Package org.eclipse.core.runtime

Class ServiceCaller<Service>

  • Type Parameters:
    Service - the service type for this caller
    public class ServiceCaller<Service>
extends Object
    ServiceCaller provides functional methods for invoking OSGi services in two different ways
    • Single invocations which happen only once or very rarely. In this case, maintaining a cache of the service is not worth the overhead.
    • Multiple invocations that happen often and rapidly. In this case, maintaining a cache of the service is worth the overhead.

    For single invocations of a service the static method callOnce(Class, Class, Consumer) can be used. This method will wrap a call to the consumer of the service with the necessary OSGi service registry calls to ensure the service exists and will do the proper get and release service operations surround the calls to the service. By wrapping a call around the service we can ensure that it is correctly released after use.

    Single invocation example: 

     ServiceCaller.callOnce(MyClass.class, ILog.class, (logger) -> logger.info("All systems go!"));

    Note that it is generally more efficient to use a long-running service utility, such as ServiceTracker or declarative services, but there are cases where a single one-shot lookup is preferable, especially if the service is not required after use. Examples might include logging unlikely conditions or processing debug options that are only read once.

    This allows boilerplate code to be reduced at call sites, which would otherwise have to do something like: 

     Bundle bundle = FrameworkUtil.getBundle(BadExample.class);
 BundleContext context = bundle == null ? null : bundle.getBundleContext();
 ServiceReference<Service> reference = context == null ? null : context.getServiceReference(serviceType);
 try {
   Service service = reference == null ? null : context.getService(reference);
   if (service != null)
     consumer.accept(service);
 } finally {
   context.ungetService(reference);
 }

    For cases where a service is used much more often a ServiceCaller instance can be used to cache and track the available service. This may be useful for cases that cannot use declarative services and that want to avoid using something like a ServiceTracker that does not easily allow for lazy instantiation of the service instance. For example, if logging is used more often then something like the following could be used: 

     static final ServiceCaller<ILog> log = new ServiceCaller(MyClass.class, ILog.class);
 static void info(String msg) {
   log.call(logger -> logger.info(msg));
 }

    Note that this class is intended for simple service usage patterns only. More advanced cases should use other mechanisms such as the ServiceTracker or declarative services.

    Since:
    3.13

    • Constructor Detail

      • ServiceCaller

        public ServiceCaller​(Class<?> caller,
                     Class<Service> serviceType)
        Creates a ServiceCaller instance for invoking an OSGi service many times with a consumer function.
        Parameters:
        caller - a class from the bundle that will consume the service
        serviceType - the OSGi service type to look up

      • ServiceCaller

        public ServiceCaller​(Class<?> caller,
                     Class<Service> serviceType,
                     String filter)
        Creates a ServiceCaller instance for invoking an OSGi service many times with a consumer function.
        Parameters:
        caller - a class from the bundle that will consume the service
        serviceType - the OSGi service type to look up
        filter - the service filter used to look up the service. May be null.

    • Method Detail

      • callOnce

        public static <Service> boolean callOnce​(Class<?> caller,
                                         Class<Service> serviceType,
                                         Consumer<Service> consumer)
        Calls an OSGi service by dynamically looking it up and passing it to the given consumer.

        If not running under OSGi, the caller bundle is not active or the service is not available, return false. If the service is found, call the service and return true.

        Any runtime exception thrown by the consumer is rethrown by this method. If the consumer throws a checked exception, it can be propagated using a sneakyThrow inside a try/catch block: 

         callOnce(MyClass.class, Callable.class, (callable) -> {
   try {
     callable.call();
   } catch (Exception e) {
     sneakyThrow(e);
   }
 });
 ...
 @SuppressWarnings("unchecked")
 static <E extends Throwable> void sneakyThrow(Throwable e) throws E {
   throw (E) e;
 }
        Type Parameters:
        Service - the OSGi service type to look up
        Parameters:
        caller - a class from the bundle that will use service
        serviceType - the OSGi service type to look up
        consumer - the consumer of the OSGi service
        Returns:
        true if the OSGi service was located and called successfully, false otherwise
        Throws:
        NullPointerException - if any of the parameters are null

      • callOnce

        public static <Service> boolean callOnce​(Class<?> caller,
                                         Class<Service> serviceType,
                                         String filter,
                                         Consumer<Service> consumer)
        As callOnce(Class, Class, Consumer) with an additional OSGi filter.
        Type Parameters:
        Service - the OSGi service type to look up
        Parameters:
        caller - a class from the bundle that will use service
        serviceType - the OSGi service type to look up
        consumer - the consumer of the OSGi service
        filter - an OSGi filter to restrict the services found
        Returns:
        true if the OSGi service was located and called successfully, false otherwise
        Throws:
        NullPointerException - if any of the parameters are null

      • call

        public boolean call​(Consumer<Service> consumer)
        Calls an OSGi service by dynamically looking it up and passing it to the given consumer. If not running under OSGi, the caller bundle is not active or the service is not available, return false. Any runtime exception thrown by the consumer is rethrown by this method. (For handling checked exceptions, see callOnce(Class, Class, Consumer) for a solution.) Subsequent calls to this method will attempt to reuse the previously acquired service instance until one of the following occurs:
        • The unget() method is called.
        • The service is unregistered.
        • The service properties change such that this ServiceCaller filter no longer matches.
        • The caller bundle is stopped.
        • The service rankings have changed.
        After one of these conditions occur subsequent calls to this method will try to acquire the another service instance.
        Parameters:
        consumer - the consumer of the OSGi service
        Returns:
        true if the OSGi service was located and called successfully, false otherwise

      • current

        public Optional<Service> current()
        Return the currently available service.
        Returns:
        the currently available service or empty if the service cannot be found.

      • unget

        public void unget()
        Releases the cached service object, if it exists. Another invocation of call(Consumer) will lazily get the service instance again and cache the new instance if found.