Main.java

package repl;

import jdk.nashorn.internal.runtime.ParserException;
import jnr.ffi.LibraryLoader;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.Collections;
import java.util.Spliterators;
import java.util.stream.Collectors;
import javax.script.ScriptContext;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class Main
{
    public static void main( String[] args ) throws Exception
    {
        ScriptEngineManager engineManager = new ScriptEngineManager();
        ScriptEngine js = engineManager.getEngineByExtension( "js" );
        BufferedReader in = new BufferedReader( new InputStreamReader( System.in ) );
        PrintWriter out = new PrintWriter( System.out, true );
        PrintWriter err = new PrintWriter( System.err, true );
        ScriptContext context = js.getContext();
        context.setReader( in );
        context.setWriter( out );
        context.setErrorWriter( err );
        loadScript( js, "nashorn:mozilla_compat.js" );
        defineClass( js, System.class );
        defineClass( js, LibraryLoader.class );
        defineClass( js, Files.class );
        defineClass( js, Paths.class );
        defineClass( js, Arrays.class );
        defineClass( js, Collections.class );
        defineClass( js, Spliterators.class );
        defineClass( js, Collectors.class );

        String pstart = "js> ";
        String pcont  = "  > ";
        String source = prompt( out, in, pstart );
        while ( source != null )
        {
            try
            {
                Object ret = js.eval( source, context );
                if ( ret != null )
                {
                    out.write( String.valueOf( ret ) );
                    out.write( '\n' );
                }
            }
            catch ( ScriptException e )
            {
                if ( e.getCause() instanceof ParserException && !source.endsWith( ";;" ) )
                {
                    source += "\n" + prompt( out, in, pcont );
                    continue;
                }
                e.printStackTrace( err );
            }
            catch ( Throwable e )
            {
                e.printStackTrace( err );
            }
            err.flush();
            out.flush();
            source = prompt( out, in, pstart );
        }
    }
    private static void loadScript( ScriptEngine js, String script ) throws ScriptException
    {
        evalPrelude( js, "load(\"" + script + "\");" );
    }

    private static void defineClass( ScriptEngine js, Class<?> cls ) throws ScriptException
    {
        evalPrelude( js, "var " + cls.getSimpleName() + " = Java.type(\"" + cls.getName() + "\");" );
    }

    private static void evalPrelude( ScriptEngine js, String script ) throws ScriptException
    {
        System.err.println( script );
        js.eval( script );
    }

    private static String prompt( PrintWriter out, BufferedReader in, String pstr ) throws IOException
    {
        out.write( pstr );
        out.flush();
        return in.readLine();
    }
}