Skip to content


Olle Jonsson edited this page Jun 7, 2021 · 4 revisions
Clone this wiki locally

Server Sockets in JRuby

JRuby uses classes from the Java platform for sockets, so our API differs slightly when it comes to the lowest-level "Socket" class.

Creating a Server Socket

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 =, Socket::SOCK_STREAM, 0)
sockaddr = ServerSocket.pack_sockaddr_in(12345, "")

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

Bind and Listen and Backlogs

The normal sequence for listening for connections on a Ruby Socket is to call new for a new Socket, bind to bind to an address and port, listen to express intent to listen for connections with the specified connection backlog, and finally accept to accept incoming connections.

On JRuby's ServerSocket, bind and listen are combined into bind, which accepts an optional backlog argument.

Here's an example:

socket =, Socket::SOCK_STREAM, 0)
sockaddr = ServerSocket.pack_sockaddr_in(12345, "")
socket.bind(sockaddr, 5)
# no call to listen...
# socket.listen(5)

For non-JRuby impls, the following simple monkey-patch makes their bind work like ours, for API-compatibility:

if RUBY_ENGINE != 'jruby'
  ServerSocket = Socket # as shown above
  class Socket
    alias :_old_bind, :bind
    def bind(addr, backlog)

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 support both connect and accept on that same object; we have to choose which of those operations will work right away.

Because connecting to remote servers is more common, 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.

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.

Details of Bind and Listen

The JDK ServerSocket class provides no listen method, instead allowing users to specify backlog at construction time (while simultaneously binding a port) or at bind time (combining the POSIX bind and listen operations). Because of this, we have no way to separate the two phases of "binding" and "listening" in the API we expose to Ruby.

For this reason, ServerSocket#bind in JRuby does both bind and listen at the same time, and accepts an optional backlog argument.

See above for an example, along with code you can use to make the API work in non-JRuby versions of Ruby.