nudge4j

Requirements

Java 8
Internet access
A modern browser (Chrome, Firefox, IE12)
Integration

copy/paste this code anywhere and restart your JVM (complete example)
// nudge4j:begin
(new java.util.function.Consumer<Object[]>() { public void accept(Object args[]) {
    try {
        javax.script.ScriptEngine engine = new javax.script.ScriptEngineManager().getEngineByName("JavaScript");
        engine.put("args", args);
        String p = "com.sun.net.httpserver.";
        Class<?> 
        HH = Class.forName(p+"HttpHandler"),
        HE = Class.forName(p+"HttpExchange"),
        HD = Class.forName(p+"Headers"), 
        HS = Class.forName(p+"HttpServer");
        java.lang.reflect.Method 
        m0 = HE.getMethod("getRequestURI"),
        m1 = HE.getMethod("getResponseHeaders"),
        m2 = HE.getMethod("sendResponseHeaders", int.class, long.class),
        m3 = HE.getMethod("getResponseBody"),
        m4 = HS.getMethod("create", java.net.InetSocketAddress.class, int.class),
        m5 = HS.getMethod("createContext", String.class, HH),
        m6 = HS.getMethod("setExecutor", java.util.concurrent.Executor.class),
        m7 = HS.getMethod("start"),
        m8 = HD.getMethod("set", String.class, String.class);
        Object server = m4.invoke(null, new java.net.InetSocketAddress((int)args[0]), 0);
        m5.invoke(server, "/", java.lang.reflect.Proxy.newProxyInstance(
            HH.getClassLoader(), 
            new Class[] { HH }, 
            new java.lang.reflect.InvocationHandler() {          
                java.nio.charset.Charset UTF8 = java.nio.charset.StandardCharsets.UTF_8;
                byte data[] = new byte[10000];
                java.util.function.Function<Object,String> stringify = (oj) -> 
                    '"'+(""+oj).replace("\\", "\\\\").
                                replace("\"", "\\\"").
                                replace("\n", "\\n").
                                replace("\b", "\\b").
                                replace("\t", "\\t").
                                replace("\r", "\\r").
                                replace("\f", "\\f") + '"';
                void send(Object httpExchange,byte array[],int max, String contentType) throws Exception {
                    m8.invoke(m1.invoke(httpExchange), "Content-Type",contentType);
                    m2.invoke(httpExchange, 200, max);
                    try (java.io.OutputStream os = (java.io.OutputStream) m3.invoke(httpExchange)) { 
                        os.write(array,0, max); 
                    }
                }
                public synchronized Object invoke(Object pxy, java.lang.reflect.Method m, Object[] args) throws Exception {
                    Object httpExchange = args[0]; 
                    String uri = m0.invoke(httpExchange).toString();
                    if (uri.startsWith("/js")) {
                        String query = ((java.net.URI) m0.invoke(httpExchange)).getQuery();
                        String id = '"'+query.substring(0, 10)+'"'; 
                        String code = query.substring(11);
                        Object result = null;
                        byte array[]; 
                        try {
                            result = engine.eval(code);
                            array = ("n4j.on("+id+",null,"+stringify.apply(result)+")").getBytes(UTF8);
                        } catch (Exception e) {
                            e.printStackTrace(new java.io.PrintStream(
                                (java.io.OutputStream)(result = new java.io.ByteArrayOutputStream())));
                            array = ("n4j.on("+id+","+stringify.apply(result)+",null)").getBytes(UTF8);
                        }
                        send(httpExchange,array,array.length,"application/javascript");
                        return null; 
                    }
                    String url = "https://lorenzoongithub.github.io/nudge4j/proxy"+uri;
                    java.net.HttpURLConnection c = (java.net.HttpURLConnection) new java.net.URL(url).openConnection();
                    c.setRequestMethod("GET");
                    int responseCode = c.getResponseCode();
                    if (responseCode != 200) {
                        m2.invoke(httpExchange,responseCode,-1);
                        return null; 
                    }
                    int count =0;
                    try (java.io.InputStream is = c.getInputStream()) {
                        for (int b=is.read();b!=-1;b=is.read()) data[count++] = (byte) b;
                    }
                    send(httpExchange,data, count, (
                         (uri.endsWith(".ico")) ? "image/x-icon" :
                         (uri.endsWith(".css")) ? "text/css" :
                         (uri.endsWith(".png")) ? "image/png" :  
                         (uri.endsWith(".js"))  ? "application/javascript" : 
                                                  "text/html"));
                    return null; 
                }
            }
        ));
        m6.invoke(server, new Object[] { null });
        m7.invoke(server);
        System.out.println("nudge4j serving on port:"+args[0]);
    } catch (Exception e) {
        throw new InternalError(e);
    }
}}).accept( new Object[] { 5050  }); 
// nudge4j:end
The way it works

The snippet of Java code starts a web server in your JVM allowing for JavaScript server side execution
You can expose Java objects as arguments. The first argument is the server's port
AJAX is used to post code to execute against the JVM (see nudge4j.js)
The web server's home page starts here https://lorenzoongithub.github.io/nudge4j/proxy/