A Clojure network REPL that provides a server and client, along with some common APIs of use to IDEs and other tools that may need to evaluate Clojure code in remote environments.
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nREPL is a Clojure network REPL that provides a REPL server and client, along with some common APIs of use to IDEs and other tools that may need to evaluate Clojure code in remote environments.



nREPL is available in Maven central. Add this :dependency to your Leiningen project.clj:

[org.clojure/tools.nrepl "0.2.0-RC1"]

Or, add this to your Maven project's pom.xml:


A list of all prior releases are available here.

Please note the changelog below.

nREPL is compatible with Clojure 1.2.0 - 1.5.0.

Please post general questions or discussion on either the clojure-dev or clojure-tools mailing lists. Bug reports and such may be filed into nREPL's JIRA.

nREPL's generated API documentation is available here. A history of nREPL builds is available, as well as a compatibility test matrix, verifying nREPL's functionality against multiple versions of Clojure and multiple JVMs.

Connecting to an nREPL server

Most of the time, you will connect to an nREPL server using an existing client/tool. Tools that support nREPL include:

If your preferred Clojure development environment supports nREPL, you're done. Use it or connect to an existing nREPL endpoint, and you're done.

Talking to an nREPL endpoint programmatically

If you want to connect to an nREPL server using the default transport, something like this will work:

=> (require '[clojure.tools.nrepl :as repl])
=> (with-open [conn (repl/connect :port 59258)]
     (-> (repl/client conn 1000)    ; message receive timeout required
       (repl/message {:op "eval" :code "(+ 2 3)"})

response-values will return only the values of evaluated expressions, read from their (by default) pr-encoded representations via read. You can see the full content of message responses easily:

=> (with-open [conn (repl/connect :port 59258)]
     (-> (repl/client conn 1000)
       (repl/message {:op :eval :code "(time (reduce + (range 1e6)))"})
       doall      ;; `message` and `client-session` all return lazy seqs
({:out "\"Elapsed time: 68.032 msecs\"\n",
  :session "2ba81681-5093-4262-81c5-edddad573201",
  :id "3124d886-7a5d-4c1e-9fc3-2946b1b3cfaa"}
 {:ns "user",
  :value "499999500000",
  :session "2ba81681-5093-4262-81c5-edddad573201",
  :id "3124d886-7a5d-4c1e-9fc3-2946b1b3cfaa"}
 {:status ["done"],
  :session "2ba81681-5093-4262-81c5-edddad573201",
  :id "3124d886-7a5d-4c1e-9fc3-2946b1b3cfaa"})

Each message must contain at least an :op (or "op") slot, which specifies the "type" of the operation to be performed. The operations supported by an nREPL endpoint are determined by the handlers and middleware stack used when starting that endpoint; the default middleware stack (described below) supports a particular set of operations, detailed here.

Embedding nREPL, starting a server

If your project uses Leiningen (v2 or higher), you already have access to an nREPL server for your project via lein repl (or, lein repl :headless if you don't need the Reply terminal-based nREPL client to connect to the resulting nREPL server).

Otherwise, it can be extremely useful to have your application host a REPL server whereever it might be deployed; this can greatly simplify debugging, sanity-checking, panicked code patching, and so on.

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:

=> (use '[clojure.tools.nrepl.server :only (start-server stop-server)])
=> (defonce server (start-server :port 7888))

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 Reply, or from another Clojure process:

=> (with-open [conn (repl/connect :port 7888)]
     (-> (repl/client conn 1000)
       (repl/message {:op :eval :code "(+ 1 1)"})

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

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.

Building nREPL

Releases are available from Maven Central, and SNAPSHOT builds from master's HEAD are automatically deployed to Sonatype's OSS repository (see this for how to configure Leiningen or Maven to use OSS-snapshots), so building nREPL shouldn't ever be necessary. But, if you insist:

  1. Clone the repo
  2. Make sure you have maven installed
  3. Run the maven build, either:
    1. mvn package: This will produce an nREPL jar file in the target directory, and run all tests against Clojure 1.2.0.
    2. mvn verify: This does the same, but also runs the tests with other Clojure "profiles" (one for each supported version of Clojure).

Why nREPL?

nREPL has been designed with the aim of ensuring that it satisfies the requirements of both application developers (in support of activities ranging from interactive remote debugging and experimentation in development contexts through to more advanced use cases such as updating deployed applications) as well as toolmakers (providing a standard way to connect to and introspect running environments as a way of informing user interfaces of all kinds, including "standard" interactive, text-based REPLs).

The default network protocol used is simple, depending neither on JVM or Clojure specifics, thereby allowing (encouraging?) the development of non-Clojure REPL clients. The REPLs operational semantics are such that essentially any non-JVM Clojure implementation should be able to implement it, with allowances for hosts that lack the concurrency primitives to support e.g. asynchronous evaluation, interrupts, etc.

For more information about the motivation, architecture, use cases, and discussion related to nREPL, see the see the original design notes, availablhere, and the notes and discussion around its recent redesign.


nREPL largely consists of three abstractions: handlers, middleware, and transports. These are roughly analogous to the handlers, middleware, and adapters of Ring, though there are some important semantic differences. Finally, nREPL is fundamentally message-oriented and asynchronous (in contrast to most REPLs that build on top of streams provided by e.g. terminals).


nREPL messages are maps. The keys and values that may be included in messages depends upon the transport being used; different transports may encode messages differently, and therefore may or may not be able to represent certain data types.

Each message sent to an nREPL endpoint constitutes a "request" to perform a particular operation, which is indicated by a "op" entry. Each operation may further require the incoming message to contain other data. Which data an operation requires or may accept varies; for example, a message to evaluate some code might look like this:

{"op" "eval" "code" "(+ 1 2 3)"}

The result(s) of performing each operation may be sent back to the nREPL client in one or more response messages, the contents of which again depend upon the operation.


Transports are roughly analogous to Ring's adapters: they provide an implementation of a common protocol (clojure.tools.nrepl.transport.Transport) to enable nREPL clients and servers to send and receive messages without regard for the underlying channel or particulars of message encoding.

nREPL includes two transports, both of which are socket-based: a "tty" transport that allows one to connect to an nREPL endpoint using e.g. telnet (which therefore supports only the most simplistic interactive evaluation of expressions), and one that uses bencode to encode nREPL messages over sockets. It is the latter that is used by default by clojure.tools.nrepl.server/start-server and clojure.tools.nrepl/connect.

Other nREPL transports are provided by the community, including:

  • nrepl-hornetq, which routes nREPL messages over HornetQ.
  • Drawbridge, Ring middleware implementing an nREPL HTTP/HTTPS endpoint and an nREPL transport to connect to such endpoints.
  • concerto, which "adds a broadcast mode to nrepl, so that when multiple users are logged into the same nrepl server they can each see what the others are doing".

(If you implement some new useful nREPL transport, let me know and I'll add it to this list.)


Handlers are functions that accept a single incoming message as an argument. An nREPL server is started with a single handler function, which will be used to process messages for the lifetime of the server. Note that handler return values are ignored; results of performing operations should be sent back to the client via the transport in use (which will be explained shortly). This may seem peculiar, but is motivated by two factors:

  • Many operations — including something as simple as code evaluation — is fundamentally asynchronous with respect to the nREPL server
  • Many operations can produce multiple results (e.g. evaluating a snippet of code like "(+ 1 2) (def a 6)).

Thus, messages provided to nREPL handlers are guaranteed to contain a :transport entry containing the transport that should be used to send all responses precipitated by a given message. (This slot is added by the nREPL server itself, thus, if a client sends any message containing a "transport" entry, it will be bashed out by the Transport that was the source of the message.) Further, all messages provided to nREPL handlers have keyword keys (as per clojure.walk/keywordize-keys).

Depending on its :op, a message might be required to contain other slots, and might optionally contain others. It is generally the case that request messages should contain a globally-unique :id. Every request must provoke at least one and potentially many response messages, each of which should contain an :id slot echoing that of the provoking request.

Once a handler has completely processed a message, a response containing a :status of :done must be sent. Some operations necessitate that additional responses related to the processing of a request are sent after a :done :status is reported (e.g. delivering content written to *out* by evaluated code that started a future). Other statuses are possible, depending upon the semantics of the :op being handled; in particular, if the message is malformed or incomplete for a particular :op, then a response with an :error :status should be sent, potentially with additional information about the nature of the problem.

It is possible for an nREPL server to send messages to a client that are not a direct response to a request (e.g. streaming content written to System/out might be started/stopped by requests, but messages containing such content can't be considered responses to those requests).

If the handler being used by an nREPL server does not recognize or cannot perform the operation indicated by a request message's :op, then it should respond with a message containing a :status of "unknown-op".

It is currently the case that the handler provided as the :handler to clojure.tools.nrepl.server/start-server is generally built up as a result of composing multiple pieces of middleware.


Middleware are higher-order functions that accept a handler and return a new handler that may compose additional functionality onto or around the original. For example, some middleware that handles a hypothetical "time?" :op by replying with the local time on the server:

(require '[clojure.tools.nrepl.transport :as t])
(use '[clojure.tools.nrepl.misc :only (response-for)])

(defn current-time
  (fn [{:keys [op transport] :as msg}]
    (if (= "time?" op)
      (t/send transport (response-for msg :status :done :time (System/currentTimeMillis)))
      (h msg))))

A little silly, but this pattern should be familiar to you if you have implemented Ring middleware before. Nearly all of the same patterns and expectations associated with Ring middleware should be applicable to nREPL middleware.

Some known third-party nREPL middleware includes:

  • Piggieback, which adds ClojureScript support to any nREPL session
  • nrepl-middleware, part of ritz that provides a variety of nREPL middleware supporting various enhanced REPL operations (including apropos, javadoc lookup, code completion, and an alternative eval implementation)
  • nrepl-project, part of ritz that provides "nREPL middleware for controlling the classpath of a REPL"
  • nrepl-codeq, part of ritz that provides "middleware for use with datomic's codeq"
  • nrepl-cljs-middleware, offering ClojureScript compilation-as-a-middleware-service

(If you implement some useful nREPL middleware, let me know so I can add it to this list.)

All of nREPL's provided default functionality is implemented in terms of middleware, even foundational bits like session and eval support. This default middleware "stack" aims to match and exceed the functionality offered by the standard Clojure REPL, and is available at clojure.tools.nrepl.server/default-middlewares. Concretely, it consists of a number of middleware functions' vars that are implicitly merged with any user-specified middleware provided to clojure.tools.nrepl.server/default-handler. To understand how that implicit merge works, we'll first need to talk about middleware "descriptors".

(See this documentation listing for details as to the operations implemented by nREPL's default middleware stack, what each operation expects in request messages, and what they emit for responses.)

Middleware descriptors and nREPL server configuration

It is generally the case that most users of nREPL will expect some minimal REPL functionality to always be available: evaluation (and the ability to interrupt evaluations), sessions, file loading, and so on. However, as with all middleware, the order in which nREPL middleware is applied to a base handler is significant; e.g., the session middleware's handler must look up a user's session and add it to the message map before delegating to the handler it wraps (so that e.g. evaluation middleware can use that session data to stand up the user's dynamic evaluation context). If middleware were "just" functions, then any customization of an nREPL middleware stack would need to explicitly repeat all of the defaults, except for the edge cases where middleware is to be appended or prepended to the default stack.

To eliminate this tedium, the vars holding nREPL middleware functions may have a descriptor applied to them to specify certain constraints in how that middleware is applied. For example, the descriptor for the clojure.tools.nrepl.middleware.session/add-stdin middleware is set thusly:

(set-descriptor! #'add-stdin
  {:requires #{#'session}
   :expects #{"eval"}
   :handles {"stdin"
             {:doc "Add content from the value of \"stdin\" to *in* in the current session."
              :requires {"stdin" "Content to add to *in*."}
              :optional {}
              :returns {"status" "A status of \"need-input\" will be sent if a session's *in* requires content in order to satisfy an attempted read operation."}}}})

Middleware descriptors are implemented as a map in var metadata under a :clojure.tools.nrepl.middleware/descriptor key. Each descriptor can contain any of three entries:

  • :requires, a set containing strings or vars identifying other middleware that must be applied at a higher level than the middleware being described. Var references indicate an implementation detail dependency; string values indicate a dependency on any middleware that handles the specified :op.
  • :expects, the same as :requires, except the referenced middleware must exist in the final stack at a lower level than the middleware being described.
  • :handles, a map that documents the operations implemented by the middleware. Each entry in this map must have as its key the string value of the handled :op and a value that contains any of four entries:
    • :doc, a human-readable docstring for the middleware
    • :requires, a map of slots that the handled operation must find in request messages with the indicated :op
    • :optional, a map of slots that the handled operation may utilize from the request messages with the indicated :op
    • :returns, a map of slots that may be found in messages sent in response to handling the indicated :op

The values in the :handles map is used to support the "describe" operation, which provides "a machine- and human-readable directory and documentation for the operations supported by an nREPL endpoint" (see clojure.tools.nrepl.middleware/describe-markdown, and the results of "describe" and describe-markdown here).

The :requires and :expects entries control the order in which middleware is applied to a base handler. In the add-stdin example above, that middleware will be applied after any middleware that handles the "eval" operation, but before the clojure.tools.nrepl.middleware.session/session middleware. In the case of add-stdin, this ensures that incoming messages hit the session middleware (thus ensuring that the user's dynamic scope — including *in* — has been added to the message) before the add-stdin's handler sees them, so that it may append the provided stdin content to the buffer underlying *in*. Additionally, add-stdin must be "above" any eval middleware, as it takes responsibility for calling clojure.main/skip-if-eol on *in* prior to each evaluation (in order to ensure functional parity with Clojure's default stream-based REPL implementation).

The specific contents of a middleware's descriptor depends entirely on its objectives: which operations it is to implement/define, how it is to modify incoming request messages, and which higher- and lower-level middlewares are to aid in accomplishing its aims.

nREPL uses the dependency information in descriptors in order to produce a linearization of a set of middleware; this linearization is exposed by clojure.tools.nrepl.middleware/linearize-middleware-stack, which is implicitly used by clojure.tools.nrepl.server/default-handler to combine the default stack of middleware with any additional provided middleware vars. The primary contribution of default-handler is to use clojure.tools.nrepl.server/unknown-op as the base handler; this ensures that unhandled messages will always produce a response message with an :unknown-op :status. Any handlers otherwise created (e.g. via direct usage of linearize-middleware-stack to obtain a ordered sequence of middleware vars) should do the same, or use a similar alternative base handler.

Change Log


Top-to-bottom redesign


Never released; initial prototype of "rich content" support that (in part) helped motivate a re-examination of the underlying protocol and design.


  • added Clojure 1.3.0 (ALPHA) compatibility


  • fixed (hacked) obtaining clojure.test output when clojure.test is initially loaded within an nREPL session
  • eliminated 1-minute default timeout on expression evaluation
  • all standard REPL var bindings are now properly established and maintained within a session


Thanks to the following Clojure masters for their helpful feedback during the initial design phases of nREPL:

  • Justin Balthrop
  • Meikel Brandmeyer
  • Hugo Duncan
  • Christophe Grand
  • Anthony Grimes
  • Phil Hagelberg
  • Rich Hickey
  • Chris Houser
  • Colin Jones
  • Laurent Petit
  • Eric Thorsen


Copyright © 2010 - 2012 Chas Emerick and contributors.

Licensed under the EPL. (See the file epl.html.)