ServerSocket

Server Sockets in JRuby

If you are here because of an error message in JRuby, here's a short explanation. A longer version comes later on this page.

In JRuby, the Ruby Socket class can only be used for client-side connections. As a result, only connect works, not accept. We provide a separate class called ServerSocket with mostly the same API as Socket, but it handles accept and not connect.

Here's a simple example of a server:

require "socket"

socket = ServerSocket.new(Socket::AF_INET, Socket::SOCK_STREAM, 0)
sockaddr = ServerSocket.pack_sockaddr_in(12345, "127.0.0.1")
socket.bind(sockaddr)
socket.accept

The API works basically like Socket, but with ServerSocket in its place. You can also include the following code to make server sockets that work on JRuby and other impls with both server and client support in Socket:

if RUBY_ENGINE != 'jruby'
  ServerSocket = Socket
end

The long description of why JRuby works like this follows below.

Details of Sockets and ServerSockets

Because JRuby is built atop the Java Development Kit and uses its classes to implement the Ruby IO subsystem, we must choose at construction time whether a socket will be used for client or server operations. Specifically, we must construct a Socket or a ServerSocket to back up the Ruby object.

This means that it's impossible for us to construct a Ruby Socket object and later either #connect or #accept on that object; we have to choose which of those operations will work right away.

Because connecting to remote servers is more common, Ruby Socket in JRuby is client-only, and therefore will only support the connect operation (not accept). We define an additional class, ServerSocket that has the same basic operations as Socket but supports accept instead of connect. By using ServerSocket when you want a server Socket.

A simple example is at the top of this page, along with code you can include to make it work on non-JRuby versions of Ruby.