Leiningen tasks are simply functions named $TASK in a leiningen.$TASK namespace. So writing a Leiningen plugin is pretty straightforward; as long as the file containing the namespace is available on the classpath, Leiningen will be able to use it.
Plugins may be installed on a per-project or user-wide basis. To use a plugin in a single project, add it to your project.clj :dev-dependencies and run "lein deps". To install it for your user, run "lein plugin install ARTIFACT-ID VERSION". In general user-level plugins are preferred when the plugin is a matter of user convenience, and :dev-dependencies are better for plugins without which the tests or packaging will not function.
Writing a Plugin
Start by generating a new project with "lein new myplugin", and add a leiningen.myplugin namespace with a myplugin function. Add :eval-in-leiningen true to your project.clj so Leiningen knows to execute its code inside the Leiningen process rather than spinning up a project subprocess.
Some tasks may only be run in the context of a project. For tasks like this, name the first argument project. Leiningen will inspect the argument list and pass in the current project if needed.
Some tasks can be run inside a project or outside, but would benefit from having the project argument if they're run from a project. For these, name the first argument something like project-or-foo, and it will be passed the project argument when appropriate.
The project is a map which is based on the project.clj file, but it also has :name, :group, :version, and :root keys added in. If you want it to take parameters from the command-line invocation, you can make the function take more arguments.
Note that Leiningen is an implied dependency of all plugins; you should not explicitly list it in the project.clj file. You also don't need to list Clojure, but you will be locked into using the same version of Clojure that Leiningen is using. For Leiningen 1.x, a dependency on Clojure Contrib is also implied, though this is gone in 2.0.
The "lein help" task will display the first line of the task function's docstring as a summary. Then "lein help $TASK" will use the task function's full docstring for detailed help. The function's arglists will also be shown, so pick argument names that are clear and descriptive. If you set :help-arglists in the function's metadata, it will be used instead for those cases where alternate arities exist that aren't intended to be exposed to the user.
If your task returns an integer, it will be used as the exit code for the process. If tasks are chained together, a nonzero integer return value will halt the chain and exit immediately. Throwing an exception will also halt execution, but returning an integer will avoid showing an unsightly stack trace.
Leiningen contains a workaround for a flaw in Clojure's agent thread pools that may cause confusion. The JVM will refuse to exit if there are active non-daemon threads. Clojure agents and futures start up a non-daemon thread pool, so if you call any code that uses agents or futures (even code that isn't inherently asynchronous like clojure.java.shell/sh), the JVM won't exit until (shutdown-agents) is called.
In order to work around this, Leiningen appends a call to (shutdown-agents) to any code that runs in a project subprocess. If it didn't, many plugins would simply never finish. But it has a few unfortunate side-effects. You may expect the fact that your plugin starts its own thread using future or send to keep its process alive, but this is not possible with Leiningen's workaround.
On the other hand, you may start up your own threads outside Clojure's thread pools. These threads will keep the process alive on their own, but they will not prevent Leiningen from running (shutdown-agents). This will cause code that's running in your threads to lose access to agents and futures since shutdown-agents is irreversible.
The solution is to block in your main thread if you need other threads to keep alive. However, if your task is being run from Leiningen's interactive task, the project subprocess will not exit and shutdown-agents will not be triggered, so it's best to block conditionally. Here's an example:
(when-not ~leiningen.core/*interactive?* (shutdown-agents))
Note again that this only applies for plugins that need to run code in the project's process, so the code above would go inside the form argument to eval-in-project. If your plugin only runs in Leiningen's process then you don't need to worry. Hopefully this will be fixed in future versions of Clojure, but the workaround will remain necessary for backwards-compatibility.
You can modify the behaviour of built-in tasks to a degree using hooks. Hook functionality is provided by the Robert Hooke library. This is an implied dependency; as long as Leiningen 1.2 or higher is used it will be available.
Inspired by clojure.test's fixtures functionality, hooks are functions which wrap tasks and may alter their behaviour by using binding, altering the return value, only running the function conditionally, etc. The add-hook function takes a var of the task it's meant to apply to and a function to perform the wrapping:
(use 'robert.hooke) (defn skip-integration-hook [task & args] (binding [clojure.test/test-var (test-var-skip :integration)] (apply task args))) (add-hook #'leiningen.test/test skip-integration-hook)
Hooks compose, so be aware that your hook may be running inside another hook. To take advantage of your hooks functionality, projects must set the :hooks key in project.clj to a seq of namespaces to load that call add-hook. Note that in Leiningen 1.2, hooks get loaded and used without being specified in project.clj; this is a bug. In 1.3 and on they are opt-in only.
If you need to use hooks from code that runs inside the project's process, you may use leiningen.util.injected/add-hook, which is an isolated copy of robert.hooke/add-hook injected into the project in order to support features like test selectors.
See the documentation for Hooke for more details.
Altering Leiningen's Classpath
Leiningen's classpath will include all plugins from :dev-dependencies as well as user plugins. To further modify the classpath of Leiningen itself, add a '.lein-classpath' file a project's root. Its contents will be prepended to Leiningen's classpath when Leiningen is invoked upon that project.
If your plugins need to do a fair amount of filesystem-y things, you may want to take a look at using Ant tasks to do them since the JDK lacks a lot of simple functionality of this kind. Using the Ant API directly is a pain, but it can be eased to a degree using Lancet. Lancet is the Clojure adapter for Ant that is developed as the sample project in the Programming Clojure book.
You can look over the Ant API documentation's listing of tasks to find an appropriate task. See the deps task for an example of how to call a task from Clojure.
Please add your plugins to the list on the wiki.
Hopefully the plugin mechanism is simple and flexible enough to let you bend Leiningen to your will.