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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:
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.
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.
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), but you can do this yourself on earlier versions:
$ export LEIN_JVM_OPTS=-XX:TieredStopAtLevel=1
You can apply the same startup boost to your project, though be aware that it could negatively affect performance in the long run:
$ export JVM_OPTS=-XX:TieredStopAtLevel=1
You can do this within project.clj as well:
:jvm-opts ["-XX:+TieredCompilation" "-XX:TieredStopAtLevel=1"]
If you do have one of the above-mentioned long-running processes and want the JVM to fully optimize, you can disable the tiered compilation by either:
$ export JVM_OPTS=
or in project.clj with:
:jvm-opts ^:replace []
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.
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.
TODO: explain
Leiningen places its own code on the JVM's bootclasspath, which allows for quicker boot by skipping bytecode verification and a few other steps. You can do the same for your own projects:
:bootclasspath true
Be aware that there are some compatibility issues with this; some libraries like Jetty assume they're being loaded from a regular classloader rather than the bootstrap classloader.
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.
TODO: make it actually work, explain