fake_service.rb

class FakeService
  def self.call(env)
    [200, { 'Content-Type' => 'application/json' }, ['{ "name": "John Doe" }']]
  end
end

service.rb

require 'net/http'
require 'json'

class MyExternalDependency
  def get_user
    data = Net::HTTP.get('example.com', '/user.json')
    JSON.parse(data)
  end
end

service_spec.rb

require 'spec_helper'
require 'service'

describe MyExternalDependency do
  describe '#get_user' do
    it 'contains the user name' do
      expect(MyExternalDependency.new.get_user['name']).to eq('John Doe')
    end
  end
end

spec_helper.rb

# spec_helper.rb
require 'webmock'
require 'rack'
require 'fake_service'

RSpec.configure do |config|
  config.before(:each) { WebMock.stub_request(:any, %r[http://example.com]).to_rack(FakeService) }
end