Reactive ReactJS: improving data flow using reactive streams

Many people that use ReactJS as their renderer are using some kind of the Flux architecture to store data, react to actions and notify components about changes. After a University project involving Scala and RxJava, I wanted to use these ideas together with ReactJS views. Besides that I found two things missing in the Flux architecture:

  1. composing different kinds of data easily
  2. interaction with the server

Of course there are ways to solve this, but perhaps reactive streams can help ease these shortcomings.

ReactJS in Java Hello World

ReactJS is becoming a hugely popular solution for building complex Web Applications. It is so nice to use because it really resembles building applications like you used to in the good old PHP days a long time ago, where you construct the HTML once on the server, and be done with it. Don't think about changing state over time, just render once, and refresh once the data changed.

ReactJS runs entirely in the Browser. You give the components data, and it constructs the DOM for you. This means that your initial page can be as simple as:

<head><title>ReactJS Page</title></head>
<body><div id="app"></div></body>
React.render(MyComponent(), document.getElementById("app"));

Considering the first principle of Guillermo Rauch's 7 Principles of Rich Web Applictions:

"Server rendered pages are not optional"

And the tl;DR

"Server rendering is not about SEO, it's about performance. Consider the additional roundtrips to get scripts, styles, and subsequent API requests. In the future, considering HTTP 2.0 PUSH of resources."

So we need to pre-render the DOM on the server too.

Fortunately this is really easy with ReactJS, using the React.renderToString function. It's exactly like the React.render function, except rendering it to in a DOM node, it returns a string. The HTML in that string contains data-react-* attributes, so if you React.render on the client-side again, it picks up the DOM that is already there and only applies the actual changes. It takes the string as initial DOM state.

The thing with ReactJS, however, or with any client-side rendering engine, is that it's written in JavaScript. That makes sense because usually it's used on the client side. With NodeJS we have this amazing server-side runtime that can run JavaScript on the server. So the usual option would be to use NodeJS to run ReactJS to generate the server-side HTML.

The downside of NodeJS is that not every server configuration has NodeJS easily available. For example a lot of servers use Ruby, Python, Java or PHP.

There are two options: run NodeJS as a "render service" or use an embedded JavaScript runtime which is possible with Java.

NodeJS as local Render Service

One option would be to run a NodeJS process on your server, and use an internal port or Unix Socket to render the ReactJS components with NodeJS and use the result in the original stack.

var http = require('http');
var parse = require('url').parse;
var React = require('react');
var MyComponent = React.createClass({
render: function() {
return React.DOM.div(null, this.props.text);
var server = http.createServer(function(req, res) {
var text = parse(req.url, true).query.text || 'hello world';
var html = React.renderToString(React.createFactory(MyComponent)({text: text}));

And then your original code, for example your Python 3 server, can use this very simple snippet to render the ReactJS component with NodeJS as service:

import http.client
conn = http.client.HTTPConnection("localhost:3000")
conn.request("GET", "/?text=bar")
r1 = conn.getresponse()
print(r1.status, r1.reason)

The downside of this approach is that you have to send your data from your program through a TCP port or a Unix Socket.

ReactJS with the Embedded JS Runtime in Java

Java has an embedded JavaScript runtime already for a long time. First there was Rhino, and now, since Java 8, there is Nashorn. In Java you have an entire JavaScript runtime at your disposal. The simple Nashorn Hello World looks something like this, directly taken from the Nashorn website at Oracle:

package sample1;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
public class Hello {
public static void main(String... args) throws Throwable {
ScriptEngineManager engineManager = new ScriptEngineManager();
ScriptEngine engine = engineManager.getEngineByName("nashorn");
engine.eval("function sum(a, b) { return a + b; }");
System.out.println(engine.eval("sum(1, 2);"));

As it turns out is is relatively easy to run ReactJS in Java, as shown here:

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
public class Test {
private ScriptEngine se;
// Constructor, sets up React and the Component
public Test() throws Throwable {
ScriptEngineManager sem = new ScriptEngineManager();
se = sem.getEngineByName("nashorn");
// React depends on the "global" variable
se.eval("var global = this");
// eval react.js
se.eval(new FileReader("node_modules/react/dist/react.js"));
// This would also be an external JS file
String component =
"var MyComponent = React.createClass({" +
" render: function() {" +
" return React.DOM.div(null, this.props.text)" +
" }" +
// Render the component, which can be called multiple times
public void render(String text) throws Throwable {
String render =
"React.renderToString(React.createFactory(MyComponent)({" +
// using JSONObject here would be cleaner obviosuly
" text: '" + text + "'" +
public static void main(String... args) throws Throwable {
Test test = new Test();
test.render("hello world");

As shown above, there are two approaches that make loading the initial page load, with pre-rendered ReactJS components on the Server super fast, as the browser has to do less, and you don't need an initial Ajax request for your initial data. The NodeJS solution is very flexible, as it can be used for any programming language, but it requires running a NodeJS process along your normal server. If you're using Java, or anything that runs on the JVM (Java, Scala, Clojure, JRuby, etc.) you can use the embedded version. Finally thanks to ReactJS, it perfectly consumes your server-generated DOM structure, so after updates, it just updates the difference, which makes it a very fluid and easy to program solution!