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Lein-Ring is a Leiningen plugin that automates common Ring tasks.

It provides commands to start a development web server, and to turn a Ring handler into a standard war file.


To use Lein-Ring, add it as a plugin to your project.clj file or your global profile:

:plugins [[lein-ring "0.12.6"]]

Then add a new :ring key to your project.clj file that contains a map of configuration options. At minimum there must be a :handler key that references your Ring handler:

:ring {:handler hello-world.core/handler}

When this is set, you can use Lein-Ring's commands.

General options

As well as the handler, you can specify several additional options via your project.clj file:

  • :init - A function to be called once before your handler starts. It should take no arguments. If you've compiled your Ring application into a war-file, this function will be called when your handler servlet is first initialized.

  • :destroy - A function called before your handler exits or is unloaded. It should take no arguments. If your Ring application has been compiled into a war-file, then this will be called when your handler servlet is destroyed.

  • :adapter - A map of options to be passed to the Ring adapter. This has no effect if you're deploying your application as a war-file.

  • :async? - If true, treat handler as an async handler. Default false.

Environment variables

Lein-Ring pays attention to several environment variables, including:

  • PORT - the port the web server uses for HTTP
  • SSLPORT - the port the web server uses for HTTPS

These will override any options specified in the project.clj file, but won't override any options specified at the command line.

Starting a web server

The following command will start a development web server, and opens a web browser to the root page:

lein ring server

If the LEIN_NO_DEV environment variable is not set, the server will monitor your source directory for file modifications, and any altered files will automatically be reloaded.

By default, this command attempts to find a free port, starting at 3000, but you can specify your own port as an argument:

lein ring server 4000

The server-headless command works like the server command, except that it doesn't open a web browser:

lein ring server-headless

lein ring server-headless 4000

Web server options

The following options affect the behavior of the web server started by lein ring server:

  • :port - The server port or port range

  • :stacktraces? - If true, display a stacktrace when an exception is thrown. Defaults to true for server or server-headless tasks. Ignored (always false) for generated artifacts.

  • :stacktrace-middleware - A symbol referencing the middleware to use for reporting errors. Defaults to ring.middleware.stacktrace/wrap-stacktrace.

  • :auto-reload? - If true, automatically reload modified source files. Defaults to true for server or server-headless tasks. Ignored (always false) for generated artifacts.

  • :reload-paths - A collection of directory paths that can trigger a reload. By default this takes all directories in the project classpath.

  • :auto-refresh? - If true, automatically refresh the browser when files are modified. Defaults to false. Ignored (always false) for generated artifacts.

  • :refresh-paths - A collection of directory paths that can trigger a refresh. Defaults to monitoring the src and resources directories.

  • :nrepl - A map of :start? and (optionally) :port and :host keys. If :start? is true, open up an nREPL server on the given port. :start? defaults to false, :port defaults to an arbitrary free port, and :host defaults to "localhost". NOTE: This option is only for development with the lein ring server task. Setting this option will not cause a generated uberjar/uberwar to run an nREPL server. If you would like to run an nREPL server in your production app, then see the project.

Executable jar files

Lein-Ring can generate executable jar files for deployment purposes:

lein ring uberjar

This generates a jar file with all dependencies. You can then copy the file to your web server and execute it with:

java -jar <project>-<version>-standalone.jar

War files


Lein-Ring can generate war files that can be loaded onto legacy Java web services such as Apache Tomcat:

lein ring war

A servlet class and web.xml file will be generated automatically, and your application packaged up in a war file.

Like the lein jar command, you can specify the filename being generated as an additional option:

lein ring war my-app.war

Also provided is a lein ring uberwar command, which packages up all the dependencies into the war:

lein ring uberwar

The following war-specific options are supported:

  • :war-exclusions - A list of regular expressions for excluding files from the target war. Defaults to excluding hidden files.

  • :servlet-class - The servlet class name.

  • :servlet-name - The name of the servlet (in web.xml). Defaults to the handler name.

  • :url-pattern - The url pattern of the servlet mapping (in web.xml). Defaults to "/*".

  • :servlet-path-info? - If true, a :path-info key is added to the request map. Defaults to true.

  • :listener-class - Class used for servlet init/destroy functions. Called listener because underneath it uses a ServletContextListener.

  • :web-xml - web.xml file to use in place of auto-generated version (relative to project root).

  • :servlet-version - The version of the servlet spec that we claim to conform to. Attributes corresponding to this version will be added to the web-app element of the web.xml. If not specified, defaults to 2.5.

  • :uberwar-name - The name of the file generated by lein ring uberwar.

  • :exploded - If true, will generate an exploded war directory instead of a war file. If not specified, defaults to false.

These keys should be placed under the :ring key in project.clj, and are optional values. If not supplied, default values will be used instead.


A war file can also include additional resource files, such as images or stylesheets. These should be placed in the directory specified by the Leiningen :resources-path key, which defaults to "resources". These resources will be placed on the classpath. To include multiple directories, use the Leiningen :resource-paths key, which should be a vector. The values in :resources-path and :resource-paths will be concatenated.

However, there is another sort of resource, one accessed through the ServletContext object. These resources are usually not on the classpath, and are instead placed in the root of the war file. If you happen to need this functionality, you can place your files in the directory specified by the :war-resources-path key (within the project map, rather than the map specified by :ring), which defaults to "war-resources". (As with normal resources, here you can use :war-resource-paths to include multiple directories.) It's recommended that you only use WAR resources for compatibility with legacy Java interfaces; under most circumstances, you should use the normal :resources-path instead.


Copyright © 2015 James Reeves, Michael Blume

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.