webthing

Implementation of an HTTP Web Thing.

Using

Maven

Add the following dependency to your project:

<dependencies>
    <dependency>
        <groupId>io.webthings</groupId>
        <artifactId>webthing</artifactId>
        <version>0.14.0</version>
    </dependency>
</dependencies>

Gradle

Add the following dependency to your project:

dependencies {
    runtime(
        [group: 'io.webthings', name: 'webthing', version: '0.14.0'],
    )
}

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 io.webthings:webthing in the search bar and search
• Select the package in the result and confirm with "OK"
• Go to "Project"
• Add , 'https://www.jitpack.io' to "Default Library Repository" field
• Click "OK" in the Project Structure dialog

Example

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("urn:dev:ops:my-lamp-1234",
                        "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("title", "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",
                                        v));

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("title", "Brightness");
brightnessDescription.put("type", "number");
brightnessDescription.put("description",
                          "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",
                                          l));

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() {
            server.stop();
        }
    });

    server.start(false);
} catch (IOException e) {
    System.out.println(e);
    System.exit(1);
}

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.

Sensor

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("urn:dev:ops:my-humidity-sensor-1234",
                         "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("title", "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(()->{
    while(true){
        try {
            Thread.sleep(3000);
            // Updates the underlying value, which in turn notifies all
            // listeners
            this.level.notifyOfExternalUpdate(readFromGPIO());
        } catch (InterruptedException e) {
            throw new IllegalStateException(e);
        }
    }
}).start();

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.

Adding to Gateway

To add your web thing to the WebThings Gateway, install the "Web Thing" add-on and follow the instructions here.