Ruby 3.1 / Rails 6.1 / Spring 4.3.0: "`initialize': Rails::Application is abstract, you cannot instantiate it directly. (RuntimeError)"

[tagliala]

Hello,

GitHub actions is getting this error when trying to update a Rails 6.1 application to Spring 4.3.

/home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/railties-6.1.7.10/lib/rails/railtie.rb:229:in `initialize': Rails::Application is abstract, you cannot instantiate it directly. (RuntimeError)
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/railties-6.1.7.10/lib/rails/engine.rb:441:in `initialize'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/railties-6.1.7.10/lib/rails/application.rb:12[9](https://github.com/diowa/ruby3-rails6-bootstrap-heroku/actions/runs/13832314292/job/38699222461?pr=1557#step:7:10):in `initialize'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/railties-6.1.7.10/lib/rails/railtie.rb:184:in `new'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/railties-6.1.7.[10](https://github.com/diowa/ruby3-rails6-bootstrap-heroku/actions/runs/13832314292/job/38699222461?pr=1557#step:7:11)/lib/rails/railtie.rb:184:in `instance'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/railties-6.1.7.10/lib/rails/application.rb:100:in `instance'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/railties-6.1.7.10/lib/rails/railtie.rb:206:in `method_missing'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/spring-4.3.0/lib/spring/application.rb:[12](https://github.com/diowa/ruby3-rails6-bootstrap-heroku/actions/runs/13832314292/job/38699222461?pr=1557#step:7:13)8:in `block in preload'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/spring-4.3.0/lib/spring/application.rb:127:in `each'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/spring-4.3.0/lib/spring/application.rb:127:in `preload'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/spring-4.3.0/lib/spring/application.rb:176:in `serve'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/spring-4.3.0/lib/spring/application.rb:158:in `block in run'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/spring-4.3.0/lib/spring/application.rb:152:in `loop'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/spring-4.3.0/lib/spring/application.rb:152:in `run'
	from /home/runner/work/ruby3-rails6-bootstrap-heroku/ruby3-rails6-bootstrap-heroku/vendor/bundle/ruby/3.1.0/gems/spring-4.3.0/lib/spring/application/boot.rb:25:in `<top (required)>'
	from <internal:/opt/hostedtoolcache/Ruby/3.1.6/x64/lib/ruby/site_ruby/3.1.0/rubygems/core_ext/kernel_require.rb>:[13](https://github.com/diowa/ruby3-rails6-bootstrap-heroku/actions/runs/13832314292/job/38699222461?pr=1557#step:7:14)6:in `require'
	from <internal:/opt/hostedtoolcache/Ruby/3.1.6/x64/lib/ruby/site_ruby/3.1.0/rubygems/core_ext/kernel_require.rb>:136:in `require'
	from -e:1:in `<main>'

Ref: https://github.com/diowa/ruby3-rails6-bootstrap-heroku/actions/runs/13832314292/job/38699222461?pr=1557

There are no issues with 4.2.1

I've tried to bisect but I didn't succeed

This is also reproducible by running rails c

$ rails c
~/.rvm/gems/ruby-3.1.6/gems/railties-6.1.7.10/lib/rails/railtie.rb:229:in `initialize': Rails::Application is abstract, you cannot instantiate it directly. (RuntimeError)

[clucas]

I am experiencing the same thing with Rails 6.1.7.10 and Ruby 3.1.5

[DanielGregolate]

Same thing here, any fixes?

[nna774]

I'm experiencing the same issue with Rails 6.1.7.9 and Ruby 3.4.2. Upon reviewing the changes at v4.2.1...v4.3.0#diff-8079047024eafaf078a5ec33c6e989295d077b3306c92c168a4cc043c9f780b2R127-R131, I found that calling watcher.add Rails::Application.paths["config/initializers"].expanded raises this RuntimeError. However, I don't understand why this happens.