thirumurthi

Execute Java code like a script using JBang

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Thirumurthi S
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Table of contents

JBang

In this blog have explained the basic usage of JBang CLI command. With additional specification we will be able to run Java code without using any source code structure.

What is JBang?

  • JBang is a CLI tool which can run java code directly from a file.
  • With JBang CLI we can execute java code from file by passing it as a argument.
    • In order to execute the Java file by JBang CLI, the file shouls include special defintions within java comments, which is explained in the example.
  • JBang can also execute .jsh (java shell) file. The .jsh file is similar to .java file, but it doesn't require class definition or main method.
    • Any code that executes in JShell REPL editor, can be places in .jsh file.
  • JBang can also be used to install JDK's and list installed JDK's, using command jbang jdk list.

Install JBang

  • JBang CLI can be installed in different ways refer installation docs.
  • In my Windows machine I used chocolatey manager.

Execute first Java code in JBang

  • JBang CLI requires the environment to be defined in very first line of the java file, like below.
  • This is similar to shell shebang, where we use #!.
///usr/bin/env jbang "$0" "$@" ; exit $?
  • Save below code snippet as addTwoNumbers.java
///usr/bin/env jbang "$0" "$@" ; exit $?

class addTwoNumbers {
    public static void main(String[] args) {
        if(args.length==0) {
            System.out.println("No arguments passed!!");
        } else {
            try{
              int num1 = Integer.parseInt(args[0]);
              int num2 = Integer.parseInt(args[1]);
              System.out.println("Sum of "+num1+" + "+num2+" = "+(num1+num2));
             } catch (NumberFormatException ex){
                 System.out.println("ERROR:- only whole number is supported");
             }

        }
    }
}
JBang command to run Java file
> jbang addTwoNumbers.java 5 5
Sum of 5 + 5 = 10

JBang can resolve Maven dependencies

  • JBang CLI can resolve Maven or Gradle dependencies from the Java file.
  • JBang requires the dependencies to be specified in a specific format which should start with //DEPS and looks like below.
//DEPS groupId:artifactId:version
  • Defining Maven BOM dependencies in the Java file. Note, the suffix @pom, is required when defining the bom dependencies.
    • Points to note when using bom dependencies,
      • The bom definition should be specified before using the related dependencies.
      • Single Java file that is executed by JBang can only contain one bom definition.
//DEPS groupId:artifactId:version@pom
  • Example for using camel bom dependencies, not nee to specify the versions for the realted dependencies.
//DEPS org.apache.camel:camel-bom:3.20.1@pom
//DEPS org.apache.camel:camel-core
//DEPS org.apache.camel:camel-main

Sample code defining maven dependencies for JBang

  • The java code below uses camel dependencies. It simply generates random number every 2.5 seconds.

  • Save below code snippet in CamelDemo.java file.

///usr/bin/env jbang "$0" "$0" : exit $?
//DEPS org.apache.camel:camel-bom:3.20.1@pom
//DEPS org.apache.camel:camel-core
//DEPS org.apache.camel:camel-main
//DEPS org.apache.camel:camel-stream
//DEPS org.slf4j:slf4j-nop:2.0.6
//DEPS org.slf4j:slf4j-api:2.0.6

import org.apache.camel.*;
import org.apache.camel.builder.*;
import org.apache.camel.main.*;
import org.apache.camel.spi.*;
import static org.apache.camel.builder.PredicateBuilder.*;
import java.util.Random;

import static java.lang.System.*;

class CamelDemo{

    public static void main(String ... args) throws Exception{
        out.println("Starting camel route...");
        setProperty("org.slf4j.simpleLogger.logFile", "System.out");
        Random random = new Random();
        Main main = new Main();

        main.configure().addRoutesBuilder(new RouteBuilder(){
            public void configure() throws Exception{
                from("timer:hello?period=2500")
               .process(exchange -> exchange.getIn().setBody(random.nextInt(500)))
               .setBody(simple("rNum: = ${body}"))
               .to("stream:out");
            }
         });
        main.run();
    }
}
Output of JBang CLI execution
  • To execute the java file using jbang use the command jbang CamelDemo.java
  • Initial run will download the defined jar dependencies and output will look like below,
  • We can use the -verbose switch to print additional logs of jbanglike, jbang --verbose CamelDemo.java
> jbang CamelDemo.java
[jbang] Resolving dependencies...
[jbang]    org.apache.camel:camel-bom:3.20.1@pom
[jbang]    org.apache.camel:camel-core:3.20.1
[jbang]    org.apache.camel:camel-main:3.20.1
[jbang]    org.apache.camel:camel-stream:3.20.1
[jbang]    org.slf4j:slf4j-nop:2.0.6
[jbang]    org.slf4j:slf4j-api:2.0.6
[jbang] Dependencies resolved
[jbang] Building jar...
Starting camel route...
rNum: = 482
rNum: = 289

image

Use JBang in Docker to run Java file

# in windows when attaching the volume better to use the complete path of the folder where CamelDemo.java exists
docker run -v C:\\simple-camel\\:/ws -w/ws -ti jbangdev/jbang-action --verbose CamelDemo.java

Note: Based on the JBang documentation, the command might looke like below, but note pwd in volume didn't work in windows

docker run -v `pwd`:/ws -w/ws -ti jbangdev/jbang-action --verbose CamelDemo.java

Use JBang to execute Java code directly using URL

  • We can use JBang CLI to execute the java code directly from the internet as well 
> jbang https://github.com/jbangdev/jbang-examples/blob/main/examples/helloworld.java

image

Consideration

  • There are certain limitation in using JBang CLI, like it can't be used for complex Java programs.
  • It can refer mulitple java file under pacakges, refer documentation
  • To use JBang in production totally depends on the usage, it can be used for small automation process but not for any complex code still this community is growing.
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