Re: using threads

"Daniel Pitts" <>
4 Jan 2007 10:55:03 -0800
Nigel Wade wrote:

Brandon McCombs wrote:

Daniel Pitts wrote:

Brandon McCombs wrote:


I have an LDAP program that uses threads to perform LDAP searches.
After actually getting the threads to properly start [I was calling
run() instead of "new Thread(p).start()"] my application responds really
fast and keeps chugging while the query in the background completes. The
problem is that I don't know how to actually retrieve the results from
the Vector that they were placed in so that I can display them in the
JList that I created. Since my program continued on, the code goes past
the point where the Vector normally would have data (without threads)
and would get loaded into the Jlist. So what's the proper way to
backtrack to get my Vector only after the thread has finished so the
data can be viewed? Do I have to end up employing another thread to
monitor the first one and then the code I have that loads the Vector
into my JList would get put into the second thread to be executed when
it detects the first thread completed?


something like this:

public class MyWorker implements Runnable {
    // fields and constructor
    public void run() {
        final Result result = doWork();
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {

You may have to move some things around in your program, Since you are
using threads, you need some sort of callback mechanism (such as the
addResultToList method, as I called it here). Its important to use
invokeLater, so that any GUI manipulation happens on the Event Dispatch
Thread. (google Swing and Threads)
Also look up SwingWorker. It wasn't a part of the standard Java
distribution, but you can find a copy of it easily online.

That direction of flow (bottom up the way I view it) is the opposite of
how I would prefer it because it makes the callback more difficult. I
have 3 classes involved in passing the data that results from the thread
  executing. I have the JList/listmodel that will display the data and
a jbutton is really calling a local method from actionPerformed() that
will call my LDAP class's search() method. The LDAP class contains all
operations that would be done with the directory server, like searching.

  The search() method ends up spawning another class that implements
Runnable to do the real work of (only) searching my directory server
(the search method uses the generic JNDI search method for an ldap
directory context so its the lowest level of my design). So somehow I
now have to make the lowest level worker object be responsible for
bypassing my stack of classes to update the GUI? That seems convoluted
to me. Or am I thinking totally the opposite of what I should be?

I seem to think I now have to tell my worker object that uses a thread
about my listmodel so that when it gets the data from its search
operation it has to be the one to update the listmodel. That means I
have to pass a reference of my listmodel through 3 classes so that the
worker object knows about it.

I also wanted the worker object to be generic so that it could also do
searches to update a JTree/treemodel (although it wasn't supposed to
have to know where the data would eventually end up) which means I'd
also have to tell the same class about my treemodel and I'll have to
tell it which to update (tree or list model) depending on what called
it. Not to mention I have JLabel in the GUI that gets updated with the #
of results that came back. Do I also have to pass in to the worker
object a reference to that label so it can update it?

There has to be a better way.

Unfortunately, there are really only two ways. One is to have a monitor thread
which waits for your worker object to finish, and then updates the component
(this, in essence, is what SwingWorker does). The other is to tell your worker
object about the component so it can update it itself when it has finished.

It's worth following up on Daniel's suggestion of SwingWorker. It takes some of
the tedium out of this, particularly the finished() method (IIRC, that's what
it's called). The unfortunate thing is that the old Tutorial for Java Swing
which explained all this has now been replaced with a new one which is only
applicable to Java 1.6. The 1.6 release now includes a [completely different]
SwingWorker class.

One way of getting around the problem of needing to update several different
types of component is to use an interface. For example, an interface called
Notifier could be defined to have one method, update(). You can then create
your own classes for each of the GUI components which need to be updated where
each class extends the base component by implementing your Notifier interface.
Your extended classes would implement the method update() to accept the new
data and update the component accordingly. Your worker object can now accept an
argument of type Notifier (rather than having multiple objects each of which
accept a specific component class) and notifies the component when it's
finished via this interface. Don't forget that you need to invoke
Notifier.update() in the EDT if the component which implements it is a Swing
component. Alternatively, the update() methods can protect themselves by
checking that they are executing in the EDT using
SwingUtilities.isEventDispatchThread() and actually perform the update in the
EDT themselves if necessary.

Of course, there's probably a thousand other ways of doing this, with varying
degrees of complexity and "correctness".

Nigel Wade, System Administrator, Space Plasma Physics Group,
            University of Leicester, Leicester, LE1 7RH, UK
E-mail :
Phone : +44 (0)116 2523548, Fax : +44 (0)116 2523555

Actually, if there are serveral different components that need to be
updated, then the Observer pattern can be applied well...

When your LDAP query finishes, it would send an event to all
LDAPQueryEventListener objects.

class LDAPQueryEvent extends java.util.EventObject {

interface LDAPQueryEventListener extends java.util.EventListener {
  void ldapQueryEventFinished(LDAPQueryEvent event);

class LDAPQuerier {
   private final Collection<LDAPQueryEventListener> listeners = new
    private class LDAPWorker implements Runnable {
         private final LDAPQuery query;
         private LDAPWorker(LDAPQuery query) {
             this.query = query;
         public void run() {
             final LDAPQueryEvent result = doWork(query);
             SwingUtilities.invokeLater(new Runnable() {
                 public void run() {
                     for (LDAPQueryEventListener listener: listeners) {

   public void runQuery(LDAPQuery query) {
      new LDAPWorker().start();

Or something to that effect.

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