Running JavaScript in Java Runtime Environment (JRE) can be done using the `javax.script` package, which provides an interface between Java applications and scripting engines. This guide will walk you through the steps required to run JavaScript code using JRE.

Step 1: Set up your development environment
Make sure you have Java Development Kit (JDK) installed on your system. You can verify this by running the `java -version` command in the terminal, which will display the JDK version you have installed. If JDK is not installed, you can download and install it from the Oracle website (https://www.oracle.com/java/technologies/javase-jdk11-downloads.html).

Step 2: Choose a scripting engine
JRE supports various scripting engines such as JavaScript, Groovy, Python, and more. For running JavaScript, we’ll use the `Nashorn` engine, which is included with JRE 8 and earlier versions. Starting from JRE 11, the Nashorn engine has been removed. If you are using JRE 11 or later, you can use alternative engines like `GraalVM`.

Step 3: Write some JavaScript code
Create a new Java class where you’ll write your Java code to run the JavaScript. Start by including the necessary `javax.script` classes:
“`java
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
“`
Then, write a method that will contain the JavaScript code you want to run. For example, let’s run a simple JavaScript function:
“`java
public class JavaScriptRunner {
public static void main(String[] args) {
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName(“nashorn”);

String javascriptCode = “function greet(name) { return ‘Hello, ‘ + name; }”;

try {
engine.eval(javascriptCode);
String result = (String) engine.eval(“greet(‘John’)”);
System.out.println(result);
} catch (ScriptException ex) {
ex.printStackTrace();
}
}
}
“`
Here, we create an instance of `ScriptEngineManager` and retrieve the `Nashorn` engine. Then, we define a JavaScript function called `greet` that takes a name as a parameter and returns a greeting. We use `engine.eval()` to execute the JavaScript code.

Step 4: Run the Java code
Compile and run your Java code from the command line or your favorite IDE. If everything goes well, you should see the output: “Hello, John”.

Congratulations! You have successfully run JavaScript code using JRE. You can now explore the capabilities of the `javax.script` package and experiment with more complex JavaScript code.

Note: If you are using JRE 11 or later, you’ll need to use alternative scripting engines like `GraalVM`. The setup and usage might differ slightly, so be sure to check the respective documentation for the engine you choose.

Remember to always sanitize any JavaScript code that you execute to prevent security risks, and make sure you understand the implications of running untrusted code within your Java application.