A document on filtering produced data.

The produced output can be filtered in two ways

• By the producer – The producer can be implemented so as to filter the data for sending (its polled method should be implemented so as to produce only values corresponding to the necessary filter). In this case, it must be registered with the WireConstants.WIREADMIN_PRODUCER_FILTERS property.
• By the connecting wire – In this case, the Wire Admin will create the Wire with the WireConstants.WIREADMIN_FILTER property; the producer must NOT have the WireConstants.WIREADMIN_PRODUCER_FILTERS property simultaneously!

As mentioned earlier in this document, the data filtering can be time-based or value-based. Value-based filtering can be applied for Number data only.

For constructing value-based filters, the following wire constants can be used:

• WireConstants.WIREVALUE_CURRENT ("wirevalue.current") – Indicates the current value available on the wire
• WireConstants.WIREVALUE_PREVIOUS ("wirevalue.previous") – Indicates the previous value passed across the wire
• WireConstants.WIREVALUE_DELTA_ABSOLUTE ("wirevalue.delta.absolute") – The absolute (always positive) difference between the last update and the current value
• WireConstants.WIREVALUE_DELTA_RELATIVE ("wirevalue.delta.relative") – The relative difference is (current_value - previous_value ) / current.

For constructing time-based filters, the WireConstants.WIREVALUE_ELAPSED ("wirevalue.elapsed") constant can be used. It represents the elapsed time in milliseconds between the current value and the previous value sent by the producer.

The following listing illustrates producer-made output filtering. It provides value-based filtering in the producer's polled method. It sends information about the current temperature only if it is higher than 25 degrees.

                    import org.osgi.framework.*;
import org.osgi.service.wireadmin.*;

import java.util.Hashtable;
  
public class ProducerThatFilters implements Producer {
    
    BundleContext bc;    
    private double currentTemperature;    

    public ProducerThatFilters(BundleContext bc) {
      this.bc = bc;
      Hashtable props = new Hashtable(3);
      Class[] flavors = new Class[] {Double.class};
      //the producer registers for Double data output
      props.put(WireConstants.WIREADMIN_PRODUCER_FLAVORS, flavors);
      props.put(WireConstants.SERVICE_PID, "producer.that.filters");
      // set this registration property with some value
      //to indicate that filtering is performed by the producer
      props.put(WireConstants.WIREADMIN_PRODUCER_FILTERS, "some.value");
      bc.registerService(Producer.class.getName(), this, props);
    }

    public void consumersConnected(Wire[] wires) {
        ...
    }
    
    //the data is sent only if it is no smaller than 25
    public Object polled(Wire wire) {
      return (currentTemperature >= 25.d) ? new Double(currentTemperature) : null;
    }
}
              

Filtering data through the Wire itself is illustrated in Listing from the Getting the WireAdmin Service part.