Using Java for scripting, with Jbang

j’bang, created by Max Andersen, enables you to use java for scripting. It supports Java 8 and onwards. It supports dependency resolution, and IDE editing in addition to many other things. Max has introduced it very well on his blog here.

To install jbang, I used sdkman and ran - sdk install jbang. I tried the the hello world example and it worked for me. Next obvious thing was to find something more interesting example to try.

I use Cloudflare for managing my domains. I have a shell script that does a Dynamic DNS update for my domain. This task requires accessing Cloudflare APIs and perform the update. This is a perfect example to create a java based script instead of a shell script.

For calling Cloudflare APIs, we need three configuration elements -

Once I have these three values, I simply created a CloudflareDDNSUpdate.java class to write the logic.

CloudflareDDNSUpdate.java source:

This class requires a path to cloudflare configuration properties file that contained these three properties -

With the java class and configuration file, we are ready to run it with j’bang. First three lines of the file has a special meaning for j’bang processing.

First line //usr/bin/env jbang "$0" "$@" ; exit $? allows this file to be run as a script on shell.

Second and third lines starting with //DEPS …​ declares the depenencies that j’bang then resolves.

Run jbang CloudflareDDNSUpdate.java ./cf.properties for executing the file.

So, j’bang not only resolved our dependencies but also successfully executed our java file.

You can also run java file from the gist or github source. For running this file from gist, get the raw file url of gist and run below command.

jbang https://gist.githubusercontent.com/manikmagar/f5a21d216ac6a5ccc7ff7e329cae382a/raw/9aa3a91985de51a1a71ba215a069d4ae32902352/CloudflareDDNSUpdate.java ./cf.properties

But, How do we make it executable?

Change permissions of CloudflareDDNSUpdate.java to make it executable and re-run without jbang command. In this case, the first line of the source helps to run this file as expected.

Woohoo! Java as an executable script and updated DNS!

This is definitely a good start and reason for continuing the "java as a scripting" exploration. As far as DDNS update is concerned, we could easily schedule that as a repeating task using cron jobs or other mechanism.

Have any thoughts, issues about j’bang? Don’t forget to let Max know or open an issue.

Have any thoughts about this article or want to share what you did with j’bang? Feel free to write in the comments below or share on twitter.