Using JBang to build Java Scripts

I have been trying to find a great solution for writing scripts for at least 5 years now. The latest addition was deno and typescript that I use to build the blog post overview for this site. In the past I tried ammonite with scala, since I am quite heavily invested in the JVM eco system.

But honestly nothing really stuck. I still use python or deno the most. And while python is great as an interactive calculator (though the lack of static typing bothers me – mypy exists, but still feels like a bit of a bandaid) I do not like to write larger programs with it. Deno is better in this regard but still has a quite immature eco system. Since I use a lot of Java in my professional life I wanted to explore if it would be a viable option. In the past Java was a bit too unwieldly as a scripting language, but since Java released versions 7 (new IO API), 8 (Lambdas + Streams) and 11 (var – local type inference) I wondered if this changed and Java could be used for scripting?

Since Java 9 there is JShell which is a REPL like environment. A great first step in the right direction, but it is still missing an important ingredient: dependency management. Another point of critisicm would be missing or incomplete IDE integration.

After doing some more research I found JBang. It tickes most of my boxes:

• Shebang support via // instead of #!
• Dependency managment via //DEPS or @Grab annotations
• Editor integration by temporarily creating a gradle project
• REPL like usage via jbang --interactive (uses JShell internally)
• (not that important but cool) GraalVM native-image support to create native executables

Now I can finally use my favorite tech stack to build scripts that start a web server.

You can get started by installing JBang (e.g. via brew) and initializing a small example:

jbang init HelloServer.java

Now for the more interesting part the content of HelloServer.java:

///usr/bin/env jbang "$0" "$@" ; exit $?
//DEPS io.javalin:javalin:4.1.1
//DEPS org.slf4j:slf4j-api:1.8.0-beta4
//DEPS org.slf4j:slf4j-simple:1.8.0-beta4

import io.javalin.Javalin;

public class HelloServer {

    public static void main(String... args) {
        var app = Javalin.create().start(7777);
        app.get("/", ctx -> ctx.result("Hello World"));
    }
}

The first line is the shebang (while still being valid java). The next three lines declare dependencies (Javalin as a webservice + SLF4j-simple for logging – no log4j necessary ;) ). What follows next is just a regular Java program to start a webserver on port 7777. To have proper IDE support the java file name needs to match the class name, though JBang itself does not care about that. Editing works via jbang edit --open=code HelloServer.java in case of VS Code. That creates a temp directory with a gradle project for the IDE. While this works great I would still prefer to edit the file directly.

[main] INFO io.javalin.Javalin -
       __                      __ _            __ __
      / /____ _ _   __ ____ _ / /(_)____      / // /
 __  / // __ `/| | / // __ `// // // __ \    / // /_
/ /_/ // /_/ / | |/ // /_/ // // // / / /   /__  __/
\____/ \__,_/  |___/ \__,_//_//_//_/ /_/      /_/

          https://javalin.io/documentation

[main] INFO org.eclipse.jetty.util.log - Logging initialized @179ms to org.eclipse.jetty.util.log.Slf4jLog
[main] INFO io.javalin.Javalin - Starting Javalin ...
[main] INFO io.javalin.Javalin - You are running Javalin 4.1.1 (released October 10, 2021. Your Javalin version is 72 days old. Consider upgrading!).
[main] INFO io.javalin.Javalin - Listening on http://localhost:7777/
[main] INFO io.javalin.Javalin - Javalin started in 115ms \o/