Developing a Transformation Service

Transformations and Profiles (see next section) are very similar in their nature. Usually if you implement a transformation, you also provide a profile for the same matter in the same bundle.

A transformation in openHAB is generally speaking a mapping from a Java String to a Java String with a given user configuration. It does not keep any state, so it is expected to return the same output for the same input.

Transformations are used for generic bindings like MQTT, HTTP, COAP and any other direct protocol interaction to transform an incoming wire string to something that a channel can process.

Let's assume that we want to write a transformation that appends the string ",bro!" to every input.

First you want to create a new bundle for example via the skeleton.

You start by writing an OSGi service class that implements TransformationService. Also check out the openHAB JavaDoc class documentation, there might be a more specialized class for your needs like AbstractFileTransformationService.

@Component(immediate = true, property = { "smarthome.transform=BRO" })
public class TheBroTransformationService implements TransformationService {
    private final Logger logger = LoggerFactory.getLogger(TheBroTransformationService.class);

    public @Nullable String transform(String config, String input) throws TransformationException {

Next you implement the transform method. You are given a user configuration and the input value. In our case we do not use the config parameter. Other services like the regex or map transformation are using this parameter for the regex (".*=(\\d*.\\d*).*") or the map ( for example.

Our implementation is as simple as this:

public @Nullable String transform(String config, String input) throws TransformationException {
    return input + ",bro!";

Developing a Profile

The communication between the framework and the Thing handlers can be influenced by "Profiles".

By their nature, profiles are correlated to links between Items and Channels (i.e. ItemChannelLinks), just like transformations.

But in contrast to transformations, if one Channel is linked to several Items it also will have several profile instances. Each instance handling the communication to exactly one of these Items. The same applies for the situation where one Item is linked to multiple Channels.

Profiles are created by ProfileFactories and are retained for the lifetime of their link. This means that they are, in contrast to transformations, allowed to retain a transient state, like e.g. the timestamp of the the last event or the last state. With this, it is possible to take into account the temporal dimension when calculating the appropriate action in any situation.

There exist two different kinds of profiles: state and trigger profiles.

State Profiles

State profiles are responsible for communication between Items and their corresponding state Channels (ChannelKind.STATE). Their purpose is to forward state updates and commands to and from the Thing handlers.

Trigger Profiles

Trigger Channels (ChannelKind.TRIGGER) by themselves do not maintain a state (as by their nature they only fire events). With the help of trigger profiles they can be linked to Items anyway. Hence the main purpose of a trigger profile is to calculate a state based on the fired events. This state then is forwarded to the linked Item by sending ItemStateEvents.

Trigger profiles are powerful means to implement some immediate, straight-forward logic without the need to write any rules.

Apart from that, they do not pass any commands or state updates to and from the Thing handler as by their nature trigger Channels are not capable of handling these.

This section explains how custom Profiles can be created. First you want to create a new bundle for example via the skeleton.

Profile Properties

A Profile is determined by its type, i.e. StateProfileType or TriggerProfileType and its ProfileTypeUID. Both types are specified via interfaces StateProfile and TriggerProfile, respectively.

The ProfileTypeUID identifies one specific type of Profile. Each Profile exists in a certain scope. There are pre-defined Profiles that are defined in scope ProfileTypeUID.SYSTEM_SCOPE, which is "system". Custom Profiles should be created in a different scope. Thus a ProfileTypeUID can be created as follows: new ProfileTypeUID("MyScope", "MyProfileName").

A StateProfile receives Commands and StateUpdates from the ThingHandler and from the Item. It has to implement four methods that specify how the Commands or StateUpdates should be handled.

A TriggerProfile makes it possible to link a TriggerChannel to an Item. This Profile receives the State of the Item and the Event that has been triggered.


Custom ProfileTypes have to be annouced by a ProfileTypeProvider to the framework via an OSGi service:

@Component(service = { ProfileTypeProvider.class })
public class MyProfileTypeProvider implements ProfileTypeProvider {
    public Collection<ProfileType> getProfileTypes(Locale locale) {
        //return custom types


The most important interface is the ProfileFactory which has to be implemented and announced as an OSGi service:

@Component(service = { ProfileFactory.class })
public class MyProfileFactory implements ProfileFactory {

Such a factory is responsible for specific ProfileTypeUIDs that should be returned by Collection<ProfileTypeUID> getSupportedProfileTypeUIDs(). Further it is capable of creating these Profiles via createProfile(ProfileTypeUID profileTypeUID, ProfileCallback callback, ProfileContext profileContext);.

For convenience, the ProfileFactory and the ProfileTypeProvider can be put into one class and announce the two services:

@Component(service = { ProfileFactory.class, ProfileTypeProvider.class })
public class MyProfileFactory implements ProfileFactory, ProfileTypeProvider {


Profiles need the opportunity to notify the framework about what they did with the Commands and StateUpdates it received from the framework. The ProfileCallback provides methods to forward the results of a Profiles processing to the ThingHandler and to the Framework. It should be injected into the Profile upon its creation.


Some more advanced Profiles which can be configured need access to their Configuration object. This is offered via the ProfileContext. A ScheduledExecutorService can also be retrieved via the ProfileContext in order to schedule long running tasks in a separate Thread. The ProfileContext may also be injected into the Profile upon its creation.


A ProfileAdvisor is an optional component which can be used to suggest a specific ProfileTypeUID for a given Channel or ChannelType. Two methods have to be implemented to achieve this:

ProfileTypeUID getSuggestedProfileTypeUID(Channel channel, @Nullable String itemType);

ProfileTypeUID getSuggestedProfileTypeUID(ChannelType channelType, @Nullable String itemType);

Using Profiles

.items file

Profiless can be specified as a parameter for a given channel:

<item-type> MyItem { channel="<bindingID>:<thing-typeID>:MyThing:myChannel"[profile="MyScope:MyProfile"]}

Existing Profiles

A few profiles are provided by openHAB itself.


If one device should "follow" the actions of another device, the FollowProfile can be used. The term "follow" in this case means that any state that is sent to an Item will be forwarded from this Item to any linked channel with the FollowProfile. The FollowProfile takes state updates on an Item and sends them as a command onto the channel. In the direction from the ThingHandler towards the Item, the FollowProfile ignores state updates.

<itemType> <itemName> { channel="<channelUID>", channel="<followChannelUID>"[profile="follow"]}


The OffsetProfile provides the possibility to adjust a value from a device before it arrives at the framework. An offset can be specified via the parameter offset which has to be a QuantityType or DecimalType. A positive offset is the amount of change from the device towards the framework, i.e. all values from the device are increased by this offset and values sent to the device are decreased by this offset. A negative offset subtracts the offset from the value sent by the device to the framework and adds the offset to values sent from the framework to the device.

Number <itemName> { channel="<bindingID>:<thing-typeID>:<thingName>:<channelName>"[profile="offset", offset="<value>"]}