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Spring is a Rails application preloader. It's trying to solve the same problem as spork, zeus and commands.


Spring is most similar to Zeus, but it's implemented in pure Ruby, and is more tightly integrated with Rails (it makes use of Rails' built-in code reloader).

Spring tries to be totally automatic. It boots up in the background the first time you run a command. Then it speeds up subsequent commands. If it detects that your pre-loaded environment has changed (maybe config/application.rb has been edited) then it will reload your environment in the background, ready for the next command. When you close your terminal session, Spring will automatically shut down. There's no "server" to manually start and stop.

Spring operates via a command line interface. Other solutions (e.g. commands) take the approach of using a special console to run commands from. This means we will have to re-implement shell features such as history, completion, etc. Whilst it's not impossible to re-implement those features, it's unnecessary work and our re-implementation won't be as feature complete as a real shell. Using a real shell also prevents the user having to constantly jump between a terminal with a real shell and a terminal running the rails "commands console".


Ruby versions supported:

  • MRI 1.9.3
  • MRI 2.0.0

Rails versions supported:

  • 3.2
  • 4.0

Spring makes extensive use of Process#fork, so won't be able to run on any platform which doesn't support that (Windows, JRuby).


Install the spring gem. You can add it to your Gemfile if you like but it's optional. You now have a spring command. Don't use it with bundle exec or it will be extremely slow.

For this walkthrough, I'm using the test app in the Spring repository:

cd /path/to/spring/test/apps/rails-3-2

We can run a test:

$ time spring testunit test/functional/posts_controller_test.rb
Run options:

# Running tests:


Finished tests in 0.127245s, 55.0121 tests/s, 78.5887 assertions/s.

7 tests, 10 assertions, 0 failures, 0 errors, 0 skips

real    0m2.165s
user    0m0.281s
sys     0m0.066s

That booted our app in the background:

$ spring status
Spring is running:

26150 spring server | rails-3-2 | started 3 secs ago
26155 spring app    | rails-3-2 | started 3 secs ago | test mode

We can see two processes, one is the Spring server, the other is the application running in the test environment. When we close the terminal, the processes will be killed automatically.

Running the test is faster next time:

$ time spring testunit test/functional/posts_controller_test.rb
Run options:

# Running tests:


Finished tests in 0.176896s, 39.5714 tests/s, 56.5305 assertions/s.

7 tests, 10 assertions, 0 failures, 0 errors, 0 skips

real    0m0.610s
user    0m0.276s
sys     0m0.059s

Running spring testunit, spring rake, spring rails, etc gets a bit tedious. It also suffers from a performance issue in Rubygems (which I am actively working on) which means the spring command takes a while to start up. The more gems you have, the longer it takes.

Spring binstubs solve both of these problems. If you will be running the testunit command regularly, run:

$ spring binstub testunit

This generates a bin/spring and a bin/testunit, which allows you to run spring and spring testunit in a way that doesn't trigger the Rubygems performance bug:

$ time bin/testunit test/functional/posts_controller_test.rb
Run options:

# Running tests:


Finished tests in 0.166585s, 42.0207 tests/s, 60.0296 assertions/s.

7 tests, 10 assertions, 0 failures, 0 errors, 0 skips

real    0m0.407s
user    0m0.077s
sys     0m0.059s

You can add "./bin" to your PATH when in your application's directory with direnv, but you should recognise and understand the security implications of using that.

Note: Don't use spring binstubs with bundle install --binstubs. If you do this, spring and bundler will overwrite each other. If you will not be using a command with spring, use bundle binstub [GEM] to generate a bundler binstub for that specific gem. If you will be using a command with spring, generate a spring binstub instead of a bundler binstub; spring will run your command inside the bundle anyway.

If we edit any of the application files, or test files, the change will be picked up on the next run, without the background process having to be restarted.

If we edit any of the preloaded files, the application needs to restart automatically. Let's "edit" config/application.rb:

$ touch config/application.rb
$ spring status
Spring is running:

26150 spring server | rails-3-2 | started 36 secs ago
26556 spring app    | rails-3-2 | started 1 sec ago | test mode

The application process detected the change and exited. The server process then detected that the application process exited, so it started a new application. All of this happened automatically. Next time we run a command we'll be running against a fresh application. We can see that the start time and PID of the app process has changed.

If we run a command that uses a different environment, then it gets booted up. For example, the rake command uses the development environment by default:

$ spring binstub rake
$ bin/rake routes
    posts GET    /posts(.:format)          posts#index
          POST   /posts(.:format)          posts#create
 new_post GET    /posts/new(.:format)      posts#new
edit_post GET    /posts/:id/edit(.:format) posts#edit
     post GET    /posts/:id(.:format)      posts#show
          PUT    /posts/:id(.:format)      posts#update
          DELETE /posts/:id(.:format)      posts#destroy

We now have 3 processes: the server, and application in test mode and the application in development mode.

$ bin/spring status
Spring is running:

26150 spring server | rails-3-2 | started 1 min ago
26556 spring app    | rails-3-2 | started 42 secs ago | test mode
26707 spring app    | rails-3-2 | started 2 secs ago | development mode

To stop the background processes:

$ bin/spring stop
Spring stopped.


The following commands are shipped by default.

Custom commands can be specified in the Spring config file. See lib/spring/commands.rb for examples.

A bunch of different test frameworks are supported at the moment in order to make it easy for people to try spring. However in the future the code to use a specific test framework should not be contained in the spring repository.


Runs a test (e.g. Test::Unit, MiniTest::Unit, etc.)

This command can also recursively run a directory of tests. For example, spring testunit test/functional will run test/functional/**/*_test.rb.

If your test helper file takes a while to load, consider preloading it (see "Running code before forking" below).


Runs an rspec spec, exactly the same as the rspec executable.

If your spec helper file takes a while to load, consider preloading it (see "Running code before forking" below).


Runs a cucumber feature.


Runs a rake task. Rake tasks run in the development environment by default. You can change this on the fly by using the RAILS_ENV environment variable. The environment is also configurable with the Spring::Commands::Rake.environment_matchers hash. This has sensible defaults, but if you need to match a specific task to a specific environment, you'd do it like this:

Spring::Commands::Rake.environment_matchers["perf_test"] = "test"
Spring::Commands::Rake.environment_matchers[/^perf/]     = "test"

rails console, rails generate, rails runner

These execute the rails command you already know and love. If you run a different sub command (e.g. rails server) then spring will automatically pass it through to the underlying rails executable (without the speed-up).


Spring will read ~/.spring.rb and config/spring.rb for custom settings, described below.

Application root

Spring must know how to find your Rails application. If you have a normal app everything works out of the box. If you are working on a project with a special setup (an engine for example), you must tell Spring where your app is located:

Spring.application_root = './test/dummy'

Running code before forking

There is no Spring.before_fork callback. To run something before the fork, you can place it in ~/.spring.rb or config/spring.rb or in any of the files which get run when your application initializers, such as config/application.rb, config/environments/*.rb or config/initializers/*.rb.

For example, if loading your test helper is slow, you might like to preload it to speed up your test runs. To do this you could put a require Rails.root.join("test/helper") in config/environments/test.rb.

Running code after forking

You might want to run code after Spring forked off the process but before the actual command is run. You might want to use an after_fork callback if you have to connect to an external service, do some general cleanup or set up dynamic configuration.

Spring.after_fork do
  # run arbitrary code

If you want to register multiple callbacks you can simply call Spring.after_fork multiple times with different blocks.

Watching files and directories

Spring will automatically detect file changes to any file loaded when the server boots. Changes will cause the affected environments to be restarted.

If there are additional files or directories which should trigger an application restart, you can specify them with Spring.watch:

Spring.watch "spec/factories"

By default Spring polls the filesystem for changes once every 0.2 seconds. This method requires zero configuration, but if you find that it's using too much CPU, then you can turn on event-based file system listening:

Spring.watch_method = :listen

You may need to add the listen gem to your Gemfile.

tmp directory

Spring needs a tmp directory. This will default to Rails.root.join('tmp', 'spring'). You can set your own configuration directory by setting the SPRING_TMP_PATH environment variable.


If you want to get more information about what spring is doing, you can specify a log file with the SPRING_LOG environment variable:

spring stop # if spring is already running
export SPRING_LOG=/tmp/spring.log
spring rake -T