Leiningen wiki: Faster

Part of what makes Leiningen's boot take a while is the fact that Leiningen's code is completely isolated from project code. This means that two JVMs are necessary to complete any task that has to execute anything in the project: one for Leiningen itself, and a subprocess for the project. There are various strategies to address this. Some of them can provide cumulative benefit, while some are mutually exclusive.

Don't Exit

The most obvious is simply to adjust your workflow so you don't start Leiningen very often. Most people simply launch a REPL once and leave it up for their whole hacking session. You still need to restart when you change your :dependencies, but working from within a single REPL session is a lot more convenient than running lein afresh over and over.

Of course, as you build up state in your process during development, there's a chance that old definitions you've removed that stick around in memory will cause bugs, so it's always a good idea to do a fresh lein test run before any major milestones like merging a long-running branch or deploying.

Fast Trampoline

As of Leiningen 2.0.0 you can perform fast trampolines. You can think of any task invocation as a pure-ish function of the command-line arguments, project.clj file, and repository state. Like any function, one way to optimize it is memoization. Setting the LEIN_FAST_TRAMPOLINE environment variable causes the bin/lein script to memoize all trampoline calls by saving off the java process invocation to disk upon the first run. This allows successive runs to skip launching a JVM for Leiningen entirely, so you will only have to wait for your own application's boot time.

Changing project.clj will invalidate the cache, as will deleting the target directory. Also note that only trampoline calls will be memoized. Since Leiningen never gets a chance to run itself, it won't check for new snapshot versions.

Tiered Compilation

Leiningen 2 uses a JVM feature called Tiered Compilation which allows the JVM to switch between compilation strategies at runtime; it can begin with a quick-start setting and switch to optimized compilation later once it has identified which sections of the code are hotspots.

Leiningen 2.1.0 onward get a speed boost by disabling the optimized compilation (which only benefits long-running processes) for both your project and Leiningen itself.

Be aware that this can negatively affect performance in the long run (or lead to inaccurate benchmarking results). If you do have a long-running processes and want the JVM to fully optimize, you can disable tiered compilation by either:

$ export LEIN_JVM_OPTS=

or in project.clj with:

:jvm-opts ^:replace []

Eval in nREPL

In Leiningen 2.1.0 and on you can add :eval-in :nrepl to re-use an existing project JVM over nREPL rather than launching a new one. This acts a bit like Cake's persistent JVMs feature, but you have to manage the lifecycle of the project JVM yourself. This can be done by simply running lein repl in a separate terminal.

This will still incur the penalty for launching Leiningen itself, just not the project JVM. If Leiningen determines there's no project nREPL server to connect to it will fall back to launching a subprocess. Note that it does not stack with fast trampolines.

Avoiding nREPL with clojure.main

A good portion of the delay involved in getting a repl up comes from launching a tools.nrepl server. Clojure ships with its own primitive repl, that lacks fancy features but still gets the basics done. In cases were you're already using fast trampoline (see above), using the clojure.main repl instead of nREPL can boost launch time by a factor of up to 5x:

$ LEIN_FAST_TRAMPOLINE=y lein trampoline run -m clojure.main
Clojure 1.6.0

Note that this is not compatible with nrepl-based tools like cider or grenchman. It also lacks tab completion and line editing. The latter can be addressed using rlwrap or by running it inside Emacs using M-x shell or inferior-lisp.


Drip is a script intended to speed up JVM start times. Installation details and an explanation of how Drip works are in the Drip Readme. Leiningen will make use of a Drip installation if the LEIN_JAVA_CMD environment variable is set to the location of the drip script.

Eval in Classloader

Setting :eval-in :classloader will run the project's code in the same JVM as Leiningen, albeit in a separate classloader. However, the bootclasspath optimizations used by Leiningen can interfere with classloader isolation, so this mode is not recommended. If you do use this mode and start to experience strange, seemingly inexplicable issues, then remove this setting.

Disable bytecode verification

Normally all code that's loaded runs thru the bytecode verifier, which is very I/O heavy. It's vanishingly unlikely that the bytecode verifier will actually find any problems that Leiningen's own checksum mechanism wouldn't, so you can turn it off with this:

:jvm-opts ["-Xverify:none"]

Check your :main

When starting the REPL, Leiningen loads the project's :main namespace. If the :main namespace takes significant time to load, the user's perception is that Leiningen is slow.


Grenchman is a fast-launching command-line client that can connect to already-running nREPL servers to evaluate code. You can use it both to avoid startup time of Leiningen itself (grench lein $TASK) or to connect directly to a project repl server.

To edit this wiki, clone the repo from git@github.com:technomancy/leiningen.wiki.git and push your changes there.