15. How do you use the new Nashorn JavaScript engine introduced in Java 8 for scripting?

Basic

15. How do you use the new Nashorn JavaScript engine introduced in Java 8 for scripting?

Overview

Nashorn, introduced in Java 8, is a JavaScript engine that allows Java applications to execute JavaScript code. It replaced Rhino, the previous JavaScript engine, offering improved performance and Java interoperability. Understanding Nashorn is crucial for developers looking to integrate JavaScript into Java applications, whether for server-side scripting, client-side GUI application development, or any other purpose where combining these two languages can be beneficial.

Key Concepts

  1. ScriptEngine API: The primary Java interface for interacting with Nashorn to evaluate JavaScript code.
  2. Invocable Interface: Enables calling functions defined in JavaScript scripts from Java code.
  3. Interoperability: Nashorn allows seamless integration between Java and JavaScript, including calling Java classes from JavaScript and vice versa.

Common Interview Questions

Basic Level

  1. How can you execute a JavaScript file from a Java application using Nashorn?
  2. What's the basic method to evaluate a JavaScript expression using Nashorn?

Intermediate Level

  1. How do you call a JavaScript function from Java code using Nashorn?

Advanced Level

  1. Discuss the interoperability between Java and JavaScript in Nashorn, including calling Java from JavaScript.

Detailed Answers

1. How can you execute a JavaScript file from a Java application using Nashorn?

Answer: To execute a JavaScript file from Java using Nashorn, you can use the ScriptEngine interface from the javax.script package. First, obtain an instance of Nashorn's ScriptEngine, and then use its eval method to execute a JavaScript file.

Key Points:
- Obtain a ScriptEngine instance specific to Nashorn.
- Use the eval method with a FileReader to execute the JavaScript file.
- Handle potential exceptions, such as FileNotFoundException and ScriptException.

Example:

// NOTE: Example code provided in Java for relevance to Nashorn
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import java.io.FileReader;
import java.io.FileNotFoundException;

public class NashornExample {
    public static void main(String[] args) {
        // Get the Nashorn script engine
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");

        try {
            // Execute the JavaScript file
            engine.eval(new FileReader("script.js"));
        } catch (ScriptException | FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}

2. What's the basic method to evaluate a JavaScript expression using Nashorn?

Answer: To evaluate a JavaScript expression, use the eval method of the ScriptEngine instance. This method parses and executes the JavaScript code provided as a string.

Key Points:
- The eval method is used for evaluating JavaScript code represented as a string.
- It returns an Object that can be cast to an appropriate type depending on the JavaScript expression's result.
- Handle ScriptException to deal with issues in the execution of the JavaScript code.

Example:

// NOTE: Example code provided in Java for relevance to Nashorn
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class EvalExample {
    public static void main(String[] args) {
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");

        try {
            // Evaluate a JavaScript expression
            Object result = engine.eval("10 + 2");
            System.out.println(result); // Outputs 12
        } catch (ScriptException e) {
            e.printStackTrace();
        }
    }
}

3. How do you call a JavaScript function from Java code using Nashorn?

Answer: To call a JavaScript function from Java, you can use the Invocable interface. First, the JavaScript code is evaluated to define the function. Then, the ScriptEngine instance is cast to Invocable, and its invokeFunction method is used to call the JavaScript function.

Key Points:
- Define functions in the JavaScript code before attempting to invoke them.
- Cast the ScriptEngine to Invocable to access the invokeFunction method.
- The invokeFunction method requires the function name and any parameters it needs.

Example:

// NOTE: Example code provided in Java for relevance to Nashorn
import javax.script.Invocable;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class InvokeFunctionExample {
    public static void main(String[] args) {
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");

        try {
            // Define a function in JavaScript
            engine.eval("function sum(a, b) { return a + b; }");

            // Call the defined JavaScript function from Java
            Invocable invocable = (Invocable) engine;
            Object result = invocable.invokeFunction("sum", 10, 5);

            System.out.println(result); // Outputs 15
        } catch (ScriptException | NoSuchMethodException e) {
            e.printStackTrace();
        }
    }
}

4. Discuss the interoperability between Java and JavaScript in Nashorn, including calling Java from JavaScript.

Answer: Nashorn provides excellent interoperability between Java and JavaScript. Java classes and methods can be accessed from JavaScript, and JavaScript functions can be invoked from Java. This interoperability allows for the creation of hybrid applications leveraging both languages' strengths.

Key Points:
- Java objects can be created and manipulated within JavaScript.
- Static Java methods can be called directly from JavaScript.
- JavaScript functions can be called from Java, allowing for two-way interaction.

Example:

// NOTE: Example code provided in Java for relevance to Nashorn
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class InteropExample {
    public static void main(String[] args) {
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");

        try {
            // Accessing Java from JavaScript
            engine.eval("var ArrayList = Java.type('java.util.ArrayList');" +
                        "var list = new ArrayList();" +
                        "list.add('Hello');" +
                        "list.add('World');" +
                        "print(list.get(0));" + // Outputs Hello
                        "print(list.get(1));");  // Outputs World
        } catch (ScriptException e) {
            e.printStackTrace();
        }
    }
}

In these examples, note that the actual Java code is used to demonstrate the concepts because Nashorn is a Java feature, and the questions are specific to Java 8 Interview Questions.

logo