load the JSON Backend lazily. If the JSON gem is already loaded, use the JSONGem backend by default.

activesupport/lib/active_support/json.rb

@@ -40,9 +40,15 @@ def self.escape(string)
     end
 
     class << self
-      attr_reader :backend
       delegate :decode, :to => :backend
-    
+
+      def backend
+        @backend || begin
+          self.backend = defined?(::JSON) ? "JSONGem" : "Yaml"
+          @backend
+        end
+      end
+
       def backend=(name)
         if name.is_a?(Module)
           @backend = name
@@ -77,6 +83,5 @@ def escape_html_entities_in_json=(value)
 end
 
 ActiveSupport.escape_html_entities_in_json = true
-ActiveSupport::JSON.backend = 'Yaml'
 
 require 'active_support/json/encoding'

activesupport/lib/active_support/json/backends/jsongem.rb

@@ -1,6 +1,8 @@
+require 'json' unless defined?(JSON)
+
 module ActiveSupport
   module JSON
-    ParseError = ::JSON::ParserError
+    ParseError = ::JSON::ParserError unless const_defined?(:ParseError)
 
     module Backends
       module JSONGem

activesupport/lib/active_support/json/backends/yaml.rb

@@ -2,7 +2,9 @@
 
 module ActiveSupport
   module JSON
-    class ParseError < StandardError
+    unless const_defined?(:ParseError)
+      class ParseError < StandardError
+      end
     end
 
     module Backends

activesupport/test/json/decoding_test.rb

@@ -36,6 +36,9 @@ class TestJSONDecoding < ActiveSupport::TestCase
     %q({"b":["\u003ci\u003e","\u003cb\u003e","\u003cu\u003e"]}) => {'b' => ["<i>","<b>","<u>"]}
   }
 
+  # load the default JSON backend
+  ActiveSupport::JSON.backend
+
   backends = %w(Yaml)
   begin
     gem 'json', '>= 1.1'