Make compressors lazily load.

Gemfile

@@ -10,7 +10,10 @@ end
 
 gem "coffee-script"
 gem "sass"
-gem "uglifier", ">= 1.0.0"
+
+# This needs to be with require false to avoid
+# it being automatically loaded by sprockets
+gem "uglifier", ">= 1.0.0", :require => false
 
 gem "rake",  ">= 0.8.7"
 gem "mocha", ">= 0.9.8"

actionpack/lib/sprockets/compressors.rb

@@ -0,0 +1,21 @@
+module Sprockets
+  class NullCompressor
+    def compress(content)
+      content
+    end
+  end
+
+  class LazyCompressor
+    def initialize(&block)
+      @block = block
+    end
+
+    def compressor
+      @compressor ||= @block.call || NullCompressor.new
+    end
+
+    def compress(content)
+      compressor.compress(content)
+    end
+  end
+end

actionpack/lib/sprockets/railtie.rb

@@ -1,5 +1,7 @@
 module Sprockets
   autoload :Helpers, "sprockets/helpers"
+  autoload :LazyCompressor, "sprockets/compressors"
+  autoload :NullCompressor, "sprockets/compressors"
 
   # TODO: Get rid of config.assets.enabled
   class Railtie < ::Rails::Railtie
@@ -66,8 +68,8 @@ module Sprockets
         if assets.compress
           # temporarily hardcode default JS compressor to uglify. Soon, it will work
           # the same as SCSS, where a default plugin sets the default.
-          env.js_compressor  = expand_js_compressor(assets.js_compressor || :uglifier)
-          env.css_compressor = expand_css_compressor(assets.css_compressor)
+          env.js_compressor  = LazyCompressor.new { expand_js_compressor(assets.js_compressor || :uglifier) }
+          env.css_compressor = LazyCompressor.new { expand_css_compressor(assets.css_compressor) }
         end
 
         env

railties/test/application/assets_test.rb

@@ -35,6 +35,17 @@ module ApplicationTests
       assert_match "alert()", last_response.body
     end
 
+    test "assets do not require compressors until it is used" do
+      app_file "app/assets/javascripts/demo.js.erb", "<%= :alert %>();"
+      ENV["RAILS_ENV"] = "production"
+      require "#{app_path}/config/environment"
+
+      assert !defined?(Uglifier)
+      get "/assets/demo.js"
+      assert_match "alert()", last_response.body
+      assert defined?(Uglifier)
+    end
+
     test "assets are compiled properly" do
       app_file "app/assets/javascripts/application.js", "alert();"
       app_file "app/assets/javascripts/foo/application.js", "alert();"