Use posix-spawn gem rather than sfl, as sfl requires fork, which can't work on JRuby.

lib/capybara/poltergeist/client.rb

@@ -1,4 +1,4 @@
-require 'sfl'
+require 'posix/spawn'
 
 module Capybara::Poltergeist
   class Client
@@ -22,7 +22,7 @@ module Capybara::Poltergeist
 
     def start
       check_phantomjs_version
-      @pid = Kernel.spawn(command)
+      @pid = POSIX::Spawn.spawn(command)
     end
 
     def stop

lib/capybara/poltergeist/inspector.rb

@@ -1,3 +1,5 @@
+require 'posix/spawn'
+
 module Capybara::Poltergeist
   class Inspector
     BROWSERS = %w(chromium chromium-browser google-chrome safari)
@@ -24,7 +26,7 @@ module Capybara::Poltergeist
 
     def open
       if browser
-        Kernel.spawn("#{browser} #{url}")
+        POSIX::Spawn.spawn("#{browser} #{url}")
       else
         raise Error, "Could not find a browser executable to open #{url}. " \
                      "You can specify one manually using e.g. `:inspector => 'chromium'` " \

poltergeist.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |s|
 
   s.add_dependency "capybara",       "~> 1.0"
   s.add_dependency "json",           "~> 1.6"
-  s.add_dependency "sfl",            "~> 2.0"
+  s.add_dependency "posix-spawn",    "~> 0.3"
   s.add_dependency "http_parser.rb", "~> 0.5.3"
   s.add_dependency "faye-websocket", "~> 0.2"
 

spec/unit/inspector_spec.rb

@@ -27,7 +27,7 @@ module Capybara::Poltergeist
 
     it 'can be opened' do
       subject.stub(:port => 1234, :browser => 'chromium')
-      Kernel.should_receive(:spawn).with("chromium http://localhost:1234/")
+      POSIX::Spawn.should_receive(:spawn).with("chromium http://localhost:1234/")
       subject.open
     end