jbang - use Java for scripting

To use jbang you as a minimum need to have Java 11+ available.

To install both java and jbang we recommend sdkman

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

sdk install java <b>(3)</b>

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

brew install maxandersen/tap/jbang

brew upgrade maxandersen/tap/jbang

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 :

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

class helloworld { <b>(2)</b>

    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 IDE’s 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 mark it executable and just run it directly:

chmod +x helloworld.java
./helloworld jbang!

You can use http(s):/ and file:/ url’s for input too:

jbang https://raw.githubusercontent.com/maxandersen/jbang/master/examples/helloworld.java

If you want to write real scripts you will want to use some java libraries. To specify dependencies you use gradle-style locators. Here is an example using log4j

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

//DEPS log4j:log4j:1.2.17 <b>(1)</b>

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(); <b>(2)</b>
		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!

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