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j’bang - Having fun with Java scripting

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JBang Logo

Intro

Want to learn, explore or use Java instantly without setup ?

Do you like Java but use python, groovy, kotlin or similar languages for your scripts ?

Ever tried out Java 10+ support for running .java files directly in your shell but felt it was a bit too cumbersome ?

Then try jbang which lets you do this:

AVwA19yijKRNKEO0bJENN2ME3

$ jbang --init=cli hello.java
$ jbang hello.java Max!
[jbang] Resolving dependencies...
[jbang]     Resolving info.picocli:picocli:4.2.0...Done
[jbang] Dependencies resolved
[jbang] Building jar...
Hello Max!
$ jbang hello.java -h
Usage: hello [-hV] <greeting>
hello made with jbang
      <greeting>   The greeting to print
  -h, --help       Show this help message and exit.
  -V, --version    Print version information and exit.

Instant cli app generated built using java and picocli as a dependency that was fetched as needed for the compilation and execution.

Table of Contents

Features

  • .java Scripting for Java 8 and upwards

  • .jsh via JShell from Java 9 and upwards

  • Works on [windows] Windows, [apple] OSX and [linux] Linux

  • Install using SDKMan ([apple]/[linux]), Homebrew ([apple]), Chocolatey ([windows]) or Scoop ([windows])

  • Dependency declarations using //DEPS <gav> for automatic dependency resolution

  • Control compile and runtime options with //JAVAC_OPTIONS <flags> and //JAVA_OPTIONS <flags>

  • Compiled jar and Dependency resolution caching

  • Launch with debug enabled for instant debugging from your favorite IDE

  • Transparent launch of JavaFX Applications on Java 8 and higher.

  • Can be used for writing plugins to other cli’s like kubectl

  • Init templates to get started easily (jbang --init=cli hello.java)

  • Generate gradle and IDE config with dependencies for easy editing in your favorite IDE (jbang --edit myfile.java)

  • (PLANNED) Lookup dependencies with a short-hand name, i.e. // DEPS log4j:1.2+,picocli for quick getting started.

To use it simply install jbang and run jbang yourscript.java

Table of Contents

Requirements

Minimum Java 8, Recommended: Java 11+

Tested and verified to use on OSX, Linux and Windows.

Installation

To use jbang Java 8 is the minimum required version, however Java 11 or higher is recommended.

SDKMan [linux] / [apple]

To install both java and jbang we recommend sdkman on Linux and OSX.

curl -s "https://get.sdkman.io" | bash # (1)
source ~/.bash_profile # (2)

sdk install java # (3)

Once Java is installed and ready, you install jbang with

sdk install jbang

To test your installation run:

jbang --help

This should print out usage information.

To update run:

sdk update jbang

Chocolatey [windows]

On Windows you can install both java and jbang` with Chocolatey.

From a command prompt with enough rights to install with choco:

choco install jdk11

Once Java in installed run:

choco install jbang

To upgrade to latest version:

choco upgrade jbang

The latest package will be published to jbang choco package page, it might be a bit delayed as the review is still manual. In case the default version is not the latest you can see the version list and install specific version using:

choco install jbang --version=<version number>

Scoop [windows]

On Windows you can also install jbang` with Scoop.

scoop bucket add maxandersen https://github.com/maxandersen/scoop-bucket
scoop install jbang

To upgrade to latest version:

scoop update jbang

Homebrew [apple]

On OSX you can install 'java' and jbang with Homebrew using custom taps.

To install Java 11:

brew tap AdoptOpenJDK/openjdk
brew cask install adoptopenjdk11

Once Java is installed you can use brew with maxandersen/tap to get jbang:

brew install maxandersen/tap/jbang

To upgrade to latest version:

brew upgrade maxandersen/tap/jbang

Installing older versions via Homebrew

If you encounter an issue in jbang that is not present in an older version, you can revert back to that older version following these steps:

  1. Find the commit id for the version to revert to (e.g. 0.8.1).

    $ cd "$(brew --repo maxandersen/tap)"
$ git log master -- Formula/jbang.rb
...
commit fd70f1bc0a7f69d81cfb5b08a0d2bb698fbd01b2
Author: Max Rydahl Andersen <[email protected]>
Date:   Tue Jan 21 00:33:05 2020 +0000

    jbang v0.8.1

  2. Checkout the the version.

    $ git checkout fd70f1bc0a7f69d81cfb5b08a0d2bb698fbd01b

  3. Unlink current jbang version.

    $ brew unlink jbang
Unlinking /usr/local/Cellar/jbang/0.13.2... 1 symlinks removed

  4. Install the older version.

    $ HOMEBREW_NO_AUTO_UPDATE=1 brew install jbang
...
🍺  /usr/local/Cellar/jbang/0.8.1: 18 files, 2.9MB, built in 6 seconds

  5. Verify the version.

    $ jbang --version
0.8.1

(Experimental) Linux packages [linux]

INFO: These builds are not fully automated yet thus might be slightly behind.

You can install rpm packages from Fedora Copr by doing the following:

dnf copr enable maxandersen/jbang
dnf install jbang

The COPR currently includes builds from various versions of CentOS, Fedora, Mageia and OpenSuse.

Docker / GitHub Action [docker]

You can run jbang via Docker:

docker run -v `pwd`:/ws --workdir=/ws maxandersen/jbang-action helloworld.java

or if you prefer using Quay.io:

docker run -v `pwd`:/ws --workdir=/ws quay.io/maxandersen/jbang-action helloworld.java

The same container images can be used with GitHub Actions, see jbang-action for details.

Manual install

Unzip the latest binary release, put the jbang-<version>/bin folder in to your $PATH and you are set.

Usage

A script is just a single .java file with a classic static main method or a .jsh file which will be passed to jshell.

Below is an (almost) minimal example you can save in helloworld.java or simply run jbang --init hellworld.java:

//usr/bin/env jbang "$0" "$@" ; exit $? // (1)

class helloworld { // (2)

    public static void main(String[] args) {
        if(args.length==0) {
            System.out.println("Hello World!");
        } else {
            System.out.println("Hello " + args[0]);
        }
    }
}

  1. By using this // style instead of shebang #! you trick bash, zsh etc. to run this as a script while still being valid java code.

  2. A classname, can be anything when using jbang but to be valid java for most IDEs you’ll want to name it the same as the source file.

Now to run this you can call it via jbang:

jbang helloworld.java

or if on Linux/OSX run it directly. If you created it manually you need to mark it as executable before running it.

chmod +x helloworld.java
./helloworld jbang!

You can use http(s):/ and file:/ url’s for input and github.com based urls will automatically download the raw version (since v0.15).

jbang https://github.com/maxandersen/jbang/blob/master/examples/helloworld.java
Tip

URL’s will follow redirects. In case you need to use it with sites with self-signed/non-trusted certificates you can if you trust the site use --insecure

Using .jsh for jshell

There are support to run .jsh via jshell. The advantage of jshell is that you do not need to have a class or static main method.

Classic jshell does not support passing in arguments nor system properties, jbang does.

In the case of .jsh files jbang injects a startup script that declares a String[] args which will contain any passed in arguments, and it sets any properties passed in with -Dkey=value as parameter to jbang.

Example:

System.out.println("Hello " + (args.length>0?args[0]:"World")); // (1)
System.setProperty("key", "value"); // (2)
/exit // (3)

  1. Line where args are accessible without previous declaration.

  2. System properties set when passed as -D arguments to jbang

  3. /exit is so the jshell app will exit. If you remove it jbang will launch into inter-active mode.

Init templates

To get started you can run jbang --init helloworld.java and a simple java class with a static main is generated.

Using jbang --init=cli helloworld.java you get a more complete Hello World CLI using picocli as dependencies.

Declare dependencies

If you want to write real scripts you will want to use some java libraries. To specify dependencies you use gradle-style locators. Below are examples for log4j.

Using //DEPS

//usr/bin/env jbang "$0" "$@" ; exit $?

//DEPS log4j:log4j:1.2.17 (1)

import static java.lang.System.out;

import org.apache.log4j.Logger;
import org.apache.log4j.BasicConfigurator;

import java.util.Arrays;

class classpath_example {

	static final Logger logger = Logger.getLogger(classpath_example.class);

	public static void main(String[] args) {
		BasicConfigurator.configure(); // (2)
		logger.info("Welcome to jbang");

		Arrays.asList(args).forEach(arg -> logger.warn("arg: " + arg));
		logger.info("Hello from Java!");
	}
}

  1. //DEPS has to be start of line and can be one or more space separated dependencies.

  2. Minimal logging setup - required by log4j.

Now when you run this the first time with no existing dependencies installed you should get an output like this:

$ ./classpath_example.java
[jbang] Resolving dependencies...
[jbang]     Resolving log4j:log4j:1.2.17...Done
[jbang] Dependencies resolved
0 [main] INFO classpath_example  - Welcome to jbang
1 [main] INFO classpath_example  - Hello from Java!

Repositories

By default jbang uses jcenter[https://jcenter.bintray.com/] as its repository as it is a superset of Maven Central and supposedly should be faster.

If that is not sufficient for you or need some custom repo you can use //REPOS id=repourl to state which repository URL to use.

For ease of use there are also a few shorthands to use popular commonly available repositories.

Short name

Description

mavencentral

Maven Central

jcenter

https://jcenter.bintray.com/

google

https://maven.google.com/

Following example enables use of Maven Central and add a custom acme repository.

//REPOS mavenCentral,acme=https://maven.acme.local/maven
Warning

If you add any //REPOS lines jbang will no longer consult jcenter thus you need to explicitly add it if needed.
Tip

For secure authentication jbang will honor ~/.m2/settings-security.xml for configuring username/passwords.

Using @Grab

There is also support for using Groovy lang style @Grab syntax.

//usr/bin/env jbang "$0" "$@" ; exit $?

import static java.lang.System.out;

import org.apache.log4j.Logger;
import org.apache.log4j.BasicConfigurator;

import java.util.Arrays;

import groovy.lang.Grab; // (1)
import groovy.lang.Grapes;
import groovy.lang.GrabResolver;

@GrabResolver("mavenCentral") // (2)
@GrabResolver(name='acme', root='https://maven.acme.local/maven')
@Grapes({ // (3)
		@Grab(group="org.codehaus.groovy", module="groovy", version="2.5.8"), // (4)
		@Grab(module = "log4j", group = "log4j", version = "1.2.17")
})
class classpath_example {

	static final Logger logger = Logger.getLogger(classpath_example.class);

	public static void main(String[] args) {
		BasicConfigurator.configure();
		Arrays.asList(args).forEach(arg -> out.println(arg));
	}
}

  1. Import needed to make the compiler be okey with @Grab annotation.

  2. Using GrabResolver to enable mavenCentral and custom acme repository

  3. In Groovy you normally put @Grab on import statements. That is not allowed in Java thus when having multiple imports you need to put them in a @Grapes annotation first.

  4. jbang will grab any @Grab annotation and assume it is declaring dependencies.

System properties and Environment variables

In dependencies you can refer to environment and system properties to parameterize the dependencies. It uses the format ${[env.]propertyname:<defaultvalue>}.

Furthermore to align with properties commonly used to make dependency resolution portable jbang exposes properties similar to what the os-maven-plugin does. Plus for ease of use for javafx dependencies it also setup a property named ${os.detected.jfxname}.

Examples:

${env.USER} = 'max'
${os.name} = 'Mac OS X'
${non.existing.key:empty} = 'empty'
${os.detected.jfxname} = 'mac'

This can be put to use in //DEPS like so:

//DEPS org.openjfx:javafx-graphics:11.0.2:${os.detected.jfxname}

Here we use the properties to avoid hardcoding your script to a specific operating system.

JavaFX

If jbang detects you have a javafx- dependency in your list of dependencies it will if you java command supports Java modules automatically set the necessary --module-path and --add-modules.

See examples/jfx.java and examples/jfxtiles.java for examples of this.

Extension-less/non-java files for cli-plugins

You can use jbang to write plugins for cli’s like kubectl, git, etc. They expect their plugins to be named like <cmd>-<plugin>, i.e. kubectl-myplugin.

Furthermore some of them, particularly kubectl currently require the file to start with #! otherwise you get a excc format error.

jbang has a bit of auto-magic to help in both cases.

kebab casing of filename

jbang lets you name your file without a .java or .jsh extension, such as kubectl-myplugin. jbang will in this case copy the file to a temporary directory using kebab-case to map the name to a proper java class name.

i.e. If you make a file called kubectl-myplugin then jbang will assume the actual class name to launch will be KubectlMyPlugin.

Note, similar is done when using --edit, here the symbolic link will be made so the IDE will treat it as regular camel cased java class.

Note: If you do not follow this naming pattern you will get a compile error as javac expect the public class name are equal to the filename.

she-bang auto-stripped

For extension less scripts, you can put #!' header in beginning to let apps recognize it is to be treated as a script. To avoid issues when compiling `jbang will remove that line before compiling.

For now this is required for kubectl plugin but not git. Issue opened on this limitation.

Editing

You can edit your script in your IDE by using jbang --edit helloworld.java. This will generate a project in a temporary location with symbolic links to your script and output the generated folder name. The easiest way to use that is to use it in a call to your IDE:

code `jbang --edit helloworld.java`

If you add additional dependencies to your file just re-run the edit command and the relevant files will be regenerated with the updated dependencies.

Note
 On Windows you might need elevated priviliges to create symbolic links. If you don’t have permissions then the --edit option will result in an error. To use it enable symbolic links for your user or run your shell/terminal as administrator to have this feature working.

Live Editing

You can also use --edit-live[=editor] and jbang will launch your editor while watching for file changes and regenerate the temporary project to pick up changes in dependencies.

The editor used will be what is specified as argument to --edit-live or default to $JBANG_EDITOR, $VISUAL or $EDITOR in that order.

IDE and Editor support

The --edit feature been tested with the following IDE’s:

Table 1. IDE’s and Editors tested with jbang

50 Visual Studio Code

50 Eclipse

50 IntelliJ Idea

50 Apache NetBeans

50 Neovim w/ spacevim Java

50 Emacs w/ Spacemacs Java

The --edit feature works with various IDE’s - it generates a build.gradle to use with IDE’s that understands Gradle directly. For speed and consistency jbang also generates IDE specific settings.

Currently launchers and project files are generated for Eclipse and vscode. Intellij just reads build.gradle for now thus to run/debug you will need to manually set it up.

Debugging

When running .java scripts with jbang you can pass the --debug-flag and the script will enable debug, suspend the execution and wait until you connect a debugger to port 4004.

jbang --debug helloworld.java
Listening for transport dt_socket at address: 4004

You can change the debug port by passing in a number to the debug argument, i.e. --debug=4321.

Note
 Be sure to put a breakpoint in your IDE/debugger before you connect to make the debugger actually stop when you need it.

Debug jbang it self

java itself will add JAVA_TOOL_OPTIONS which will apply to jbang too.

For finer and more explicit control the scripts for jbang will add JBANG_JAVA_OPTIONS to the call to jbang itself. Thus if you want to enable debug or other details for jbang set that environment variable.

java and javac Options

If you want to tweak memory settings or enable preview features you can setup the necessary options using //JAVA_OPTS and //COMPILER_OPTS as in the following example using Java 14 experimental record feature:

//usr/bin/env jbang "$0" "$@" ; exit $?
//JAVAC_OPTIONS --enable-preview -source 14 (1)
//JAVA_OPTIONS --enable-preview // (2)

import static java.lang.System.*;

public class records {

    record Point(int x, int y) {}

    public static void main(String[] args) {
        var p = new Point(2,4);
        out.println(p);
    }
}

Since Java 9 JDK_JAVA_OPTIONS and JDK_JAVAC_OPTIONS are also picked up by the Java runtime and compiler automatically.

For Java 8 and if you want to set explicilty only for jbang you can also add flags by setting JBANG_JAVA_OPTIONS and JBANG_JAVAC_OPTIONS respectively.

JAVA_HOME vs PATH

If JAVA_HOME environment variable is set that will be used over what is in the path.

Allows you to expliclity control what JDK you are using for jbang, i.e.

JAVA_HOME=~/sdkman/candidates/java/14.ea.302-open/bin/java jbang examples/records.java

This will use Java 14 from sdkman no matter what your PATH or JAVA_HOME contains in the shell environment.

Bash/Zsh auto-completion

If you are using bash or zsh in your terminal you can get auto-completion by running the following:

source <(jbang --completion)

Caching

In previous versions of jbang Java 10+ direct launch of .java was used, but since v0.6 jbang works with Java 8 and thus it needs to do a separate compile step. Besides now working with Java 8 it also allow us to cache the compile step and thus launch faster on consecutive runs.

The caching goes to ~/.jbang by default, you can run jbang --clear-cache to remove all cache data from this folder.

FAQ

  1. Why the name j’bang?

    I was reading up on how to use the new shebang (#!) feature support in Java 10 and came up with the idea of port kscript to Java and needed a name. From there came j’bang which is a "bad" spelling of how shebang is pronounced in french.

  2. Why use gradle resource locators rather than ?

    kscript used it and it’s nice as it is a one-liner and easily parsable.

  3. Why use of gradle resource locators rather than ?

    kscript used it and its nice as it is a one-liner and easily parsable.

  4. How does this compare to ?

    After doing jbang I’ve learned about similar projects and thought it would be nice with some comparison;

    jgo: an alternative way to launch jars using maven coordinates. Implemented in python, depends on Java and Maven to be available. Not really for scripting but a novel way to launch java apps already packaged as a maven dependency.

  5. Why would I use Java to write scripts ? Java sucks for that…​ Use gradle, kotlin, scala, etc. instead!

    Well, does it really suck ? With Java 8 streams, static imports and greatly improved standard java libraries it is very close to what kscript and grape look like. With the following advantages:

    • works with plain Java without installing additional compiler/build tools

    • all IDE’s support editing .java files very well, content assist etc.

    • great debugging

      And to be honest I built jbang just to see if I could and get my Java skills refreshed for the newer features in the language. Use it at your own risk :)

  6. Why not use normal shebang(#!) in the header ?

    You can use normal shebang (#!/usr/bin/env jbang) and Java 10+ will actually work with it from the command line. Not recommended though as many tools and especially IDE’s will start complaining about syntax errors as they don’t ignore the first line in this case.

    By using the // form it is treated as both a bash/shell file AND a valid java file and thus works everywhere a java file will work.

    Its worth noting that Go uses a similar approach which is also where I learned it from.

Thanks

jbang was heavily inspired by how kscript by Holger Brand works.

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Unleash the power of Java for shell scripting

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