I'm trying to learn how to write simple servlets to use with a Raspberry Pi. I want to control the board I/O via web.I'm using the Pi4J library which is a wrapper for the WiringPi C library.It works when I use it to blink a led locally so I assume that I'm doing something wrong coding my servlet.

This is the code I wrote:

package com.luca.servlet;

import com.pi4j.io.gpio.GpioController;
import com.pi4j.io.gpio.GpioFactory;
import com.pi4j.io.gpio.GpioPinDigitalOutput;
import com.pi4j.io.gpio.RaspiPin;
import com.pi4j.io.gpio.PinState;

public class MyServlet extends javax.servlet.http.HttpServlet {

  private GpioController gpio=GpioFactory.getInstance();
  private GpioPinDigitalOutput redLed=gpio.provisionDigitalOutputPin(RaspiPin.GPIO_23,PinState.LOW);
  private GpioPinDigitalOutput greenLed=gpio.provisionDigitalOutputPin(RaspiPin.GPIO_22,PinState.LOW);
  private GpioPinDigitalOutput blueLed=gpio.provisionDigitalOutputPin(RaspiPin.GPIO_21,PinState.LOW);
  private GpioPinDigitalOutput[] pins=new GpioPinDigitalOutput[]{redLed,greenLed,blueLed};

  public void doGet(javax.servlet.http.HttpServletRequest request,javax.servlet.http.HttpServletResponse response) throws java.io.IOException {
    java.io.PrintWriter print=response.getWriter();
                   "<p> Choose a color! </p>"+
                   "<form action=\"first\" method=\"POST\">"+
                   "<input type=\"submit\" name=\"button\" value=\"red\"/>"+
                   "<form action=\"first\" method=\"POST\">"+
                   "<input type=\"submit\" name=\"button\" value=\"green\"/>"+
                   "<form action=\"first\" method=\"POST\">"+
                   "<input type=\"submit\" name=\"button\" value=\"blue\"/>"+         

  public void doPost(javax.servlet.http.HttpServletRequest request,javax.servlet.http.HttpServletResponse response) throws java.io.IOException {
    java.io.PrintWriter pw=response.getWriter();
    String act=request.getParameter("button"); 
    switch(act) {
      case "red":
        pw.write("<p>the led is red!</p>");
      case "green":
        pw.write("<p>the led is green</p>");
      case "blue":
        pw.write("<p>the led is blue!</p>");

  private void togglePin() {
    for (GpioPinDigitalOutput pin : pins) 
      if (pin.isHigh()) pin.toggle();

it compiles fine and I manually deploy it inside tomcat,with the deployment descriptor and all. But when I connect it says to me that the resource is unavailable. If I remove the GPIO related code it works fine.

  • 2
    Is it a permissions problem? wiringPi requires root privileges to access the GPIO. Does your applet have root privileges?
    – joan
    Dec 8, 2015 at 15:35
  • It is a permission problem as @joan indicated. The applet runs in the context of the webserver and has (thus) no root privileges. Dec 8, 2015 at 16:26
  • @DiederikdeHaas I start tomcat as sudo ./startup.sh
    – Luca
    Dec 8, 2015 at 16:44
  • 1
    @Luca I assumed that you were using raspbian tomcat packages. Note that it is incredibly dangerous to run a webserver as root. One security problem in either tomcat or your code and the intruder has root privileges on your system. Dec 8, 2015 at 18:03
  • I have a software package which I think does what you are trying to achieve - as a former engineer, it might suit your needs - see grapl.16mb.com
    – user38056
    Dec 10, 2015 at 13:02

1 Answer 1


I finally found out what the problem was! I forgot to add the Pi4J libraries (the jars) in the lib subfolder of my web application. That was stupid :D . Thanks anyway!


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