Launch Java code from Python and the CLI, installation-free.
jgo: painless Java component execution
Maven is a great tool. It manages dependencies so that Java projects become reusable "building blocks" in a much more robust way than many other languages offer. And the Maven Central repository contains a tremendous wealth of code, ripe for reuse in your own projects.
But shockingly, Maven provides no easy way to actually launch code from the
beautifully managed dependencies stored so lovingly into
This project fills that gap:
jgo launches Java code. You do not need to
download or install any JARs; you just specify an "endpoint" consisting of a
Maven artifact identifier, plus
a main class if needed/desired, and
jgo uses Maven to obtain and run it.
There are two implementations from which to choose! Each has pros and cons.
java for the heavy lifting.
The shell script version needs some common utilities (e.g.,
If you are missing anything, the script will tell you.
The shell script
Installing the shell script
Just clone this repo and symlink
jgo.sh into your favorite
For example, assuming
~/bin is on your PATH:
cd git clone https://github.com/scijava/jgo cd bin ln -s ../jgo/jgo.sh jgo jgo --help
The Python module
jgo/jgo.py module requires Python. It offers a
jgo console script,
as well as a
jgo module for programmatically creating endpoints.
Installing with pip
pip install jgo
Installing with conda
conda install -c conda-forge jgo
Installing from source
git clone https://github.com/scijava/jgo cd jgo # install globally (not recommended unless using conda or other virtual environment) pip install . # install into $HOME/.local (see pip install --help for details) pip install --user . # install into $PREFIX pip install --prefix=$PREFIX .
Usage: jgo [-v] [-u] [-U] [-m] <jvm-args> <endpoint> <main-args> -v : verbose mode flag -u : update/regenerate cached environment -U : force update from remote Maven repositories (implies -u) -m : use endpoints for dependency management (see "Details" below) <jvm-args> : any list of arguments to the JVM <endpoint> : the artifact(s) + main class to execute <main-args> : any list of arguments to the main class The endpoint should have one of the following formats: - groupId:artifactId - groupId:artifactId:version - groupId:artifactId:mainClass - groupId:artifactId:version:mainClass - groupId:artifactId:version:classifier:mainClass If version is omitted, then RELEASE is used. If mainClass is omitted, it is auto-detected. You can also write part of a class beginning with an @ sign, and it will be auto-completed. Multiple artifacts can be concatenated with pluses, and all of them will be included on the classpath. However, you should not specify multiple main classes.
|SciJava REPL with JRuby||
|SciJava REPL with Jython||
|SciJava REPL with Groovy||
|SciJava REPL with Clojure||
Note the usage of the
+ syntax as needed to append elements to the classpath.
- Is it fast?
Endpoints are synthesized in a local cache under
~/.jgo. So invoking the same endpoint a second time is really quick.
- What does "no installation" mean?
Classpath elements are hard-linked
~/.m2/repositoryrather than copied, so the
~/.jgofolder has a tiny footprint even if you execute lots of different endpoints.
- What if an endpoint has a new version?
jgoto rebuild the endpoint. Note that unlike
jgodoes not check for updates otherwise.
You can configure the behavior of
jgo using the
You can define additional remote Maven repositories, from which artifacts will be retrieved. E.g.:
[repositories] imagej.public = https://maven.imagej.net/content/groups/public
If you need more control over where artifacts come from—for example, if you
want to use your own remote Maven repository as a mirror of Maven Central—you
can do it using Maven's usual
~/.m2/settings.xml; see Using Mirrors for
You can define shortcuts for launching commonly used programs:
[shortcuts] repl = imagej:org.scijava.script.ScriptREPL imagej = net.imagej:imagej fiji = sc.fiji:fiji:LATEST scifio = io.scif:scifio-cli
Shortcuts are substituted verbatim from the beginning of the endpoint, single-pass in the order they are defined. So e.g. now you can run:
Note that with the
repl shortcut above, the main class
org.scijava.script.ScriptREPL) comes from a different artifact than
the toplevel artifact (
net.imagej:imagej). This is intentional, so that
all of ImageJ, including all of the various SciJava
scripting-<foo> plugins, is included in the classpath of the REPL.
There are a few configurable settings:
[settings] m2Repo = /path/to/.m2Repo (default ~/.m2/repository) cacheDir = /path/to/.jgo (default ~/.jgo) links = soft (options: hard, soft, none; default hard)
Note that the
jgo cache dir can also be set via the
variable when using Python
jgo. The precedence of reading the cache dir, from
highest to lowest:
- default to
Maven has a feature whereby a project can override the versions of transitive
(a.k.a. inherited) dependencies, via a
The problem is: a library may then believe it depends on components at
particular versions as defined by its
<dependencyManagement>, but downstream
projects which depend on that library will resolve to different versions.
See this SO thread and
for full details.
To work around this issue, you can pass
-m to jgo, which
causes it to add all endpoints to the synthesized POM's
<dependencyManagement> section using
By doing this, the versions of transitive dependencies used in the synthesized
project should more precisely match those of each endpoint itself—although in
the case of multiple endpoints concatenated via the
+ operator with
conflicting dependency management, the earlier endpoints will win because they
will be declared earlier in the POM. See also
issue #9 in the jgo issue tracker.
There is JPM4J, but it did not work too well for me:
It wants to maintain its own local repository of JARs outside of Maven—why? Everyone should use Maven repositories, a thoroughly established standard.
For each artifact, you have to choose a single main class as its sole command which gets linked into a shell command that runs it.
It does not seem well-synced with Maven Central, and/or does not seem to deal with dependencies in the expected way; e.g.:
$ jpm install -l -f -m org.scijava.script.ScriptREPL org.scijava:scijava-common Errors 0. Target specifies Class-Path in JAR but the indicated file .../repo/scijava-expression-parser-3.0.0.jar is not found 1. Target specifies Class-Path in JAR but the indicated file .../repo/gentyref-1.1.0.jar is not found 2. Target specifies Class-Path in JAR but the indicated file .../repo/eventbus-1.4.jar is not found
Since April 2017, the web site is offline.
There is also Mop, a more general tool with similar features, but it has not been developed since 2010, and its website is also offline.