j2se 5.0
j2se 6.0
at least Linux-i586 affected (presumably it's "general" bug that affects all systems)

when no audio card installed, MidiSystem.getSequencer() returns RealTimeSequencer with Transmitter connected to RealTimeSequencer$SequencerReceiver (i.e. to another RealTimeSequencer) instead throwing MidiUnavailableException (due spec).

MidiSystem.getReceiver() returns RealTimeSequencer$SequencerReceiver (should return Synthesizer or MIDI OUT port)

below is the test (exception should be thrown w/o installed sound card):

import javax.sound.midi.*;
import java.util.*;

public class defSequencer {
    
    public defSequencer() {
    }

    public void main(String[] args) {
        Sequencer seq = null;
        try {
            System.out.println("getting sequencer...");
            seq = MidiSystem.getSequencer();
            System.out.println("OK: " + seq);
            
            List<Receiver> recvList = seq.getReceivers();
            System.out.println("Receivers"
                    + " (count=" + recvList.size() + ","
                    + " max=" + seq.getMaxReceivers() + "):");
            Iterator<Receiver> recvIter = recvList.iterator();
            while (recvIter.hasNext()) {
                Receiver recv = recvIter.next();
                System.out.println(" - " + recv);
            }
            
            List<Transmitter> transList = seq.getTransmitters();
            System.out.println("Transmitters"
                    + " (count=" + transList.size() + ","
                    + " max=" + seq.getMaxTransmitters());
            Iterator<Transmitter> transIter = transList.iterator();
            while (transIter.hasNext()) {
                Transmitter trans = transIter.next();
                System.out.println(" - " + trans);
                Receiver recv = trans.getReceiver();
                System.out.println(" => " + recv);
            }
            
            System.out.println("opening sequencer...");
            seq.open();
            System.out.println("OK.");
            
            System.out.println("closing...");
            seq.close();
            System.out.println("OK.");
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}