nREPL Server

Starting a server

There are many ways to start an nREPL server. Most often you’d start it using some build tool, but you can also embed the server in your application and start it from it. Here we’ll outline the most popular options available for you today.

Using Clojure CLI tools

This section assumes you’re using Clojure 1.9+ and nREPL 0.4.4+.

If you’re into the clj command you can take advantage of nREPL’s built-in command-line interface (nrepl.cmdline).

Add this alias to ~/.clojure/deps.edn:

;; ...
:aliases {:nREPL
            {nrepl/nrepl {:mvn/version "1.2.0"}}}}

Then you can simply run the nREPL server in headless mode like this:

$ clj -M:nREPL -m nrepl.cmdline

A good practice is add whatever nREPL middleware you want to use to the nREPL profile, so you can easily activate them when needed. Here’s how you can easily start a ClojureScript capable nREPL:

;; ...
:aliases {:nREPL
           {nrepl/nrepl {:mvn/version "1.2.0"}
            cider/piggieback {:mvn/version "0.4.2"}}}}
$ clj -M:nREPL -m nrepl.cmdline --middleware "[cider.piggieback/wrap-cljs-repl]"

By default, nREPL listens for connections on a randomly chosen local port with no authentication, but you can specify the address or port, or, if you’re using JDK 16 or newer, or you add a junixsocket dependency, you can ask it to listen on a UNIX domain (filesystem) socket instead:

;; ...
:aliases {:nREPL
           ;; UNIX domain socket support was added in 1.0.0
           {nrepl/nrepl {:mvn/version "1.2.0"}
            com.kohlschutter.junixsocket/junixsocket-core {:mvn/version "2.3.2"}}}}
$ mkdir -m go-rwx nrepl-test
$ clj -M:nREPL -m nrepl.cmdline --socket nrepl-test/socket
UNIX domain sockets know that they’re running locally, so they can avoid some checks and operations (like routing), which makes them faster and lighter than TCP/IP sockets. In practice this means they are a better option for local development.

Here’s a listing of all the options available via nREPL’s command-line interface (this output was simply generated with --help):

-i/--interactive            Start nREPL and connect to it with the built-in client.
-c/--connect                Connect to a running nREPL with the built-in client.
-C/--color                  Use colors to differentiate values from output in the REPL. Must be combined with --interactive.
-b/--bind ADDR              Bind address, by default "".
-h/--host ADDR              Host address to connect to when using --connect. Defaults to "".
-p/--port PORT              Start nREPL on PORT. Defaults to 0 (random port) if not specified.
-s/--socket PATH            Start nREPL on filesystem socket at PATH.
--ack ACK-PORT              Acknowledge the port of this server to another nREPL server running on ACK-PORT.
-n/--handler HANDLER        The nREPL message handler to use for each incoming connection; defaults to the result of `(nrepl.server/default-handler)`. Must be expressed as a namespace-qualified symbol. The underlying var will be automatically `require`d.
-m/--middleware MIDDLEWARE  A sequence of vars (expressed as namespace-qualified symbols), representing middleware you wish to mix in to the nREPL handler. The underlying vars will be automatically `require`d.
-t/--transport TRANSPORT    The transport to use (default `nrepl.transport/bencode`), expressed as a namespace-qualified symbol. The underlying var will be automatically `require`d.
--help                      Show this help message.

When defining an alias in deps.edn with :main-opts, spaces between sequence values will not work. Use comma separated values instead. Here’s a simple example:

:main-opts ["-m" "nrepl.cmdline"
            "-b" ""
            "-p" "12345"
            "--middleware" "[cider.nrepl/cider-middleware,refactor-nrepl.middleware/wrap-refactor]"]

Some additional details on the subject can be found here.

Using Leiningen

Leiningen has built-in support for nREPL. Just do:

$ lein repl

And you’re all set. By default Leiningen will also connect to the running nREPL server using the popular command-line nREPL client REPL-y. If you don’t need the terminal REPL you can also start nREPL in headless mode:

$ lein repl :headless

This will start nREPL listening on localhost and a random port. You can specify the host and port explicitly like this:

$ lein repl :headless :host some-host :port 1234

Alternatively you bind nREPL to an Unix domain socket like this:

$ lein repl :headless :socket path/to/socket
You’ll need Leiningen 2.9.9 or newer for the command above to work.
As noted earlier - using Unix sockets improves your security, as that’s a connection that’s guaranteed to be local. You should always be careful not to start nREPL on a public host and port.

You can also specify a lot of nREPL’s configuration options in the :repl profile in your project.clj file:

;; Options to change the way the REPL behaves.
:repl-options { ;; Specify the string to print when prompting for input.
               ;; defaults to something like (fn [ns] (str *ns* "=> "))
               :prompt (fn [ns] (str "your command for <" ns ">? " ))
               ;; What to print when the repl session starts.
               :welcome (println "Welcome to the magical world of the repl!")
               ;; Specify the ns to start the REPL in (overrides :main in
               ;; this case only)
               ;; This expression will run when first opening a REPL, in the
               ;; namespace from :init-ns or :main if specified.
               :init (println "here we are in" *ns*)
               ;; Print stack traces on exceptions (highly recommended, but
               ;; currently overwrites *1, *2, etc).
               :caught clj-stacktrace.repl/pst+
               ;; Skip's the default requires and printed help message.
               :skip-default-init false
               ;; Customize the socket the repl task listens on and
               ;; attaches to.  Specify either a filesystem :socket
               ;; (where the parent directory should be used to
               ;; control access since POSIX doesn't require
               ;; respecting the socket permissions):
               :socket "/path/to/the/socket"
               ;; or a network :host and/or :port
               ;;   :host ""
               ;;   :port 4001
               ;; If nREPL takes too long to load it may timeout,
               ;; increase this to wait longer before timing out.
               ;; Defaults to 30000 (30 seconds)
               :timeout 40000
               ;; nREPL server customization
               ;; Only one of #{:nrepl-handler :nrepl-middleware}
               ;; may be used at a time.
               ;; Use a different server-side nREPL handler.
               :nrepl-handler (nrepl.server/default-handler)
               ;; Add server-side middleware to nREPL stack.
               :nrepl-middleware [my.nrepl.thing/wrap-amazingness
                                  ;; TODO: link to more detailed documentation.
                                  ;; Middleware without appropriate metadata
                                  ;; (see nrepl.middleware/set-descriptor!
                                  ;; for details) will simply be appended to the stack
                                  ;; of middleware (rather than ordered based on its
                                  ;; expectations and requirements).
                                  (fn [handler]
                                    (fn [& args]
                                      (prn :middle args)
                                      (apply handler args)))]}

Refer to Leiningen’s sample.project.clj for an up-to-date version of those options.

You can see the version of nREPL used by Leiningen in the message that it will display once the REPL has been started.

nREPL server started on port 56249 on host - nrepl://
REPL-y 0.4.3, nREPL 1.2.0
Clojure 1.10.0
Java HotSpot(TM) 64-Bit Server VM 10.0.1+10
    Docs: (doc function-name-here)
          (find-doc "part-of-name-here")
  Source: (source function-name-here)
 Javadoc: (javadoc java-object-or-class-here)
    Exit: Control+D or (exit) or (quit)
 Results: Stored in vars *1, *2, *3, an exception in *e


Here you can see that Leiningen has started an nREPL 1.2.0 server and has connected to it using REPL-y 0.4.3.

Using Gradle

The Clojurephant plugin provides Clojure/ClojureScript support for Gradle, including built-in nREPL support:

$ ./gradlew clojureRepl
Clojurephant will only start an nREPL server. You will need to use a separate nREPL client, such as your editor.

See Clojurephant’s docs for other configuration options.

Embedding nREPL

All the above options are typically used during the development of an application. It can also be extremely useful to have your application host a REPL server wherever it might be deployed; this can greatly simplify debugging, sanity-checking, panicked code patching, and so on.

You should think long and hard before hot-patching code in production, but that’s a subject for an unrelated discussion.

nREPL provides a socket-based server that you can trivially start from your application. Add it to your project’s dependencies, and add code like this to your app:

=> (require '[nrepl.server :refer [start-server stop-server]])
=> (defonce server (start-server :port 7888))

If you want your nREPL server to listen on a particular address instead of the default one, you can use the :bind keyword to specify the address to listen on. E.g., to make the nREPL server listen on address and port 4001:

=> (require '[nrepl.server :refer [start-server stop-server]])
=> (defonce server (start-server :bind "" :port 4001))
Keep in mind that running a nREPL server on a public address is an epic security hole! As the connections are insecure (no authentication, no authorization) by default, anyone can connect to your app and modify its behaviour or run code on the remote host.

You can also ask nREPL to listen on a UNIX domain (filesystem) socket with the :socket keyword (if you’re using JDK 16 or newer add a junixsocket dependency), which should be as secure as the access to the socket's parent directories (POSIX doesn’t specify the effect of the socket file’s permissions (if any), and some systems have ignored them):

=> (require '[nrepl.server :refer [start-server stop-server]])
=> (defonce server (start-server :socket "/some/where/safe/nrepl"))

Depending on what the lifecycle of your application is, whether you want to be able to easily restart the server, etc., you might want to put the value start-server returns into an atom or somesuch. Anyway, once your app is running an nREPL server, you can connect to it from a tool like Leiningen or Counterclockwise or REPL-y, or from another Clojure process, as shown here.

You can stop the server with (stop-server server).

Embedding in a Java application

Embedding nREPL in an existing Java application can also be useful - if your Java code allows for it you can still introspect values and call methods while the app is running. Since nREPL currently does not have a Java API, you’ll need to use Clojure’s interop features.

import clojure.lang.IFn;

public class App {
    public static void main(String[] args) {
        IFn require = Clojure.var("clojure.core", "require");
        IFn start = Clojure.var("nrepl.server", "start-server");
        int port = 7888;
        System.out.println("nrepl server started on port " + port);

You can pull in the needed dependencies by adding this to your pom.xml:


A more complete example, including the use of custom middleware, can be found here.

Server options

Note that nREPL is not limited to its default messaging protocol, nor to its default use of sockets. nREPL provides a transport abstraction for implementing support for alternative protocols and connection methods. Alternative transport implementations are available, and implementing your own is not difficult; read more about transports here.

Server Configuration

Starting with version 0.5 you can configure certain aspects of the nREPL server’s behaviour via configuration files.

There are two configuration files:

  • Global configuration file .nrepl/nrepl.edn

  • Local configuration file .nrepl.edn

The global configuration file is useful for setting options that you’d like to use for all the nREPL servers that you start (e.g. a common bind-address, transport, handler, etc).

You can alter the location of the global configuration file via the environment variable NREPL_CONFIG_DIR.

The local configuration file should be placed in the directory from which you’re starting the server (normally the root directory of your project). Its purpose is to set project-specific settings (e.g. a common port you always want to use with a particular project). Any setting in .nrepl.edn will take precedence over a setting in .nrepl/nrepl.edn.

Settings passed via the command-line interface will take precedence over settings specified via the configuration files.

Here’s an example global configuration file:

{:bind         "::"
 :transport    nrepl.transport/tty
 :middleware   [some.ns/mw1 some.ns/mw1]}
You should refer to vars only as symbols.

And this is an example of a local config file:

{:bind         "localhost"
 :port         12345
 :ack          23456
 :handler      some.ns/awesome-handler
 :transport    nrepl.transport/bencode}