Swing invokeandwait - swing

Question

I am learning Swing and created a sample GUI. I am trying to achieve the following in exact order...
The user enters text into some text fields.
The user clicks a "launch" button.
The "launch" button becomes disabled.
A background thread spawns and processes the text from text fields.
the background thread finishes.
the "launch" button becomes enabled again.
I am trying to use invokeandwait as can be seen below but I get "Cannot call invokeAndWait from the event dispatcher thread". My main method is in the same .class file and I'm not too sure what exactly the "event dispatcher thread" is. Whats the best approach for something like this, do I need to setup some kind of alert in my worker thread to route back to the "event dispatcher thread"?
LaunchButton code
private void launchButtonActionPerformed(java.awt.event.ActionEvent evt) {
launchButton.setEnabled(false);
try {
java.awt.EventQueue.invokeAndWait(new MyTestThread());
} catch (Exception e) {
}
launchButton.setEnabled(true);
}
Worker Thread
public class MyTestThread extends Thread {
private int i = 0;
public void run() {
while (i < 5) {
try {
System.out.println(i++);
sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
Solution
Worker Thread
public class WorkerThread extends SwingWorker<Integer[], Void> {
#Override
public Integer[] doInBackground() {
System.out.println("Doing in background");
for (int i = 0; i < 5; i++) {
try {
Thread.sleep(1000);
System.out.println("Doing in background" + i);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
return null;
}
#Override
public void done() {
System.out.println("Swingworker is Done");
}
}
Starting the worker thread from my Event Dispatch Thread (EDT)
new WorkerThread().execute();

Answer 1

The event dispatch thread (EDT) is the only thread allowed to access Swing classes and (almost all) methods. All the initizalization and events of the GUI are executed by the EDT. To execute something in the EDT you have to use SwingUtilities.invokeLater or invokeAndWait (this is equivalent to EventQueue.invokeXXX). This is why all Swing programs start with SwingUtilities.invokeLater() in the main: to execute the GUI initialization in the EDT.
While the EDT is busy the UI freezes that's why background threads are useful. When you need to do a big amount of work independent from the UI (calculations, I/O, transmission, ...) you have to use "worker threads".
For more about threading in Swing see this tutorial: http://docs.oracle.com/javase/tutorial/uiswing/concurrency/
Now, what you are trying to accomplish is right but the tools you are using aren't.
You need to know two things: how to handle events (like button presses) and how to create background threads.
For the first one see this tutorial:
http://docs.oracle.com/javase/tutorial/uiswing/components/button.html
You just need to add anActionListener to a button and whenever the button throws an event the listener's actionPerformed method will be executed, in the EDT.
Minimal example:
JButton button = new JButton("Test button");
button.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent event)
{
System.out.println("The button was pressed.");
}
}
The event variable contains useful information, for example event.getSource() return the object that threw the event, in this case the button.
Now what you want to do when the button is pressed is create a worker thread. Worker threads are created using the SwingWorker class, as you've seen in the concurrency tutorial. There you can define a piece of code that will be executed in the background thread (in the doInBackground() method) and a piece of code that will be executed in the EDT after the work in the background is done (in the done() method).
So you'd want to do something like this:
private static JButton _button;
//...
_button = new JButton("Test button");
_button.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent event)
{
System.out.println("The button was pressed.");
_button.setEnabled(false);
SwingWorker worker = new SwingWorker()
{
#Override
protected Object doInBackground() throws Exception
{
//do something useful in the background thread
return null;
}
#Override
protected void done()
{
_button.setEnabled(true);
}
}
worker.execute();
}
}
There's a lot of information out there about this. Read the Java reference for the classes, read the official tutorials and search in SO. Another good tutorial about SwingWorkers is: http://www.javacreed.com/swing-worker-example/