Try a cloneNode followed by an adoptNode to use a child element as the root element of a new Document.

    String xml = "<hello><world /></hello>";
    try {
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = dbf.newDocumentBuilder();

        Document document0 = db.parse(new InputSource(new StringReader(xml)));
        Document document1 = db.newDocument();


        NodeList nodeList = document0.getDocumentElement().getChildNodes();
        document1.appendChild(document1.adoptNode(nodeList.item(0).cloneNode(true)));

        // Serialize the two documents to check that the copy was correct.
        ByteArrayOutputStream output = new ByteArrayOutputStream();
        Transformer transformer = TransformerFactory.newInstance().newTransformer();
        transformer.transform(new DOMSource(document0), new StreamResult(output));
        System.out.println(output.toString("UTF-8"));

        output.reset();
        transformer.transform(new DOMSource(document1), new StreamResult(output));
        System.out.println(output.toString("UTF-8"));
    } catch (TransformerException e) {
    } catch (IOException e) {
    } catch (SAXException e) {
    } catch (ParserConfigurationException e) {
    }

What I did was basically follow the OP's answer, but I had beforehand to create the .jar file. To do so, I just prepared my custom class in test.java:

import java.lang.System;

public class Test {
    public static void main(String[] args) {
        System.out.println("Hello, world!");
    }
}

And then

javac Test.java
jar cf test.jar Test.class

Now you should see the .jar created. From this point, you are able to access it from Javascript:

let java= require('java');
java.classpath.push('test.jar');

let test = java.import("Test");
test.main(null) // should print "Hello, World!"

It may be better to just create a java server to do the computations and communicate with your node.js application over a messaging queue. Here is an example which shows how to do that - http://blog.james-carr.org/2010/09/09/rabbitmq-nodejs-and-java-goodness/

To better way to use 'node-java' package for inoking the java methods instead of node child process. 'node-java' package allows you to invoke java methods and use java classes within node js application.

For example: First create the java file with name TestJava.java.

public class TestJava {
    public String helloJava(String name) {
        return "Test, " + name + "!";
    }
}

Then compile the java file to generate the .class file. After the generating the class file then create the node.js script with name callJavaMethod.js.

// callJavaMethod.js
const java = require('java');

// Add the current directory to the classpath
java.classpath.push('.');

// Import the Java class
java.import('TestJava');

// Create an instance of the Java class
const testJava= new java.newInstanceSync('TestJava');

// Call the Java method
const result = testJava.helloJava('Java');

// Print the result
console.log(result);

after that you run the node js script and you will get the following answer

Test Java

Node copiedNode = doc1.importNode(element, true);

should be

Node copiedNode = doc2.importNode(element, true);

The node comes from doc1, and you want to import it into doc2. Not into doc1, where it already comes from.