Automatically prefer Yajl or JSON backend over Yaml, if available

activesupport/CHANGELOG

@@ -1,6 +1,6 @@
 *Rails 3.0 (pending)*
 
-* JSON backend for YAJL.  #2666 [Brian Lopez]
+* JSON backend for YAJL. Preferred if available.  #2666 [Brian Lopez]
 
 
 *Rails 3.0.0 [beta] (February 4, 2010)*

activesupport/lib/active_support/json/decoding.rb

@@ -6,12 +6,15 @@ module ActiveSupport
   mattr_accessor :parse_json_times
 
   module JSON
+    # Listed in order of preference.
+    DECODERS = %w(Yajl JSONGem Yaml)
+
     class << self
       attr_reader :parse_error
       delegate :decode, :to => :backend
 
       def backend
-        self.backend = "Yaml" unless defined?(@backend)
+        set_default_backend unless defined?(@backend)
         @backend
       end
 
@@ -31,6 +34,18 @@ module ActiveSupport
       ensure
         self.backend = old_backend
       end
+
+      def set_default_backend
+        DECODERS.find do |name|
+          begin
+            self.backend = name
+            true
+          rescue LoadError
+            # Try next decoder.
+            false
+          end
+        end
+      end
     end
   end
 end

activesupport/test/json/decoding_test.rb

@@ -45,7 +45,7 @@ class TestJSONDecoding < ActiveSupport::TestCase
   }
 
   # load the default JSON backend
-  ActiveSupport::JSON.backend
+  ActiveSupport::JSON.backend = 'Yaml'
 
   backends = %w(Yaml)
   backends << "JSONGem" if defined?(::JSON)