Java implementation of a Web Thing server
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Implementation of an HTTP Web Thing.



Add the following dependency to your project:



Add the following dependency to your project:

dependencies {
        [group: 'org.mozilla.iot', name: 'webthing', version: 'LATEST'],

Android Studio

  • Open File → Project Structure
  • Select the module you want to add this as a dependency to
  • Go to the "Dependencies" tab
  • Click green "+" button
  • Select "Library dependency"
  • Enter org.mozilla.iot:webthing in the search bar and search
  • Select the package in the result and confirm with "OK"
  • Go to "Project"
  • Add , '' to "Default Library Repository" field
  • Click "OK" in the Project Structure dialog


In this example we will set up a dimmable light and a humidity sensor (both using fake data, of course). Both working examples can be found in here.

Dimmable Light

Imagine you have a dimmable light that you want to expose via the web of things API. The light can be turned on/off and the brightness can be set from 0% to 100%. Besides the name, description, and type, a Light is required to expose two properties:

  • on: the state of the light, whether it is turned on or off
    • Setting this property via a PUT {"on": true/false} call to the REST API toggles the light.
  • brightness: the brightness level of the light from 0-100%
    • Setting this property via a PUT call to the REST API sets the brightness level of this light.

First we create a new Thing:

Thing light = new Thing("My Lamp",
                        new JSONArray(Arrays.asList("OnOffSwitch", "Light")),
                        "A web connected lamp");

Now we can add the required properties.

The on property reports and sets the on/off state of the light. For this, we need to have a Value object which holds the actual state and also a method to turn the light on/off. For our purposes, we just want to log the new state if the light is switched on/off.

JSONObject onDescription = new JSONObject();
onDescription.put("@type", "OnOffProperty");
onDescription.put("label", "On/Off");
onDescription.put("type", "boolean");
onDescription.put("description", "Whether the lamp is turned on");

Value<Boolean> on = new Value<>(true,
                                // Here, you could send a signal to
                                // the GPIO that switches the lamp
                                // off
                                v -> System.out.printf(
                                        "On-State is now %s\n",

light.addProperty(new Property(light, "on", on, onDescription));

The brightness property reports the brightness level of the light and sets the level. Like before, instead of actually setting the level of a light, we just log the level.

JSONObject brightnessDescription = new JSONObject();
brightnessDescription.put("@type", "BrightnessProperty");
brightnessDescription.put("label", "Brightness");
brightnessDescription.put("type", "number");
                          "The level of light from 0-100");
brightnessDescription.put("minimum", 0);
brightnessDescription.put("maximum", 100);
brightnessDescription.put("unit", "percent");

Value<Double> level = new Value<>(0.0,
                                  // Here, you could send a signal
                                  // to the GPIO that controls the
                                  // brightness
                                  l -> System.out.printf(
                                          "Brightness is now %s\n",

light.addProperty(new Property(light, "level", level, brightnessDescription));

Now we can add our newly created thing to the server and start it:

try {
    // If adding more than one thing, use MultipleThings() with a name.
    // In the single thing case, the thing's name will be broadcast.
    WebThingServer server = new WebThingServer(new SingleThing(light), 8888);

    Runtime.getRuntime().addShutdownHook(new Thread() {
        public void run() {

} catch (IOException e) {

This will start the server, making the light available via the WoT REST API and announcing it as a discoverable resource on your local network via mDNS.


Let's now also connect a humidity sensor to the server we set up for our light.

A MultiLevelSensor (a sensor that returns a level instead of just on/off) has one required property (besides the name, type, and optional description): level. We want to monitor this property and get notified if the value changes.

First we create a new Thing:

Thing sensor = new Thing("My Humidity Sensor",
                         new JSONArray(Arrays.asList("MultiLevelSensor")),
                         "A web connected humidity sensor");

Then we create and add the appropriate property:

  • level: tells us what the sensor is actually reading

    • Contrary to the light, the value cannot be set via an API call, as it wouldn't make much sense, to SET what a sensor is reading. Therefore, we are creating a readOnly property.
    JSONObject levelDescription = new JSONObject();
    levelDescription.put("@type", "LevelProperty");
    levelDescription.put("label", "Humidity");
    levelDescription.put("type", "number");
    levelDescription.put("description", "The current humidity in %");
    levelDescription.put("minimum", 0);
    levelDescription.put("maximum", 100);
    levelDescription.put("unit", "percent");
    levelDescription.put("readOnly", true);
    this.level = new Value<>(0.0);
    sensor.addProperty(new Property(sensor, "level", level, levelDescription));

Now we have a sensor that constantly reports 0%. To make it usable, we need a thread or some kind of input when the sensor has a new reading available. For this purpose we start a thread that queries the physical sensor every few seconds. For our purposes, it just calls a fake method.

// Start a thread that polls the sensor reading every 3 seconds
new Thread(()->{
        try {
            // Spdates the underlying value, which in turn notifies all
            // listeners
        } catch (InterruptedException e) {
            throw new IllegalStateException(e);

This will update our Value object with the sensor readings via the this.level.notifyOfExternalUpdate(readFromGPIO()); call. The Value object now notifies the property and the thing that the value has changed, which in turn notifies all websocket listeners.