```ruby
module Asynchronic
  module QueueEngine
    class InMemory

      attr_reader :default_queue

      def initialize(default_queue=nil)
        @default_queue = default_queue || :asynchronic
        @queues ||= Hash.new { |h,k| h[k] = Queue.new }
      end

      def [](name)
        @queues[name]
      end

      def queues
        @queues.keys.map(&:to_sym)
      end

      def clear
        @queues.clear
      end


      class Queue < ::Queue

        def to_a
          @que.dup
        end

        def pop
          super rescue nil
        end

      end


      class Listener

        def listen(queue, &block)
          @stopping = false

          loop do
            break if @stopping
            item = queue.pop
            next unless item
            block.call item
          end
        end

        def stop
          @stopping = true
        end

      end

    end
  end
end```

Refactor in memory queue for error in JRuby
user
```ruby
module Asynchronic
  module QueueEngine
    class InMemory

      attr_reader :default_queue

      def initialize(default_queue=nil)
        @default_queue = default_queue || :asynchronic
        @queues ||= Hash.new { |h,k| h[k] = Queue.new }
      end

      def [](name)
        @queues[name]
      end

      def queues
        @queues.keys.map(&:to_sym)
      end

      def clear
        @queues.clear
      end


      class Queue

        extend Forwardable

        def_delegators :@queue, :size, :empty?, :to_a

        def initialize
          @queue = []
          @mutex = Mutex.new
        end

        def pop
          @mutex.synchronize { @queue.shift }
        end

        def push(message)
          @mutex.synchronize { @queue.push message }
        end

      end


      class Listener

        def listen(queue, &block)
          @stopping = false

          loop do
            break if @stopping
            item = queue.pop
            next unless item
            block.call item
          end
        end

        def stop
          @stopping = true
        end

      end

    end
  end
end```
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