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Q: How do you pronounce Leiningen?
A: It's LINE-ing-en. ['laɪnɪŋən]

Q: What's a group ID? How do snapshots work?
A: See the tutorial for background.

Q: How should I pick my version numbers?
A: Use semantic versioning to communicate intentions to downstream users of your library, but don't make assumptions that libraries you use stick with it consistently. Remember that the difference between a breaking change and a bug fix is often subjective.

Q: What if my project depends on jars that aren't in any repository?
A: You will need to get them in a repository. The deploy guide explains how to set up a private repository. In general it's easiest to deploy them to a static HTTP server or a private S3 bucket. Once the repo is set up, lein deploy private-repo com.mycorp/somejar 1.0.0 somejar.jar pom.xml will push the artifacts out. If you don't have a pom, you can create a dummy project with lein new and generate a pom from that. If you are just doing exploratory coding you can deploy to file:///$HOME/.m2/repository and the jars will be available locally.

Q: I want to hack a project and one of its dependencies, but it's annoying to switch between them.
A: Leiningen provides a feature called checkout dependencies to make this smoother. See the tutorial to learn more.

Q: Is it possible to exclude indirect dependencies?
A: Yes. Some libraries, such as log4j, depend on projects that are not included in public repositories and unnecessary for basic functionality. Projects listed as :dependencies may exclude any of their dependencies by using the :exclusions key. See lein help sample for details.

Q: I specified a dependency on version X but am getting version Y; what's up?
A: One of your dependencies' dependencies has declared a dependency on a hard version range, which overrides your "soft" declaration. Running lein deps :tree will identify which of your dependencies are responsible for the version range. You can add an :exclusions clause to prevent that from affecting the rest of your dependencies. See lein help sample for how exclusions work. You may also want to report a bug with the dependency that uses hard version ranges as they cause all kinds of problems and exhibit unintuitive behaviour.

Q: I have two dependencies, X and Y, which depends on Z. How is the version of Z decided?
A: The decision depends on which depth and which order the dependencies come in the :dependencies vector: The dependency at the lowest depth will be picked. If there are multiple versions of a single group/artifact at that depth, the first of those will be picked. For instance, in the dependency graph

[Z "1.0.9"]
[X "1.3.2"]
  [Z "2.0.1"]

the direct dependency ([Z "1.0.9"]) is picked, as it is closest to the root. For the dependency graph

[X "1.3.2"]
  [Z "2.0.1"]
[Y "1.0.5"]
  [Z "2.1.3"]

the dependency X comes first, and therefore [Z "2.0.1"] is picked. If we place Y before X however, [Z "2.1.3"] will be picked.

Note that this only applies to soft dependencies, and lein deps :tree will only warn if the latest version is not chosen.

Q: I'm behind an HTTP proxy; how can I fetch my dependencies?
A: Set the $http_proxy environment variable in Leiningen 2.x. You can also set $http_no_proxy for a list of hosts that should be reached directly, bypassing the proxy. This is a list of patterns separated by | and may start or end with a * for wildcard, e.g. localhost|*.mydomain.com. For Leiningen 1.x versions, see the instructions for configuring a Maven proxy using ~/.m2/settings.xml.

Q: What can be done to speed up launch?
A: The main delay involved in Leiningen comes from starting two JVMs: one for your project and one for Leiningen itself. Most people use a development cycle that involves keeping a single project REPL process running for as long as they're working on that project. Depending on your editor you may be able to do this via its Clojure integration. (See cider or fireplace, for example.) Otherwise you can use the basic lein repl.

Q: Version 2.8.0 seems a bit slower; why is that?
A: We have long used a hack of putting Leiningen on the JVM's bootclasspath to speed up boot time, but the module system in Java 9 breaks this. We have switched to another method of speeding it up (-Xverify:none) which gives anywhere from 95% to 70% of the same speed boost depending on the machine on which you're running it. So some users will notice a performance regression. We hope to go back to the old method once Clojure 1.9.0 is released with a workaround, but in the mean time if you are not using Java 9, you can go back to the bootclasspath hack with this setting:


Q: Still too slow; what else can make startup faster?
A: The wiki has a page covering ways to improve startup time.

Q: What if I care more about long-term performance than startup time?
A: Leiningen 2.1.0 onward get a speed boost by disabling optimized compilation (which only benefits long-running processes). This can negatively affect performance in the long run, or lead to inaccurate benchmarking results. If want the JVM to fully optimize, you can switch profiles with lein with-profiles production run ....

Q: I'm attempting to run a project as a background process (lein run &), but the process suspends until it is in the foreground. How do I run a program in the background?
A: For long-lasting processes, it's better to create an uberjar and run that or use lein trampoline run &. For short-lived ones, both lein run <&- & and bash -c "lein run &" will work fine.

Q: How do I determine my project's version at runtime?
A: Leiningen writes a file called pom.properties into target/classes which contains the version number and current git revision of the project. In previous versions of Leiningen this was only available when running from jar files, but as of 2.4.1 it's available during lein run ..., etc. You can read it by running this code (replace "group" and "artifact" with values appropriate to your project):

(with-open [pom-properties-reader (io/reader (io/resource "META-INF/maven/group/artifact/pom.properties"))]
  (doto (java.util.Properties.)
    (.load pom-properties-reader)))

Q: How can I read my project map at runtime?
A: Usually you do not need the complete project map, only a specific subset of some values. If you want different configuration based on different tasks, then environ is probably a good fit. If you want information like the project's version number or git revision, read the question and answer above.

Generally those solutions are sufficient, but if you need more than this, you should rather read the project.clj yourself. The project map changes based on the task you use, and so different tasks (repl, jar, uberjar to name a few) will make it hard to make the testing- and production project map identical. project.clj is added as a resource in META-INF/leiningen/group/artifact/project.clj (replace "group" and "artifact" with values appropriate to your project). You can read it as follows:

(read-string (slurp (io/resource "META-INF/leiningen/group/artifact/project.clj")))

Q: I need to do AOT for an uberjar; can I avoid it during development?
A: Yes, it is strongly recommended to do AOT only in the uberjar task if possible. But by default the AOT'd files will still be visible during development unless you also change :target-path to something like "target/uberjar" in the :uberjar profile as well.

Q: Is there a way to use an uberjar without AOT?
A: As of Leiningen 2.4.0, if you omit :main in project.clj, your uberjars will use clojure.main as their entry point. You can launch with java -jar my-app-standalone.jar -m my.entry.namespace arg1 arg2 [...] without any AOT, but it will take longer to launch.

Q: Why does lein jar package some namespaces from dependencies into my jar?
A: This is likely because you have AOT-compiled namespaces. An AOT-compiled namespace can only depend on AOT-compiled namespaces. Therefore, if you depend on a namespace in a dependency that is not AOT-compiled, it will be AOT-compiled and bundled with the jar. It is strongly recommended not to perform AOT other than during the creation of an uberjar.

Q: I'd like to have certain config active only on a certain OS.
A: You can do this by using unquote in the :dev profile:

:profiles {:dev [:dev/all ~(leiningen.core.utils/get-os)]
           :dev/all {...}
           :linux {...}
           :windows {...}
           :macosx {...}}

You can also check things like (System/getProperty "java.specification.version") to use the JVM version or any other property.

Q: What does Received fatal alert: protocol_version mean when trying to access Clojars?
A: This usually means your JVM is not configured to use TLSv1.2, which is used by Clojars' CDN. It's strongly recommended to upgrade to at least Java 8, but if this is not feasible, you can fix it by exporting LEIN_JVM_OPTS=-Dhttps.protocols=TLSv1.2 as an environment variable.

Q: I get a java.security.KeyException or sun.security.provider.certpath.SunCertPathBuilderException when running lein
A: The java.security.KeyException indicates an ssl error when trying to communicate with the HTTPS server via Java. This could be because you need to update the JDK, or some other package (e.g. with old versions of the nss package).

  • On Fedora, you might just try running a sudo yum update to update all of your packages or sudo yum update nss.
  • On Debian/Ubuntu, sudo update-ca-certificates -f might help, or sudo /var/lib/dpkg/info/ca-certificates-java.postinst configure
  • You should also check your system clock and make sure the time is accurate; it's possible to run into SSL connection failures if your clock is way out of sync.
  • If it still doesn't work, please see if any of these 'ssl' labelled issues might help

Q: I got Tried to use insecure HTTP repository without TLS, what is that about?
A: This means your project was configured to download dependencies from a repository that does not use TLS encryption. This is very insecure and exposes you to trivially-executed man-in-the-middle attacks. In the rare event that you don't care about the security of the machines running your project or can ensure that the only http traffic is going out over a trusted network, you can re-enable support for unsafe repositories by putting this in your project.clj file:

;; never do this
(require 'cemerick.pomegranate.aether)
 "http" #(org.apache.maven.wagon.providers.http.HttpWagon.))

It's also possible you have a dependency which includes a reference to an insecure repository for retrieving its own dependencies. If this happens it is strongly recommended to add an :exclusion and report a bug with the dependency which does this.

Q: lein/lein.bat won't download leiningen-x.y.z-SNAPSHOT.jar
A: You probably downloaded lein/lein.bat from the main branch. Unless you plan to build leiningen yourself or help develop it, we suggest you use the latest stable version: lein/lein.bat

Q: I have a dependency whose group ID and/or artifact ID starts with a number (which is invalid for symbols in Clojure). How can I add it to my project's dependencies?
A: As of version 2.8.0, Leiningen supports string dependency names like this:

:dependencies [["net.3scale/3scale-api" "3.0.2"]]

Prior to version 2.8.0, this is the workaround:

:dependencies [[~(symbol "net.3scale" "3scale-api") "3.0.2"]]

Q: I'm getting warnings for implicit hooks.
A: Hooks are a deprecated feature where plugins can modify the behavior of built-in Leiningen functionality; they result in situations which can be very difficult to debug and usually point to situations in which the original API is not flexible enough.

Adding :implicits false to project.clj will disable all implicit features.

Q: What causes "WARNING: An illegal reflective access operation has occurred"?
A: This is due to changes introduced in Java 9+. At the time of this writing, it's recommended to use Java 8 if possible. Otherwise you can use workarounds including adding {:user {:jvm-opts ["--illegal-access=deny"]}} to your ~/.lein/profile.clj.