linkedlist Queue in java not working with threads

Question

This is the code that I am using to implement the queue. Here queue poll is always returning null even when queue is not empty.

Runnable runnable = new Runnable() {

    @Override
    public void run() {
        service.schedule(runnable, 500, TimeUnit.MILLISECONDS);
        process();
    }

    public void process() {
        try {
            String tt = nextItem();
            //System.out.println("SQ:"+tt);
        } catch (Exception e) {//Catch exception if any
            System.out.println("2Error: " + e.getMessage());
        }
    }
};

public String nextItem() {
    Object poll;
    try {
        synchronized (queue) {
            System.out.println("SQ:" + queue.poll());
            //if (poll != null) {
            //    return poll.toString();
            //} else {
            return "";
            //}
        }
    } catch (Exception e) {
        e.printStackTrace();
        return "";
    }
}

public void run() {
    try {
        Class.forName("com.mysql.jdbc.Driver");
        String url =
                "jdbc:mysql://1xxx:3306/ayan";
        Connection con =
                DriverManager.getConnection(
                url, "[user]", "[pass]");

        Queue queue = new LinkedList();
        service = Executors.newScheduledThreadPool(1000);
        service.schedule(runnable, 0, TimeUnit.MILLISECONDS);
        while (true) {
            Statement statement = con.createStatement();
            statement.setFetchSize(1);
            ResultSet resultSet = statement.executeQuery("SELECT * from query_q");
            while (resultSet.next()) {
                // process results. each call to next() should fetch the next row
                String id = resultSet.getString("id");
                String query = resultSet.getString("query");
                String msisdn = resultSet.getString("msisdn");
                String pass = id + "|" + query + "|" + msisdn;
                System.out.println("MQ:" + pass);
                //String str = "foo";
                //Queue<Character> charsQueue = new LinkedList<Character>();
                boolean inserted = false;
                for (char c : pass.toCharArray()) {
                    inserted = queue.offer(c);
                }

                if (inserted != false) {
                    // Statement stats = con.createStatement();
                    //stats.executeUpdate("delete from query_q where id=" + id);
                }
            }
            Thread.sleep(10000);
        }
        //con.close();
}

Answer 1

LinkedList is the only non-thread safe Queue. Any other implementation would have been a better choice. Your offer is not synchronized. ;)

An ExecutorService has a built in queue. You can make use of that and not create a queue of your own at all. Just execute(Runnable) tasks as you need something to be done.

Answer 2

That's because you are not synchronizing the queue for your queue.offer(). You need to synchronize all access to the queue.

The simplest way to do this is to use a LinkedBlockingQueue which will take care of all the synchronization for you.

Answer 3

Note that you call offer() and poll() on different queue 's - offer() 's queue is a local variable, whereas the poll() 's one is probably a field:

Queue queue = new LinkedList(); 

Also, syncrhonization is required, as suggested in other answers.