class Closure::Compiler

much to see here.
The Closure::Compiler is a basic wrapper around the actual JAR. There’s not

def command 

def command
  [@java, '-jar', "\"#{@jar}\"", @options].flatten.join(' ')
end

def compile(io)

block, for streaming.
JavaScript as a string or yields an IO object containing the response to a
Can compile a JavaScript string or open IO object. Returns the compiled 
def compile(io)
  tempfile = Tempfile.new('closure_compiler')
  if io.respond_to? :read
    while buffer = io.read(4096) do
      tempfile.write(buffer)
    end
  else
    tempfile.write(io.to_s)
  end
  tempfile.flush
  begin
    result = `#{command} --js #{tempfile.path}`
  rescue Exception
    raise Error, "compression failed"
  ensure
    tempfile.close!
  end
  unless $?.exitstatus.zero?
    raise Error, result
  end
  yield(StringIO.new(result)) if block_given?
  result
end

def initialize(options={})

When you create a Compiler, pass in the flags and options. 
def initialize(options={})
  @java     = options.delete(:java)     || JAVA_COMMAND
  @jar      = options.delete(:jar_file) || COMPILER_JAR
  @options  = serialize_options(DEFAULT_OPTIONS.merge(options))
end

def serialize_options(options)

Serialize hash options to the command-line format. 
def serialize_options(options)
  options.map do |k, v|
    if (v.is_a?(Array))
      v.map {|v2| ["--#{k}", v2.to_s]}
    else
      ["--#{k}", v.to_s]
    end
  end.flatten
end