How to handle wait() notify()?

Question

Here I have two run methods which should synchronize each other.

Poller Class:

     */
public void run() {
    int seqId = 0;
    while(true) {
    List<KpiMessage> list = null;

        try{
            if(!accumulator.isUsed){                
                try {
                    list = fullPoll(seqId);

                    if (!list.isEmpty()) {
                        seqId = list.get(0).getSequence();
                        accumulator.manageIngoing(list);
                    }
                    System.out.println("Updated");                      
                    wait(); 
                } catch (Exception e1) {
                    e1.printStackTrace();

                }
            }

        } catch (Exception e){
            // TODO:
            System.err.println(e.getMessage());
            e.printStackTrace();                
        }
    }

}


/**
 * Method which defines polling of the database and also count the number of Queries
 * @param lastSeq 
 * @return pojo col
 * @throws Exception
 */
public List<KpiMessage> fullPoll(int lastSeq) throws Exception {
    Statement st = dbConnection.createStatement();
    System.out.println("Polling");
    ResultSet rs = st.executeQuery("Select * from msg_new_to_bde where ACTION = 814 and 
    STATUS = 200 order by SEQ DESC");
    List<KpiMessage> pojoCol = new ArrayList<KpiMessage>();
    try {


        while (rs.next()) {
            KpiMessage filedClass = convertRecordsetToPojo(rs);
            pojoCol.add(filedClass);
        }

        for (KpiMessage pojoClass : pojoCol) {
            System.out.print(" " + pojoClass.getSequence());
            System.out.print(" " + pojoClass.getTableName());
            System.out.print(" " + pojoClass.getAction());
            System.out.print(" " + pojoClass.getKeyInfo1());
            System.out.print(" " + pojoClass.getKeyInfo2());
            System.out.print(" "+ pojoClass.getStatus());
            System.out.println(" " + pojoClass.getEntryTime());

        }


    } finally  {
        try {
            st.close();
            rs.close();
        } catch (Exception e) {
            System.err.println(e.getMessage());
            e.printStackTrace();
        }
    }       

Processing and Updating Class:

        public void run() {
    while(true){
        try {
            while(!accumulator.isUsed)
            {
                try {
                System.out.println("Waiting for new outgoingmessages"); 
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
     Collection<KpiMessage> outgoingQueue = generate(accumulator.outgoingQueue); 
            accumulator.manageOutgoing(outgoingQueue, dbConnection);
        } catch (Exception e) {
            System.err.println(e.getMessage());
            e.printStackTrace();
        }
    }
}
}   

I have a logical error:

The poller is polling not only for new messsage but also reads the DB again and again from the first.

Also Updates again and again.

How to solve this synchronization problem .

Answer 1

You should synchronize or rather hold the lock or monitor for the object that you are calling wait() or notify() on.

Here is what will help you : wait() throwing IllegalArgumentException

synchronized(lockObject){

     lockObject.wait(); //you should hold the lock to be able to call wait()
}

Answer 2

Alternatively you could use a BlockingQueue to transfer the data between threads.

See BlockingQueue for details.

// The end of the list.
private static final Integer End = -1;

static class Producer implements Runnable {
  final Queue<Integer> queue;
  private int i = 0;

  public Producer(Queue<Integer> queue) {
    this.queue = queue;
  }

  @Override
  public void run() {
    try {
      for (int i = 0; i < 1000; i++) {
        queue.add(i++);
        Thread.sleep(1);
      }
      // Finish the queue.
      queue.add(End);
    } catch (InterruptedException ex) {
      // Just exit.
    }
  }
}

static class Consumer implements Runnable {
  final Queue<Integer> queue;
  private int i = 0;

  public Consumer(Queue<Integer> queue) {
    this.queue = queue;
  }

  @Override
  public void run() {
    boolean ended = false;
    while (!ended) {
      Integer i = queue.poll();
      if ( i != null ) {
        ended = i == End;
        System.out.println(i);
      }
    }
  }
}

public void test() throws InterruptedException {
  Queue queue = new LinkedBlockingQueue();
  Producer p = new Producer(queue);
  Consumer c = new Consumer(queue);
  Thread pt = new Thread(p);
  Thread ct = new Thread(c);
  // Start it all going.
  pt.start();
  ct.start();
  // Close it down
  pt.join();
  ct.join();
}