simplecov hangs after all spec passed when running on jruby. the temporary solution is to add at_exit hook like this.

rspec.rb

#!/usr/bin/env ruby
system("jruby -S bundle exec rspec spec")
exit 0

spec_helper.rb

require 'sc'
require 'pp'
require 'rspec/mocks'
require 'simplecov'
require 'simplecov-rcov'

SimpleCov.formatter = SimpleCov::Formatter::RcovFormatter
SimpleCov.start do
  add_filter "/vendor/bundle/"
end

at_exit do
  if $! #was an exception thrown?
    @exit_status = $!.is_a?(SystemExit) ? $!.status : SimpleCov::ExitCodes::EXCEPTION
  end

  SimpleCov.at_exit.call

  if SimpleCov.result? # Result has been computed
    covered_percent = SimpleCov.result.covered_percent.round(2)

    if @exit_status.to_i == 0 # No other errors
      @exit_status = if covered_percent < SimpleCov.minimum_coverage
        $stderr.puts "Coverage (%.2f%%) is below the expected minimum coverage (%.2f%%)." % \
                     [covered_percent, SimpleCov.minimum_coverage]

        SimpleCov::ExitCodes::MINIMUM_COVERAGE

      elsif (last_run = SimpleCov::LastRun.read)
        diff = last_run['result']['covered_percent'] - covered_percent
        if diff > SimpleCov.maximum_coverage_drop
          $stderr.puts "Coverage has dropped by %.2f%% since the last time (maximum allowed: %.2f%%)." % \
                       [diff, SimpleCov.maximum_coverage_drop]

          SimpleCov::ExitCodes::MAXIMUM_COVERAGE_DROP
        end
      end
    end

    metrics = {
      :result => { :covered_percent => covered_percent }
    }
    SimpleCov::LastRun.write(metrics)
  end

  # HACK: temporary solution.
  #exit @exit_status if @exit_status # Force exit with stored status (see github issue #5)
  exit! !($!.nil? || $!.is_a?(SystemExit) && $!.success?)
end

RSpec.configure do |config|
  # some (optional) config here
end

For me, the culprit was not actually jruby, but the fact that I was running on a virtual box vm with a synched directory.

When simplecov exists, by default it will try to merge the recorded coverage with what's on disk. To avoid corruption it uses a lock file to guard this merge. Because my virtual box shared fs is not actually posix compliant, the lock file would never acquire, and silently block forever here.

Since I don't care about merging coverage results, the solution for me was to simply disable this behavior with the use_merging flag:

SimpleCov.start do
  use_merging false
end