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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.
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
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.
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
script to memoize all trampoline calls by saving off the
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.
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.
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=
: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
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
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 user=>
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
Drip is a script intended to speed
up JVM start times. Installation details and an explanation of how
Drip works are in the
Leiningen will make use of a Drip installation if the
environment variable is set to the location of the drip script.
Eval in Classloader
: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:
Check your :main
When starting the REPL, Leiningen loads the project's
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.