The standard Rails development log contains a lot of noise that is rarely meaningful for debugging. The Quiet Assets gem is a mandatory part of my Rails development process as it removes the logging noise of the asset pipeline.
Also, if WEBrick is used as a development server, the following entry is logged for each asset pipeline log entry (whether silenced or not by Quiet Assets):
WARN Could not determine content-length of response body. Set content-length of the response or set Response#chunked = true
While some have suggested monkey-patching WEBrick or installing Thin as an alternate server (which doesn’t work under JRuby), the simplest way to remove those statements is to add WEBrick specifically in your Gemfile:
gem 'webrick', '~> 1.3.1', group: :development