Java Versions

jbang will by default use JAVA_HOME and if not available, check the PATH to locate the java executable to run the script with.

If your script requires a specific or minimal version of Java you can use //JAVA <version>(+).

If JBang finds a java executable using JAVA_HOME or PATH which satisfies the stated java version jbang will use it. If no such version is found it will automatically download and install it.

Examples:

//JAVA 11 will force use of Java 11.
//JAVA 13+ will require at least java 13. Java 13 or higher will be used.

In case no matching java is found jbang will fail.

You can always force running with specific version of java using --java command line option, i.e. jbang --java 8 hello.java

Managing JDKs

In the previous section it was mentioned that JBang will automatically download and install JDKs when necessary. You can use the jdk command to manage JDKs, for example you can run the following:

jbang jdk list

which will list all the JDKs that are currently installed by JBang.

It’s easy to install additional JDKs by running:

jbang jdk install 14

which will download and install JDK version 14 into JBang’s cache (~/.jbang/cache/jdks by default). The list of versions that are available for installation can be found here: https://adoptopenjdk.net/releases.html

The first JDK that gets installed by JBang will be set as the "default" JDK. This is from then on the JDK that will be used by JBang if no Java could be found on the system (meaning javac wasn’t found on the PATH and no JAVA_HOME is set). You can change the default JDK by running:

jbang jdk default 12

Running it without an argument will return the version of the JDK that is currently set as the default.

On Windows you might need elevated privileges to create symbolic links. If you don’t have permissions then running the above command 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.

When you uninstall a JDK by running:

jbang jdk uninstall 12

and that JDK was set as the default, JBang will set the next higher version JDK as the default. If no higher version is available it will select the next lower version.

Using managed JDKs yourself

Given the fact that JBang is able to easily download and install JDKs we thought that it might be a good option for our users to be able to access those JDKs for their own use instead of having to install yet another version themselves.

To make that easy we added a couple of useful commands. The first can be used to set retrieve to location where the JDK is installed:

jbang jdk home

This will return the path to the "default" JDK (by default ~/.jbang/currentjdk), if you want to know the location of a specific JDK you can pass the version as an argument: jbang jdk home 14. This command could be used by scripts to find a JDK to use to run a Java program for example (eg: JAVA_HOME=$(jbang jdk home).

For setting up your current command line environment there’s something simpler. You can run:

jbang jdk java-env

On Linux, Mac and AIX this will output something like:

export PATH="/home/user/.jbang/currentjdk/bin:$PATH"
export JAVA_HOME="/home/user/.jbang/currentjdk"
# Run this command to configure your shell:
# eval $(jbang jdk java-env)

And the output itself shows how to properly use it to configure your command line to use the JDK. In this case it’s by running:

eval $(jbang jdk java-env)

To do this by default for all shells you start simply add the above line to your ~/.bashrc file.

Unfortunately on Windows using CMD things are not as easy as is show by the output of jbang jdk java-env on that platform:

set PATH=C:\Users\user\.jbang\currentjdk\bin;%PATH%
set JAVA_HOME=C:\Users\user\.jbang\currentjdk
rem Copy & paste the above commands in your CMD window or add
rem them to your Environment Variables in the System Settings.

Instead of copying and pasting lines you could also redirect the output to a .bat file and execute that instead:

> jbang jdk java-env > setenv.bat
> setenv