The Kernel#require from before RubyGems was loaded.
Use Kernel#gem to activate a specific version of gem_name.
requirements is a list of version requirements that the specified gem must match, most commonly “= example.version.number“. See Gem::Requirement for how to specify a version requirement.
If you will be activating the latest version of a gem, there is no need to call Kernel#gem, Kernel#require will do the right thing for you.
Kernel#gem returns true if the gem was activated, otherwise false. If the gem could not be found, didn’t match the version requirements, or a different version was already activated, an exception will be raised.
Kernel#gem should be called before any require statements (otherwise RubyGems may load a conflicting library version).
In older RubyGems versions, the environment variable GEM_SKIP could be used to skip activation of specified gems, for example to test out changes that haven’t been installed yet. Now RubyGems defers to -I and the RUBYLIB environment variable to skip activation of a gem.
Example:
GEM_SKIP=libA:libB ruby -I../libA -I../libB ./mycode.rb
# File lib/rubygems.rb, line 1200 1200: def gem(gem_name, *requirements) # :doc: 1201: skip_list = (ENV['GEM_SKIP'] || "").split(/:/) 1202: raise Gem::LoadError, "skipping #{gem_name}" if skip_list.include? gem_name 1203: spec = Gem::Dependency.new(gem_name, *requirements).to_spec 1204: spec.activate if spec 1205: end
When RubyGems is required, Kernel#require is replaced with our own which is capable of loading gems on demand.
When you call require 'x', this is what happens:
If the file can be loaded from the existing Ruby loadpath, it is.
Otherwise, installed gems are searched for a file that matches. If it’s found in gem ‘y’, that gem is activated (added to the loadpath).
The normal require functionality of returning false if that file has already been loaded is preserved.
# File lib/rubygems/custom_require.rb, line 34 34: def require path 35: if Gem.unresolved_deps.empty? or Gem.loaded_path? path then 36: gem_original_require path 37: else 38: spec = Gem::Specification.find { |s| 39: s.activated? and s.contains_requirable_file? path 40: } 41: 42: unless spec then 43: found_specs = Gem::Specification.find_in_unresolved path 44: unless found_specs.empty? then 45: found_specs = [found_specs.last] 46: else 47: found_specs = Gem::Specification.find_in_unresolved_tree path 48: end 49: 50: found_specs.each do |found_spec| 51: found_spec.activate 52: end 53: end 54: 55: return gem_original_require path 56: end 57: rescue LoadError => load_error 58: if load_error.message.end_with?(path) and Gem.try_activate(path) then 59: return gem_original_require(path) 60: end 61: 62: raise load_error 63: end
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