A read-eval print loop in Java using Rhino.

REP.java

package com.comphenix.testing;

import java.io.PrintStream;
import java.util.Scanner;

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class REP {
  private boolean running = false;
    
    public static void main(String args[]) throws ScriptException {
        Test test = new Test();
        test.start();
    }
    
    private void start() {
        final ScriptEngineManager manager = new ScriptEngineManager();
        final ScriptEngine engine = manager.getEngineByName("js");
        
        final Scanner input = new Scanner(System.in);
        final PrintStream output = System.out;
        
        StringBuilder builder = new StringBuilder();
        int lines = 0;
        
        engine.put("program", this);
        
        // Start the loop
        running = true;
        
        while (running) {
            // Aquire a line
            builder.append(input.nextLine());
            
            if (evaluate(engine, output, builder.toString(), ++lines)) {
                // Current input has been completed
                lines = 0;
                builder.setLength(0);
            }
        }
        
        // Clean up
        input.close();
    }
    
    /**
     * Evaulate the given statements.
     * @param engine - the current engine.
     * @param output - output stream.
     * @param statements - statements to evaluate.
     * @param lines - number of statements.
     * @return TRUE if we successfully evaluated the statement, FALSE if we need more lines.
     */
    private boolean evaluate(ScriptEngine engine, PrintStream output, String statements, int lines) {
        try {
            output.println("> " + engine.eval(statements));
            return true;
            
        } catch (ScriptException e) {
            if (e.getLineNumber() != lines) {
                // An error ocured in the 
                e.printStackTrace(output);
                return true;
            }
        }
        return false;
    }
    
    public void exit() {
        running = false;
    }
}