Pembahasan Soal SO no 4

Pembahasan soal ujian mid Sistem Operasi No 4
(Dosen Pengampu: Dr. Tech. Khabib Mustofa, S.Si, M.Kom)
Pseudocode dari soal no 4 aku kodekan dalam bahasa Java (dengan beberapa penyesuaian).

Ternyata memang ada masalah alias pseudocode (soal no 4) Producer dan Consumer TIDAK AMAN jika dijalankan secara kongkuren. Untuk lebih jelasnya, silakan teman-teman mengEKSPLORE SENDIRI, source code dan output (CONTOH KASUS) aku sertakan.

Mohon masukan dari teman-teman, barangkali ada kesalahan di programnya.

Source Code:

public class Buffer {
    private Vector buffer;
    private int bufferSize;
    private int count;

    public Buffer(int bufferSize) {
        this.buffer = new Vector(bufferSize);
        this.bufferSize = bufferSize;
        this.count = 0;
    }

    public synchronized void enterItem(String item) {
        buffer.add(item);
        System.out.println(item + " added.");
        if (buffer.size()>bufferSize) {
            System.err.println("\tBuffer overflow!!...");
        }
    }

    public synchronized String removeItem() {
        Object result;
        try {
        result = buffer.remove(0);
        } catch (Exception e) {
            System.err.println("\tBuffer underflow!!...");
            return "";
        }

        System.out.println("" + result + " removed.");
        return "" + result;
    }

    public void incCount() {
        count++;
    }

    public void decCount() {
        count--;
    }

    public int getBufferSize() {
        return bufferSize;
    }

    public int getCount() {
        return count;
    }

    public synchronized String toString() {
        StringBuffer sb = new StringBuffer(buffer.size());
        for (Object o:buffer) {
            if (o!=null) {
                sb.append("# ");
            }
        }

        return "" + count + ": " + sb.toString();
    }
}

public class Producer extends Thread {
    private Buffer buffer;
    private long tts;

    private Consumer consumer;

    //--
    String item;
    private int msgId = 0;

    public Producer(Buffer buffer, long tts) {
        this.buffer = buffer;
        this.tts = tts;

        setDaemon(false);
    }

    public void setConsumer(Consumer consumer) {
        this.consumer = consumer;
    }

    public void run() {
        while (true) {
            item = "msg " + ++msgId;
            if (buffer.getCount() == buffer.getBufferSize()) {
                try {
                    Thread.sleep(tts);
                } catch (InterruptedException e) {
                    //e.printStackTrace();
                }
            }

            buffer.enterItem(item);
            buffer.incCount();

            if (buffer.getCount() == 1)
                synchronized (consumer) {
                    consumer.notifyAll();
                }
        }
    }
}

public class Consumer extends Thread {
    private Buffer buffer;
    private long tts;

    private Producer producer;

    //--
    private String item;

    public Consumer(Buffer buffer, long tts) {
        this.buffer = buffer;
        this.tts = tts;

        setDaemon(false);
    }

    public void setProducer(Producer producer) {
        this.producer = producer;
    }

    public void run() {
        while (true) {
            if (buffer.getCount() == 0) {
                try {
                    Thread.sleep(tts);
                } catch (InterruptedException e) {
                    //e.printStackTrace();
                }
            }

            item = buffer.removeItem();
            buffer.decCount();

            if (buffer.getCount() == buffer.getBufferSize() - 1) {
                synchronized (producer) {
                    producer.notifyAll();
                }
            }

            System.out.println("Consumer consumes " + item);
        }
    }
}

Output:

Untuk kasus buffer size=1:

msg 1 added.
msg 1 removed.
Consumer consumes msg 1
msg 2 added.
msg 2 removed.
Consumer consumes msg 2
msg 3 added.
msg 3 removed.
Consumer consumes msg 3
msg 4 added.
msg 4 removed.
Consumer consumes msg 4
msg 5 added.
msg 5 removed.
Consumer consumes msg 5
msg 6 added.
msg 6 removed.
Consumer consumes msg 6
msg 7 added.
msg 7 removed.
Consumer consumes msg 7
msg 8 added.
msg 8 removed.
Consumer consumes msg 8
msg 9 added.
msg 9 removed.
Consumer consumes msg 9
msg 10 added.
msg 10 removed.
Consumer consumes msg 10
msg 11 added.
msg 11 removed.
Consumer consumes msg 11
msg 12 added.
msg 12 removed.
Consumer consumes msg 12
msg 13 added.
Buffer overflow!!…