diff --git a/lib/rhino/context.rb b/lib/rhino/context.rb
index b17cc73..6868e81 100644
--- a/lib/rhino/context.rb
+++ b/lib/rhino/context.rb
@@ -46,9 +46,18 @@ module Rhino
       def eval(javascript)
         new.eval(javascript)
       end
-
+      
     end
-
+    
+    @@default_factory = nil
+    def self.default_factory
+      @@default_factory ||= ContextFactory.new
+    end
+    
+    def self.default_factory=(factory)
+      @@default_factory = factory
+    end
+    
     attr_reader :scope
     
     # Create a new javascript environment for executing javascript and ruby code.
@@ -56,8 +65,9 @@ module Rhino
     # * <tt>:with</tt> - use this ruby object as the root scope for all javascript that is evaluated
     # * <tt>:java</tt> - if true, java packages will be accessible from within javascript
     def initialize(options = {}) #:nodoc:
-      @factory = ContextFactory.new
-      @factory.call do |context|
+      factory = options[:factory] || 
+        (options[:restrictable] ? RestrictableContextFactory.instance : self.class.default_factory)
+      factory.call do |context|
         @native = context
         @global = @native.initStandardObjects(nil, options[:sealed] == true)
         if with = options[:with]
@@ -73,7 +83,12 @@ module Rhino
         end
       end
     end
-
+    
+    # Returns the ContextFactory used while creating this context.
+    def factory
+      @native.getFactory
+    end
+    
     # Read a value from the global scope of this context
     def [](key)
       @scope[key]
@@ -117,14 +132,28 @@ module Rhino
       end
     end
 
-    # Set the maximum number of instructions that this context will execute.
-    # If this instruction limit is exceeded, then a Rhino::RunawayScriptError
-    # will be raised
-    def instruction_limit=(limit)
-      @native.setInstructionObserverThreshold(limit)
-      @factory.instruction_limit = limit
+    # Returns true if this context supports restrictions.
+    def restrictable?
+      @native.is_a?(RestrictableContextFactory::Context)
     end
-
+    
+    def instruction_limit
+      restrictable? ? @native.instruction_limit : false
+    end
+    
+    # Set the maximum number of instructions that this context will execute.
+    # If this instruction limit is exceeded, then a #Rhino::RunawayScriptError
+    # will be raised.
+    def instruction_limit=(limit)
+      if restrictable?
+        @native.instruction_limit = limit
+      else
+        warn "setting an instruction_limit has no effect on this context, use " + 
+             "Context.open(:restricted => true) to gain a restrictable instance"
+        nil
+      end
+    end
+    
     def optimization_level
       @native.getOptimizationLevel
     end
@@ -134,7 +163,7 @@ module Rhino
     # By using the -1 optimization level, you tell Rhino to run in interpretative mode,
     # taking a hit to performance but escaping the Java bytecode limit.
     def optimization_level=(level)
-      if @native.class.isValidOptimizationLevel(level)
+      if JS::Context.isValidOptimizationLevel(level)
         @native.setOptimizationLevel(level)
         level
       else
@@ -157,13 +186,12 @@ module Rhino
     def version=(version)
       const = version.to_s.gsub('.', '_').upcase
       const = "VERSION_#{const}" if const[0, 7] != 'VERSION'
-      js_context = @native.class # Context
-      if js_context.constants.include?(const)
-        const_value = js_context.const_get(const)
+      if JS::Context.constants.include?(const)
+        const_value = JS::Context.const_get(const)
         @native.setLanguageVersion(const_value)
         const_value
       else
-        @native.setLanguageVersion(js_context::VERSION_DEFAULT)
+        @native.setLanguageVersion(JS::Context::VERSION_DEFAULT)
         nil
       end
     end
@@ -179,11 +207,11 @@ module Rhino
     private
     
       def do_open
+        factory.enterContext(@native)
         begin
-          @factory.enterContext(@native)
           yield self
         ensure
-          JS::Context.exit
+          factory.exit
         end      
       end
     
@@ -216,18 +244,72 @@ module Rhino
     
   end
 
-  class ContextFactory < JS::ContextFactory # :nodoc:
+  ContextFactory = JS::ContextFactory # :nodoc: backward compatibility
 
-    def observeInstructionCount(cxt, count)
-      raise RunawayScriptError, "script exceeded allowable instruction count" if count > @limit
+  class RestrictableContextFactory < ContextFactory # :nodoc:
+
+    @@instance = nil
+    def self.instance
+      @@instance ||= new
     end
+    
+    # protected Context makeContext()
+    def makeContext
+      Context.new(self)
+    end
+    
+    # protected void observeInstructionCount(Context context, int instructionCount)
+    def observeInstructionCount(context, count)
+      if context.is_a?(Context)
+        context.instruction_count += count
+        context.check!
+      end
+    end
+    
+    # protected Object doTopCall(Callable callable, Context context, 
+    #                            Scriptable scope, Scriptable thisObj, Object[] args)
+    def doTopCall(callable, context, scope, this, args)
+      context.instruction_count = 0 if context.is_a?(Context)
+      super
+    end
+    
+    class Context < JS::Context # :nodoc:
+
+      def initialize(factory)
+        super(factory)
+        reset!
+      end
+
+      attr_reader :instruction_limit
+      
+      def instruction_limit=(limit)
+        treshold = getInstructionObserverThreshold
+        if limit && (treshold == 0 || treshold > limit)
+          setInstructionObserverThreshold(limit)
+        end
+        @instruction_limit = limit
+      end
+
+      attr_accessor :instruction_count
+
+      def check!
+        if instruction_limit && instruction_count > instruction_limit
+          raise RunawayScriptError, "script exceeded allowable instruction count: #{instruction_limit}"
+        end
+      end
+
+      private
+
+        def reset!
+          self.instruction_count = 0
+          self.instruction_limit = nil
+          self
+        end
 
-    def instruction_limit=(count)
-      @limit = count
     end
     
   end
-
+    
   class ContextError < StandardError # :nodoc:
   end
 
diff --git a/spec/rhino/context_spec.rb b/spec/rhino/context_spec.rb
index 7ff0f7a..7d76f6e 100644
--- a/spec/rhino/context_spec.rb
+++ b/spec/rhino/context_spec.rb
@@ -65,5 +65,29 @@ describe Rhino::Context do
     context.version = '1.7'
     context.version.should == 1.7
   end
+
+  it "should have a (shared) factory by default" do
+    context1 = Rhino::Context.new
+    context1.factory.should_not be nil
+    context1.factory.should be_a(Rhino::JS::ContextFactory)
+    
+    context1.factory.should be Rhino::Context.default_factory
+    
+    context2 = Rhino::Context.new
+    context2.factory.should be context1.factory
+  end
+
+  it "allows limiting instruction count" do
+    context = Rhino::Context.new :restrictable => true
+    context.instruction_limit = 100
+    lambda {
+      context.eval %Q{ for (var i = 0; i < 100; i++) Number(i).toString(); }
+    }.should raise_error(Rhino::RunawayScriptError)
+    
+    context.instruction_limit = nil
+    lambda {
+      context.eval %Q{ for (var i = 0; i < 100; i++) Number(i).toString(); }
+    }.should_not raise_error(Rhino::RunawayScriptError)
+  end
   
 end
\ No newline at end of file