Delegate some Java processing to JavaScript

This is a personal memo as a result of investigating "Can some Java processing be injected from the outside?"


When I looked it up, there was a ** JavaScript scripting engine (Nashorn) ** on Java, so I tried using it. Nashorn can be called from the Java SE 8 javax.script package.

Sample code

This is a sample code that delegates the filtering process of List to JavaScript.

public class Main {

    public static void main(String[] args) throws Exception {
	    List<Person> list = Arrays.asList(new Person("a", 1), new Person("b", 2), new Person("c", 3));
        //Delegate JavaScript filtering
		List<Person> resultList = -> eval(e)).collect(Collectors.toList());
		//Show list before filter
		System.out.println("Before filter: " + list);
		//Show filtered list
		System.out.println("After filtering: " + resultList);

	 *Evaluate JavaScript execution results
	 * @param person
	 * @return true/false
	public static boolean eval(Person person) {
    	boolean result = false;
		try {
			ScriptEngineManager manager = new ScriptEngineManager();
			//Create a JavaScript engine
			ScriptEngine engine = manager.getEngineByName("javascript");
			//Set Java object to JavaScript variable
			engine.put("person", person);
			//Read a JavaScript file from a file
			Reader script = new InputStreamReader(Main.class.getResourceAsStream("sample.js"));
			//Execute JavaScript
			result = Boolean.valueOf(true).equals(engine.eval(script));
		} catch(ScriptException ex) {
		return result;


A Java object that is an element in the list. We have defined a public getter so that we can access field variables from JavaScript.

public class Person {
    private String name;
    private Integer age;

    public Person(String name, Integer age) { = name;
        this.age = age;
    public String getName() {
        return name;
    public Integer getAge() {
        return age;
    public String toString() {
        return "{ name: " + + ", age: " + this.age + "}";

The following sample.js is processing the filter method on the Java side. I was also able to access the fields of the person object passed by Java.


//person is an element of the list passed from the Java side
//Access the name field and use it in the judgment formula === 'a' || === 'b';

Processing result

The processing result of the above sample code is as follows. From the evaluation result of sample.js, you can see that the contents of List have been filtered.

Before filter: [{ name: a, age: 1}, { name: b, age: 2}, { name: c, age: 3}]
After filtering: [{ name: a, age: 1}, { name: b, age: 2}]


If you use it like this case, For example, I found it convenient to create a list on the screen where the user can change the filtering conditions each time.


Below is a reference link. Basics of JavaScript engine Nashorn on Java OpenJDK wiki >> Nashorn

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