Invalid JSON string error in Rails

Question

I'm on Rails 2.3.5. In a typical user controller create action

class UsersController
  def create
    @user = User.new(params[:user])
    respond_to do |format]
      if @user.save ...
      else
        format.json ....
      end        
  end
end

When a client passes an invalid / malformed JSON string for input, Rails throw a 500 internal server error saying "Invalid JSON string". Is it possible for me to trap the error so I can give a custom message?

UPDATED Here's the stacktrace as requested. Just so I'm clear, I know this is a malformed JSON string, my question is not how to fix the JSON string but rather how to trap this particular error so I can send back a more meaningful error message than HTTP 500 Internal Server Error. Thanks in advance for your help

Error occurred while parsing request parameters.
Contents:

user: {login: "John", email: "[email protected]", password: "111"}}
/!\ FAILSAFE /!\  Mon Nov 08 02:01:04 -0800 2010

  Status: 500 Internal Server Error
  Invalid JSON string
    c:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/json/backends/yaml.rb:14:in `decode'
    c:1:in `__send__'
    c:1:in `decode'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/params_parser.rb:42:in `parse_formatted_parameters'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/params_parser.rb:11:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/cookie_store.rb:93:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/failsafe.rb:26:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `synchronize'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:114:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/reloader.rb:34:in `run'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:108:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/rack/static.rb:31:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:46:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:40:in `each'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:40:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/rack/log_tailer.rb:17:in `call'
    ...
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/handler/mongrel.rb:34:in `run'
    c:/ruby/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/commands/server.rb:111
    c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
    c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
    script/server:3
re.rb:31:in `require'
    script/server:3

Answer 1

I've found this rails plugin to work on rails 3.0.5: https://github.com/kares/request_exception_handler

As Rob Cameron suggests it loads your rails application and raises the parseError from your application code. I haven't tested it's performance impact, but it does seem to work well.