Category Archives: Java

DIY Java & Kubernetes (Videos and Samples)

This year, on three occasions, I presented about Kubernetes and how you can use it on the Google Cloud Platform, or your own home-built cluster.

Kubernetes RaspberryPi cluster
Kubernetes RaspberryPi cluster


Two of these presentations were recorded, and are available on youtube. The presentations, alongside with the code samples explained below, can be a great quick-start guide for making Java microservices or apps with Spring Boot and deploying them on a Kubernetes cluster.

The code samples (a bit modified though) are available on github:
In order to smoothly migrate from a single app deployment and basics of running Kubernetes, to splitting the app into microservices and deploying a full distributed scenario, I made two implementations:
– kub-calublog is a monolithic Spring Boot application which works with in-memory database and represents a very simple blog platform.
– kub-calublog-ui and kub-calublog-service are ‘microserviced’ pieces of the same application which use HATEOAS RESTful calls between them.
– kub-calublog-deployment contain the scripts and resources for deploying the app or services on a cluster. /simple is for basic single app deployement with docker-like commands, while /kubernetes is for the reccomended declarative methods. /kubernetes-arm is just a showcase if a RaspberryPi ARM based deployment is to be done.

Things to do in order to get started:
– Sign up for a Google Container Engine free trial.
– Create a project, allocate machines (up to 4 shared will do)
– Install gcloud SDK on your machine
– Watch the video(s) and experiment on your own, having the samples for help. (make sure that you replace properly the docker.prefix places in both the pom.xml files and deployment scripts)

– jPrime ’16:
– Voxxed Days Belgrade ’16:

Other resources:
– Devoxx presentation done by Ray Tsang (@saturnism) and Arjen Wassink (@ArjenWassink):
– gcp-live-k8s-visualizer:
– Setting up Kubernetes on a RaspberryPi cluster:
– RaspberryPi Kubeternetes stack 3d design:

OpenJFX with RaspberryPi

JavaFX on the Raspberry Pi is a particularly nifty platform to use when you need a nice looking GUI on a regular monitor or a touch-screen. The platform used to ship along with JDK8 for ARM directly and was bundled (last I saw it still was) with out of the box Raspbian.

However, from update 33,  Oracle decided to drop the support for JavaFX on the ARM distribution of their JDK, and stopped shipping it within as well.

However the story doesn’t end here. This change just expedited my idea to give OpenJFX a try, after which I wished I did that way sooner.

My test bed was a RaspberryPi with PiTFT and a touchscreen adapted JavaFX application. Previously I had implemented a headless start of the JavaFX application with fbcp running in the background, and having all the parameters for the touchscreen set right in order to get a nice and correct projection of the FX framebuffer. With OpenJFX, this was no longer needed, cause it is directly supported.

In order to quickly get and use OpenJFX on Raspbian, follow these steps:
– Get and flash a fresh Raspbian image. Make sure you have java and javac present.
– Download a built OpenJFX package (OpenJFX 8u## stable for armv6hf). I used @chriswhocodes OpenJFX builds. There are others listed at the OpenJFX community builds page.
– Extract the contents of the zip file. Copy the /jre/lib folder contents somewhere in your project (I simply copied everything to the project root folder).
– (include the /ext/jfxrt.jar file in your classpath if you compile in an environment without JavaFX present in the JDK)

Finally, when executing your java application, pass the following arguments to the JVM:
-Djava.ext.dirs=dir/to/jfx/ext (I used .dirs=ext because I unpacked the lib contents in the project root)
-Dmonocle.screen.fb=/dev/fb1  (only if you use a touchscreen (like the PiTFT)
-Dprism.order=sw (again only for touchscreen, but I’m not really sure. If you experience problems with eventual hardware rendering, use this)

The outcome was pretty pleasant. The UI was looking good and it adapted just fine, although be careful with the dimensions and the general conditions (see tips here).  Also, I had no need to calibrate the screen, it was working correctly from the first run. And last but not least, having not to use fbcp in the background is a huge performance boost and resource saver.


Main screen with buttons LED light screen with color picker


DoorNFX: Touchscreen JavaFX 8 on Raspberry Pi

As of March 2014, Java8 is finally out there. Bunch of new features and improvements, not that they weren’t known previously, but good that they went official. The ones that I’m targeting with this blogpost are JavaFX, JDK8 on ARM devices, and their joint functionality.

The new JDK for ARM is targeted specifically for v6/v7 ARM HardFloat ABI devices running on Linux. The best and world-wide accepted example for this is the Raspberry Pi running on an OS like Raspbian. This JDK was around for some time with the early access program, so I had the chance to play around with it previously. However, for the example below, I’m using the official version.

JavaFX is, as the definition says, a set of graphics and media packages that enables developers to design, create, test, debug and deploy rich client applications that operate consistently across diverse platforms. In short, it’s a Java framework building Rich Internet or Desktop applications. Some of it’s features include:
– Pure Java API integrated in JavaSE: as from Java8, JavaFX is an integral part of the JRE and JDK. It’s API i in pure Java so it can be used any language that runs on the JVM.
– UI can be defined either programmatically or declaratively via FXML
– Interoperable with the old Swing
– All UI components can be styled with CSS
– New theme ‘Modena’ which makes the UI look very nice fora change
– General 3D features, hardware acceleration support
– WebView component which allows two-way interfacing (Java to JavaScript and vice-versa)
– Canvas and printing API, support for RichText
The easiest way to explore JavaFX is to play around with the Ensamble app on the Oracle web site.



So, as an example, I decided to make my NFC PN532 Java port to some usage and make some exact device out of it. My idea was to make a protected door access node which reads NFC Tags, prompts for a user code, authenticates it against some remote server and grants or declines access based on the output.

The core of the device is a RaspberryPi model B. The GPIO section has more than enough options for connecting multiple external devices. For the device, I’m using two such devices which are made specifically for the RaspberryPi: PiTFT and ITEAD PN532 NFC module.


DoorNFC - Device

The touchscreen used is the adafriut 2.8” PiTFT resistive touchscreen with 320×240 resolution. It is a actually a Pi HAT device with a socket the same as the raspberry pi. Its assembly is very easy, and it’s usage with the Raspbian OS is relatively simple. For communicating with the RaspberryPi it uses the SPI interface.

The ITEAD NFC Module is a PN532 based board with an integrated antenna. It exposes the same functionality as all the other PN532 boards and uses either SPI, I2C or UART for communication. However, this device also has a native RaspberryPi header interface. One bad thing is, this interface can only work with SPI. Since the SPI and the same channel is already taken by the PiTFT, I made some alteration of the NFC module in order to patch it to work with the same header but by using I2C. I’ve described that procedure in my previous blogpost.


As the core for starting the device, I used the pre-built adafruit image of the Raspbian OS. This image is described in details in the adafruit tutorials section. Basically, it is a Raspbian OS with the patched kernel, driver and necessary configuration to enable and use the PiTFT. Besides all that, it also comes with JDK8 and nicely split GPU/CPU memory which is the core need for running JavaFX applications on the Pi.

With only the image however, the job for configuring the device is not done. First, the FrameBufferCopy tool (fbcp) will be needed:

Then,  the start-up console needs to be disabled. Do this by removing the fbcon map and fbcon font settings in /boot/cmdline.txt.

Next, and this is the trickiest part, the display needs to be adaptad to be with the same format as the PiTFT. The touchscreen is designed to be in portrait mode with resolution of 240×320. The original configuration of the X server done here is by rotating the display and re-calibrating the touchscreen. JavaFX runs in a framebuffer and it’s not connected to X whatsoever. Therefore, the display and the touchscreen behavior work differently and wrong. This can be fixed by force-adjusting the display resolution to 240×320 and not rotating the screen by default. In order to do so, alter the settings in /boot/config.txt:

and by resetting the rotation in /etc/modprobe.d/: rotate=0

At the end, in order to enable I2C, modify /etc/modules by adding:

and comment out i2c in /etc/modprobe.b/raspi-blacklist.conf.


The software for the device is already on github:

There are two packages present:

Writing JavaFX code for the Pi is rather straight forward. The  most important aspects have to be met at start, as they are more environment related. Others are just tips. Some that I can mention:

  • The CPU/GPU memory split needs to configured correctly in order to achieve nicer performances (or even to get the JavaFX app up and running). 128MB for the GPU is a decent amount.
  • The JavaFX app will run in a framebuffer. This is maybe the biggest difference that you must have in mind. Running JavaFX apps on the Pi is not conditioned by the presence of an X server: they don’t run in a widget or a frame and can be invoked straight from a console. Even better, running a JavaFX app from an X session will most likely break it and freeze the UI after you exit it. Always execute the JavaFX app from a console, local or remote.
  • Because the app will run in a Framebuffer, make sure that you use and manage all the visual space that you have in your disposal and run it in full screen. You can still run it with fixed size, but then it will most probably end up centered on the screen.
  • JavaFX will register it’s own Keyboard and mouse handler and render a mouse pointer. If you have some settings done in X that change the behavior of the mouse or the keyboard, they will not be present here. E.g.: a major problem with the touchscreen was the initial rotation. The screen was rotated, but the touchscreen was only calibrated for that in X. That’s why the settings here are reversed and the display is in portrait.
  • Last but not least: JavaFX runs in its own thread. If you are to populate other heavier operations from the main routine or an event handler, do it in a different thread. If you need to alter something UI related from a different thread, use Platform.runLater.

You can see some examples already implemented in the source code. It’s not very pragmatic or anything, but enough to get the idea and to get the device working.

A general frame of a basic JavaFX app looks something like this:

The other part of the code is the pi4j usage. Here I’m using the managed way to access the hardware aspect of the Pi and send/receive data through it:

  • I2C is used for communicating with the NFC PN532 module. The API is simple but the hard part is maintaining the protocol set by the device manufacturer:

    The adress of the device can be given by the manufacturer of the device,  or you can look it up with the tool i2cdetect or something similar. See some tips in this adafruit tutorial.
  • General I/O pin provisioning can be also combined, regardless that both SPI and I2C are used. Just make sure that you don’t provision a PIN somehow that will break the other two interfaces. Again, here I’m using the managed API of pi4j:

Some remarks about using pi4j:

  • Since pins need to be provisioned (exported), the java process MUST be started with sudo, or else it will fail.
  • I2C is used for communication, so make sure that the device is enabled and not blacklisted
  • The communication is not reliable. Your app should be prepared for that and easily recover from misscomunications.


For ease of access, I’ve added two shell scripts ( and to make my compile&test experience on the Pi bearable. The pi4j library is automatically added in the classpath in both compile and test, the java process in run with sudo and fbcp is run in parallel.

The performance of the app itself is so-so. I can’t really deduct a conclusion since fbcp is an important parameter, and it may alter the visual response. Overall it is usable, but still not on that level that I want to see.

Always bear in mind that the device is quite limited with resources, and the platform itself is still catching up. It would be great if some ideas done in OpenJFX like setting a target framebuffer or altering the touchscreen input are implemented in the Oracle JDK too. That way, the output will be independent and I would presume more efficient.

P.S. I’ve also done a different JavaFX app which reads RFID tags, runs on an HDMI monitor, and is used as a poll. The output is quite bigger, the solution is simpler, but the overall user experience is still similar.

ITEAD PN532 NFC Module and RaspberryPi via I2C and Java

In my last post I explained how I got the ITEAD PN532 NFC Module up and running with a RaspberryPi by using plain Java code with pi4j and SPI as an interface for communication.

However, for a different idea, I needed the SPI interface to be available completely, and that meant that I must somehow change the implementation for the NFC module. One option is to use I2C as an interface and to adapt the hardware and the software to use it instead.

The ITEAD PN532 NFC Module, has only the SPI pins connected at the RaspberryPi connector. I believe this is some compromise that the guys at ITEAD must have did. However, the PN532 supports SPI, I2C and Serial, and all these interfaces can be accessed through the other connector, having in mind that they share the same pins.

In order to achieve I2C communication with a RaspberryPi, the following steps need to be made:

1. Change the switches on the NFC module to indicate that I2C will be used: SET0 set to H and SET1 set to L.

2. The I2C pins need to be rewired manually to the I2C pins of the RaspberryPi connector. My gruesome workaround is this:

itead back rewire

3. The SPI pins of the RaspberryPi connector are still conected to the same shared pins, and with the rewiring, the I2C and SPI are interconnected and they won’t be functioning properly. To avoid this, the SPI pins need to be removed. Again, my workaround by cutting the 5 pins from the connector:

itead front rewire

As of now, the device is ready to work with I2C. In order to test it, you can use the libnfc as described in this ITEAD studio blogpost. (see from number 8 onward).

The final step was to make all this functioning with Java code. For that, I extended my library by porting the elechouse PN532 implementation for I2C to Java with pi4j. After some struggle with the proper API usage, the implementation got quite clean. It is already added in the same project at GitHub.

As of this point, since I got everything working, my idea is to resume with this projec, port the whole PN532 implementation to Java, and making the API more clear to use. That I will cover in a future blog post.

NFC with RaspberryPi and Java

For a presentation I did, I needed to come up with a neat example of directly connecting a RaspberryPi with some add-on device. The ITEAD PN532 NFC Module looked perfect for this case, and I started working with it. My end goal was to operate it with Java code.

RaspberryPi with ITEAD PN532 NFC Module
RaspberryPi with ITEAD PN532 NFC Module

The ITEAD blog has a nice example plus a library done in C for interfacing the NFC Module.  It uses the SPI interface and operates on a low level by using the WiringPi library.

The pi4j library is a JNI to the WiringPi C library, so in basic terms it should be able to do the same thing.

I started by porting an Arduino code first (github link) and using the Serial interface for communication, but I failed. Then I started porting the ITEAD provided library and used the SPI interface as in the original code. After some struggle, I managed to devise some working code:


  • there are two interface implementations: PN532Spi and PN532Serial. The second one is marked as @Deprecated because it’s not functioning for some reasons
  • the SPI implementation at the moment can only get the firmware version and read the passive target id of an NFC tag. Hopefully I’ll implement the other functions soon.
  • before using the SPI interface, make sure that it is enabled. See the notes in the ITEAD blogpost and in the github repo readme.
  • when starting the example, execute java with sudo in order pi4j to work properly.

WebSockets in Java

At the second JavaDay in Skopje in 2013, I talked about WebSockets and how to do them in Java. As a base for explaining, I’ve created a simple chat room application along with three different WebSocket implementations: JavaWebSocket, JavaEE7 and Spring4.


The application prompts for a username on start, shows all the present users in the room, provides basic chat functionality and does automatic log out when the browser tab/window is closed.

The slides of the presentation are available at SlideShare.
The talk is available (in Macedonian only) at Parleys.
The whole source code (explained in short below) is avalable at Github.


Client application

The client application is built with jQuery 1.10.2 and Bootstrap 3.0.3. For the first two examples, native WebSocket API is used for the communication.

At the start, a bootstrap modal dialog is shown for entering the desired nickname. When it’s confirmed, the whole application is being initialized, the communication procedure is started up, the events are bound and the jQuery assisted DOM manipulation is defined.

The JavaScript WebSocket API is quite simple and it involves just creating a web socket connection and binding the four communication events:

  • onOpen – when the connection has been successfully opened
  • onMessage – when a server sent message has been received
  • onClose – when the connection has been properly closed
  • onError – when a communication error has occured and the connection has been severed

The basic shape of a javascript WebSocket handler is:

In the implementation, the messages that the client sends to the server are pure text. By convention, the first message sent is the username, while every next one is a message that the user is sending to everyone else.

For better manipulation, the messages that are being received from the server are placed in JSON objects. There are three types of messages that the server can send:

  • addUser message – a message that has only one field ‘addUser’ containing the username to be added to the list, i.e. a new user joins the chat
  • removeUser message – a message that has only one field ‘removeUser’ containing the username to be removed from the user list, i.e. a user closed the chat
  • message message – a message that has two fields: nickname and message, describing a sent chat message from the user ‘nickname’ and contents ‘message’

There is no logout option, since the tab/window closing automatically closes the WebSocket and the server gets this event.


JavaWebSocket implementation

One of the simplest ways to start-up a WebSocket server is by using a standalone implementation like the Java WebSocket (github link).

In order to use it, you need to create a standard Java7 application, which defines an extension of the WebSocketServer class, instantiates it and starts in the main routine (or anywhere applicable). The WebSocket server will run as a background thread as long as it is not explicitly stopped or the application is terminated.

The class that resembles the WebSocket server should pass a proper base constructor for the target IP and port, and override the four methods that are basically the event handlers in the WebSocket communication life cycle:

Several notes here:

  • There is just one instance of the WebSocket server. It handles all the connections and the communication.
  • The WebSocket server implements the very same methods / event handlers as present in the client side: onOpen, onClose, onMessage and onError, having now, the onMessage is triggered when a message is received from some client.
  • For every opened websocket connection, there is a single WebSocket object instantiated. That object will remain the same and present until the connection is closed somehow. It can be referenced anywhere from the code and used as the WebSocket handler for that specific connection.

In my example, I store every WebSocket object in a set, and map them in a Map to their nickNames for future usage. When a connection is opened, the WebSocket instance is stored in the set, and when the nickname is passed, in the Map as well. When a message is received, the set is being iterated and the messages is sent to all active clients. In case if a connection closes or it’s being terminated by an error, the WebSocket instance is removed from the map and the set, and the ‘removeUser’ message is sent to all other clients.

For more details, see the JavaWebSocket sample in the source code. In order to run this sample, you need just a Java7 runtime. Start the server as a Java application and then open the chat.html file with your browser. (note: this is an Eclipse project)


JavaEE implementation

JSR 356 defines how the Java API should look like in the enterprise application servers in order to provide server side and Java client side WebSocket functionality. It is already implemented in the most popular application servers.

Apache Tomcat originally implemented JSR 356 and dropped its own WebSocket implementation in Tomcat 8. After a while though, the same functionality was back-ported to Tomcat 7.0.43. It’s usage again is bound by using Java 7.

There are two ways of how a WebSocket server can be defined in JavaEE7. As in many of the other Java APIs, there’s the annotation driven approach, which provides very simple means for creating basic server side constructions. There’s also the extension (interface based) approach by extending the base WebSocket classes and overriding the methods implementations.

A very basic annotation driven WebSocket implementation in Tomcat is comprised by using the following annotations:
– @ServerEndpoint(path) – for defining a class as a WebSocket endpoint at the specified relative path
– @OnOpen(Session), @OnClose(Session, CloseReason), @OnMessage(String), @OnError(Session, Throwable) – for defining the methods handlers for the four events in the WebSocket communication lifecycle.

Sending message back to the client is performed by getting a remote object from the session, and sending data through it. E.g.: session.getBasicRemote().sendText(text);

The session object is unique and persistent for one client throughout it’s communication life-cycle. It can be stored and used in any other place your application.

On client side, everything is the same, except that now the port is 8080 (the same port at which Tomcat is configured to listen).

For more details, see the EESockets sample in the source code. In order to run the sample, you’ll need a Java7 Runtime along with a newer version of Tomcat 7 (> 7.0.43). It is again an eclipse project, so you may need to fine-tune the settings.


Spring 4 implementation

The freshly out Spring4 framework comes with it’s own and improved out of the box WebSockets implementation. It comprises of both well connected server and client side modules which provide native WebSocket communication and give a seamless fallback if no such option is possible.

In order to use the WebSockets implementation in spring,  the context configuration needs to enable both SpringWebMVC and WebSocket, afterwards to define the beans for the WebSocketHandlers and register them. If the spring configuration is done by Java code, it should look something like this:

Several things are important here:
– A web socket server endpoint in Spring is a implementation of a kind from WebSocketHandler
– PerConnectionWebSocketHandler is an extensions which basically says that a new instance from the WebSocketHandler class will be created for each WebSocket client.
– .withSockJS() says that this endpoint should be enabled for, and expect calls from the SockJS client library.

The WebSocket server implementation is done by extending and overriding the WebSocketHandler class. There is a small hierarchy already present in Spring, so e.g.: if only textual data needs to be interchanged, then the implementation class should extend TextWebSocketHandler, since there the encoding/decoding stuff is already handled:

Again here, the same four methods are being overridden, and similar pattern is used for sending data back to the client. The WebSocketSession object is again unique and persistent, and can be saved and used throughout the application.

One crucial difference now is the JavaScript client side implementation. Since the WebSocketHandler is sockJS enabled, the same library can be used for interfacing with it. The difference is minimal: instead of instantiating a WebSocket, include the Sock.JS JavaScript client library, make a SockJS object and bind the same functions to it:

What happens now is, that SockJS will first check if a WebSocket communication can be established (by polling a info sub-path first). If successful, then it is resumed as such. If not, then a fallback option is applied (e.g.: http long polling) and the communication is abstracted on top of it. At the end, only one WebSocket like handler is needed for defining the whole communication. Like this, regardless of the client browser, only one code base is needed to handle all types of communication protocols.

Please note that now when passing the address, “http” and not “ws” is used. This is not an error, since SockJS will try to upgrade the connection afterwards.

For more details, see the springsockets sample in the source code. In order to run the sample, you’ll need a Java7 Runtime along with a newer version of Tomcat 7 (> 7.0.43). It is again an eclipse project, so you may need to fine-tune the settings. In the maven settings, the proper repositories are also added, so if you’re behind a local nexus mirror, please make sure that they will be accessible. Also, I’m using Spring 4.0.0.RC2 here. You can change it to the most recent Spring version.

pi4jmultimeter: Raspberry Pi + Arduino + d3js electrical multimeter

As a fun project and as an example for my recent talk at the Jazoon conference, I’ve made this device. I call it my pi4jmultimeter.

 What it does its some basic electrical multimeter features:

  • DC voltage measurement
  • AC waveline preview
  • AC spectrum analysis
  • Electrical resistance measurement

The device is a combination of a Raspberry Pi and an Arduino. The Arduino does the analog readings and uses Fast Fourier Transformation for producing the spectrum graph data. The Raspberry Pi hosts the main process and runs a lighttd web server. Both devices are connected with their extension headers (Raspberry Pi’s GPIO and Arduino’s Vin, GND and serial port pins) and have a paired serial port interface. The end result is visualized in a JavaScript web application which renders optimized d3js animated SVG graphs.

The Arduino loops a program which does simple analog reads on three inputs, periodical FFT and sends all this data to the Raspberry Pi via the serial interface. The Raspberry Pi, has a running Java8 SE Embedded process based on the pi4j and Java WebSocket libraries. It reads everything from the serial port with the help of the pi4j library, packs everything in a nice JSON format and broadcasts the data to every opened WebSocket connection by the use of the Java WebSocket library.

The whole source code with the build instructions can be found on GitHub:
The slides of my presentation are available at the jazoon guide webpage:  (or directly at slideshare)

The d3js graphs can be observed and used individually. All the graphs populated with some random data can be found here: