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require 'rspec'
require 'rspec/core/formatters/json_formatter'

config = RSpec.configuration

formatter = RSpec::Core::Formatters::JsonFormatter.new(config.output_stream)

# create reporter with json formatter
reporter =  RSpec::Core::Reporter.new(config)
config.instance_variable_set(:@reporter, reporter)

# internal hack
# api may not be stable, make sure lock down Rspec version
loader = config.send(:formatter_loader)
notifications = loader.send(:notifications_for, RSpec::Core::Formatters::JsonFormatter)

reporter.register_listener(formatter, *notifications)

RSpec::Core::Runner.run(['spec.rb'])

# here's your json hash
p formatter.output_hash