Run a Java Function through NodeJS

You can execute java commands from nodejs. You can run exec commands via expressjs router. For the best case scenario, I would create another API with Java and make request to that endpoint (microservices). But if you don't want to do that you can try this code sample;

const express = require('express')
const app = express()
const port = 3000
const exec = require('child_process').exec
app.get('/', (req, res) => {
  const child = exec('/usr/bin/java ~/example.jar', => (error, stdout, stderr) {
    if (err) {
        console.error(err);
        res.json({error: err, status: 500, errorOutput: stderr})
        return
    }
    // it is important to have json structure in your output or you need to create a logic which parse the output
    res.json(stdout)
  })
})

app.listen(port, () => console.log(`Example app listening on port ${port}!`)) 

You can use Node.js 'exec' to call an external Java program like this:

Javascript program

const exec = require('child_process').exec;

// Number 7 is a command line argument to pass to the Java program
exec('java MyJavaApplication 7', function callback(error, stdout, stderr){
    console.log(stdout);
});

Java program

public class MyJavaApplication {
    public static void main(String[] args) {
        int input = Integer.parseInt(args[0]);
        int output = calculate(input);
        System.out.println(Integer.toString(output));
    }

    private static int calculate(int input) {
        // Do some complex calculation
        return input * input;
    }
}

On Node.js you can capture whatever the Java program wrote to the standard output. Depending on how complex your input is you might want to pass a file name as an argument to the Java program. That file could have the input content in a JSON format, for instance.

You can use a library called a "bridgeService". This exposes java methods as REST objects. You can deploy it locally or on a server.

For a method like "a.b.c.d.E.f(String argument)"

The payload will be something like:

{
  "callContent": {
    "<ID>": {
      "class": "a.b.c.d.E",
      "method": "f",
      "args": [
        "argument"
      ]
    }
  }
}

For more information on the bridgeService you can refer to https://github.com/adobe/bridgeService