new opt-in.
Co-authored-by: AI (Pi/Claude Opus 4.7) <noreply@pi.dev>
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rafaelfranca
approved these changes
May 26, 2026
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Adds
Spring.allow_reloading_disabledas an opt-in. The default:ensure_reloading_is_enabledinitializer is unchanged and is the right default. Rails apps almost universally depend onRails.application.reloader.reload!between Spring serves to pick up edits (route redraw, I18n reload,to_preparecallbacks, etc..). Booting Spring with that disabled is wrong for those apps and the check correctly refuses.why?
The opt-in is useful, when running test, for apps where every resource that would require a reloader (translations, routes, ...) are lazy-loaded inside the forked child, and thus not loaded in the Spring parent.
In such apps, running with
config.enable_reloadingsaves quite a bit of time on every test run, particularly on very large apps, where the reloaders have tostattens of thousands of files on every test run to figure out if reloading is required.