# Framework Tools
Find a few framework utilities listed in this chapter.
# Unique Instance ID
When communicating with external systems, it is often desirable to have a unique identifier.
The org.openhab.core.id bundle is a mean to generate such an id, which is automatically persisted.
The persistence is done in the configured userdata directory as a file called uuid.
The id is provided through a static method and can be retrieved through
String uuid = InstanceUUID.get();
# Network Address Service
The NetworkAddressService is an OSGi service that can be used like any other OSGi service by adding a service reference to it.
Its OSGi service name is org.openhab.core.network.
A user can configure his default network address via UI under Settings -> Network Settings.
One can obtain the configured address via the getPrimaryIpv4HostAddress() method on the service.
This service is useful for example in the ThingHandlerFactory or an AudioSink where one needs a specific IP address of the host system to provide something like a callback URL.
Some static methods like getAllBroadcastAddresses() for retrieving all interface broadcast addresses or getInterfaceAddresses() for retrieving all assigned interface addresses might be usefull as well for discovery services.
# Network Address Change Listener
The NetworkAddressChangeListener is a consumer type OSGi service interface.
If listeners want to be notified about network interface address changes, they can implement NetworkAddressChangeListener and register as an OSGi service.
Please be aware that not all network interface changes are notified to the listeners, only "useful" network interfaces :-- When a network interface status changes from "up" to "down", it is considered as "removed". When a "loopback" or "down" interface is added, the listeners are not notified.
# Caching
The framework provides some caching solutions for common scenarios.
A common usage case is in a ThingHandler to encapsulate one value of an internal state and attach an expire time on that value.
A cache action will be called to refresh the value if it is expired.
This is what ExpiringCache implements.
If handleCommand(ChannelUID channelUID, Command command) is called with the "RefreshType" command, you just return cache.getValue().
It is a good practice to return as fast as possible from the handleCommand(ChannelUID channelUID, Command command) method to not block callers especially UIs.
Use this type of cache only, if your refresh action is a quick to compute, blocking operation.
If you deal with network calls, consider the asynchronously reloading cache implementation instead.
# Expiring and asynchronously reloading cache
If we refreshed a value of the internal state in a ThingHandler just recently, we can return it immediately via the usual updateState(channel, state) method in response to a "RefreshType" command.
If the state is too old, we need to fetch it first and this may involve network calls, interprocess operations or anything else that will would block for a considerable amout of time.
A common usage case of the ExpiringCacheAsync cache type is in a ThingHandler to encapsulate one value of an internal state and attach an expire time on that value.
A handleCommand implementation with the interesting RefreshType could look like this:
public void handleCommand(ChannelUID channelUID, Command command) {
if (command instanceof RefreshType) {
switch (channelUID.getId()) {
case CHANNEL_1:
cache1.getValue(updater).thenAccept(value -> updateState(CHANNEL_1, value));
break;
...
}
}
}
The interesting part is the updater.
If the value is not yet expired, the returned CompletableFuture will complete immediately and the given code is executed.
If the value is expired, the updater will be used to request a refreshed value.
An updater can be any class or lambda that implements the funtional interface of Supplier<CompletableFuture<VALUE_TYPE>>.
In the following example the method CompletableFuture<VALUE_TYPE> get() is accordingly implemented.
The example assumes that we deal
with a still very common callback based device refreshing method doSuperImportantAsyncStuffHereToGetRefreshedValue(listener).
The listener is the class
itself, which implements DeviceStateUpdateListener.
We will be called back with a refreshed device state in asyncCallbackFromDeviceStateRefresh
and mark the Future as complete.
class FetchValueFromDevice implements Supplier<CompletableFuture<double>>, DeviceStateUpdateListener {
CompletableFuture<double> c;
@Override
CompletableFuture<double> get() {
if (c != null) {
c = new CompletableFuture<double>();
doSuperImportantAsyncStuffHereToGetRefreshedValue( (DeviceStateUpdateListener)this );
}
return c;
}
// Here you process the callback from your device refresh method
@Override
void asyncCallbackFromDeviceStateRefresh(double newValue) {
// Notify the future that we have something
if (c != null) {
c.complete(newValue);
c = null;
}
}
}
If you deal with a newer implementation with a CompletableFuture support, it is even easier. You would just return your CompletableFuture.