Sync advice

Question

hi all. bit of a boring one - i'd really appreciate it if anyone has the time....
i'm looking for some pretty general guidance really. i have some data flowing in from an xml source and i am using e a queue (LinkedList, whatever) to accept new objects at one end and for these to be constantly removed and processed (to a database) from the other end. i am a little confused about synchronization and threading, and wondered if anyone could give me an outline of how to get it working.
my current technique is to have a Thread subclass that has a reference to the LinkedList, and uses while(! list.isEmpty())to pop objects off the end. i am synchronizing on the list when my xml reader adds objects to it, and also when this thread takes them off. the problem is that when the reader adds them it doesn't seem to trigger the other end to take them off.
i tried putting the while(! list.isEmpty()) inside a while(true) but the processor goes crazy. i think i need to call wait() and notify, but i'm not sure what on.
can anyone help?!
cheers

Answer 1

Here's something I pulled from a book that might help...
import java.util.*;
* Server adds items to a Queue.  Client takes items off the
* Queue for processing (asynchronously.)
* Taken from Producer/Consumer pattern in "Patterns In Java"
class Queue {
    private List data = new ArrayList();
    synchronized public void enqueue(String s) {
     data.add(s);
     notify();  // use notifyAll() if > 1 Consumer possible
    synchronized public String dequeue() {
     while(data.size() == 0) {
         try {
          wait();
         catch(InterruptedException ignore) {}
     String s = (String)data.get(0);
     data.remove(0);
     return s;
class Consumer implements Runnable {
    private Queue queue;
    Consumer(Queue _queue) { queue = _queue; }
    public void run() {
     String s;
     s = queue.dequeue();
     while(!s.equals("END_OF_INPUT")) {
         System.out.println("consumer processing: " + s);
         s = queue.dequeue();
class Server implements Runnable {
    private Queue queue;
    Server(Queue _queue) { queue = _queue; }
    String[] data = {"coke", "pepsi", "7-up", "orange", "END_OF_INPUT"};
    public void run() {
     for(int i=0; i<data.length; i++) {
         System.out.println("server queuing: " + data);
     queue.enqueue(data[i]);
public class prodcom
public static void main(String[] args)
     Queue q = new Queue();
     // note: starting consumer first
     new Thread(new Consumer(q)).start();
     new Thread(new Server(q)).start();