Develop Reference

The proper way to handle exceptions thrown by the SwingWorker.doInBackground

The proper way to handle exceptions thrown by the doInBackground method of SwingWorker class is to invoke the get method from within the done method, as explained here and here.
The documentation for the get method states the following:
Waits if necessary for the computation to complete, and then retrieves
its result.
Note: calling get on the Event Dispatch Thread blocks all
events, including repaints, from being processed until this
SwingWorker is complete.
Therefore, if the get method causes a waiting within the done method, in fact it would block the Event Dispatch Thread, since the done method is executed on EDT.
However, after performing a simple test of the proposed solution, you may notice that the EDT is not blocked: this behavior occurs because the get method is invoked within the done method, so get is invoked after the result of the operation was calculated, and therefore the call to it will not block the EDT. Is this motivation correct?

Therefore, if the get method causes a waiting within the done method, in fact it would block the Event Dispatch Thread, since the done method is executed on EDT.
Actually, if done is called, the doInBackground has already returned, therefore calling get within done will NOT block the Event Dispatching Thread.
The same goes if you are using the PropertyChangeListener support and monitoring the change in state to DONE
Updated
So, after having a look at SwingWorker calls 'done' before the 'doInBackground' is finished, which would mean that calling get on a cancelled worker would block the EDT indeventially, the basic work around is actually to ignore the return result by checking the SwingWorker#isCancelled state. Let's face it, if the worker is cancelled, the return result is unknown/undefined, so it's best NOT to try and get it.
As an example (based on the code from the bug)
import java.util.concurrent.ExecutionException;
import javax.swing.SwingWorker;
public class Main {
public static void main(String[] args) throws InterruptedException {
SwingWorker<String, String> worker = new SwingWorker<String, String>() {
#Override
protected String doInBackground() throws Exception {
try {
while (!Thread.currentThread().isInterrupted()) {
System.out.println("Working...");
Thread.sleep(1000);
}
} catch (InterruptedException ex) {
System.out.println("Got interrupted!");
}
try {
System.out.println("Cleaning up");
Thread.sleep(10000);
System.out.println("Done cleaning");
} catch (InterruptedException ex) {
System.out.println("Got interrupted second time!");
}
return null;
}
#Override
protected void done() {
System.out.println("Done");
if (!isCancelled()) {
long start = System.currentTimeMillis();
try {
get();
} catch (InterruptedException | ExecutionException ex) {
ex.printStackTrace();
}
long end = System.currentTimeMillis();
System.out.println("Took " + ((end - start) / 1000d));
} else {
System.out.println("Was cancelled");
}
}
};
worker.execute();
Thread.sleep(10000);
worker.cancel(true);
Thread.sleep(20000);
}
}

#Override
protected void done()
{
try
{
if(!super.isCancelled())
{
super.get();
}
}
catch(Exception ex)
{
ex.printStackTrace();
}
}

Java Swing Thread changing UI - complications

I tried this for many hours.. I have a thread that changes a JTextField of my UI, which completely destroys the UI. The Thread (lets call it Thread A) is generated by an ActionListener. The .setText() function call is in a extra thread (B) created by Thread A. Thread B is the Parameter of SwingUtilitis.invokeAll() and/or SwingUtilities.invokeAndWait(). I tried them both. Here's some code to make it more clear.
This is my ActionListener which creates Thread A - shortened of course:
public void actionPerformed(ActionEvent evt) {
Object source = evt.getSource();
if (source == window.getBtn_Search()) {
Refresher refresh = new Refresher();
refresh.start();
}
}
This is my Thread A, which later puts Thread B into the EDT Queue:
public class Refresher extends Thread implements Runnable {
private int counter = 0;
private UI window = null;
private int defRefresh = 0;
#Override
public void run() {
while(true){
-bazillion lines of code-
do {
try {
Refresher.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if(window.canceled()) break;
UI.updateCounter(window.getLbl_Status(), (Configuration.getRefreshTime()-counter));
counter++;
} while (counter <= Configuration.getRefreshTime());
- more code-
}
}
}
The UI.updateCounter(...) will queue Thread B into the EDT.
public static void updateCounter(final JLabel label, final int i) {
try {
SwingUtilities.invokeAndWait(
new Runnable() {
public void run() {
label.setText("Refreshing in: " + i + " seconds.");
}
}
);
} catch (InvocationTargetException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
Now when the last function gets called, everything gets messed up. I tried different stuff for hours and nothing worked. I also tried using SwingWorker, but the some or nothing at all happened.

The invokeAndWait() tried allows to post a Runnable task to be executed on the EDT, but it blocks the current thread and waits until the EDT is done executing the task.
But there is deadlock potential in invokeAndWait(), as there is in any code that creates a thread interdependency.
If the calling code holds some lock (explicitly or implicitly) that the code called
through invokeAndWait() requires, then the EDT code will wait for the non-
EDT code to release the lock, which cannot happen because the non-EDT code
is waiting for the EDT code to complete, and the application will hang.
As we can see here, modifying the JLabel component passed by the waiting non-
EDT code.
Instead we can use
invokeLater() takes
care of creating and queuing a special event that contains the Runnable. This event is processed on the EDT in the order it was received, just like any other event.
When its time comes, it is dispatched by running the Runnable’s run() method.
SwingUtilities.invokeLater(new Runnable() {
public void run() {
label.setText("Refreshing in: " + i + " seconds.");
}
});
OR
isEventDispatchThread() that returns true if the calling code is currently being executed on the EDT, false otherwise.
Runnable code= new Runnable() {
public void run() {
label.setText("Refreshing in: " + i + " seconds.");
}
}
);
if (SwingUtilities.isEventDispatchThread()) {
code.run();
} else {
SwingUtilities.invokeLater(code);
}

In general, labels are not very good at displaying text which change: their width change, and the layout with it.
Using a read-only JTextField, perhaps with proper changes in style, could be a better solution.

I think the intermediate JPanels you've created may count as validation roots. Therefore the revalidate() that automagically happens when you call setText() does not cause any layout changes higher than the level of the JPanel parent.
I don't think you actually need the panels, since a JLabel can contain both an Icon and text. See the tutorial.
So my advice is to remove the panels or, if they serve a purpose, make sure isValidateRoot() on the panels returns false.

When changing the label's text you should at least call repaint()/revalidate() on the label's topmost container, triggering a relayout, assuming the label calls invalidate()/revalidate() correctly on text change.

Infinite loop in a JPanel

I want to program a simple Snake.
Therefore I have programmed a custom JPanel, which can hold a Scene.
A Scene just draws something, and you can thread it with the public void run() method, so it implements Runnable.
Now, when I initialise the Scene, I create a Thread of the instance.
if (this.getThread() == null) {
Thread sceneThread = new Thread(this);
this.setThread(sceneThread);
this.getThread().run();
} else {
System.err.println("Scene is already running");
}
And the scene finally begins to be executed in a separate thread:
// Run thread
public void run () {
try {
while (true) {
this.update();
this.getGamePanel().sceneShouldRepaint();
Thread.sleep(this.getFps());
}
}
catch (Exception e) {
System.err.println(e);
}
}
Somehow this is blocking the windows thread.
It does not appear anymore.
Can anyone tell me why?

You are not starting the thread but directly invoke its run method, thus you are blocking the event thread itself in an endless loop - try starting it by calling start() instead.
Plus be sure to read about multithreading in Swing applications as pointed out by Qwerky.

SwingWorker ends, PropertyChangeListener listens, but how do I get back to EDT?

I've already browsed many threads on this topic, but nothing seems to fit my specific situation.
I have a swing application which analyzes QR-Codes, extracts the found loginNames and makes DB calls to fetch data for that user. To make sure the capture of the QR-Codes can be canceled and my application is still accessible while capturing, I used a SwingWorker for this purpose. Everything works fine so far, I've included a PropertyChangeListener so the application knows when my SwingWorker successfully read a code. But since I don't want to have the PropertyChangeListener as a nested class within my mainClass (to keep it well structured), I've created a new class for it outside. Now I want to return to my main class from this PropertyChangeListener class to switch to the appropriate panel which displays the fetched data. I have different codes which can be read, so depending on the code I have different panels to switch to (so I can't do a static switch to the same panel over and over again). So how can I delegate the PropertyChangeListener to give the control back to my EDT ?
I've tried using wait() and notify() to let my EDT know the SwingWorker finished. But obviously wait() blocks my EDT and the use of SwingWorker is pointless.
I hope I could explain my problem in enough detail, and some of you have a good idea to deal with this.
For any code snippets please ask, I'll then add the necessary ones. But since my project is a bit more complex I'll just post what is asked for.
Thanks in advance for any help :)
EDIT: Here is a code excerpt to illustrate what my SwingWorker is doing.
SwingWorker class:
public class CodeDetector extends SwingWorker<byte[], String> {
String s; // read String
byte[] completeCode; // byte[] which is returned by doInBackground()
BufferedImage resizedImg;
IplImage img;
JLabel labelForStream;
JLabel result;
FrameGrabber grabber = new VideoInputFrameGrabber(); // using JavaCV.
public CodeDetector(JLabel labelForStream, JLabel result) {
this.labelForStream = labelForStream;
this.resultLabel = result;
}
#Override
protected byte[] doInBackground() throws Exception {
try {
grabber.start(); //
while (true) {
// End if current thread was canceled.
if (Thread.currentThread().isInterrupted()) {
return null;
}
// Grab each image, save it, scan for code and display it.
img = grabber.grab();
resizedImg = // resizing image to fit labelForStream.
// save resizedImg to file
// read barcode from saved file
if (isBadgeCode(tmp) || isDeviceCode(tmp)) {
s = tmp;
} else {
continue;
}
break;
} catch (NotFoundException e) {
// Code could not be encoded yet.
}
...
// end worker after timeout
// show image on window
if (img != null) {
labelForStream.setIcon(new ImageIcon(resizedImg));
}
}
}
} catch (Exception e) {
System.err.println("Error: " + e.getMessage() + " - " + e.getStackTrace() + " - " + e.getClass());
}
return s != null ? s.getBytes() : null;
}
#Override
protected void done() {
try {
completeCode = get();
if (completeCode != null) {
String code = new String(completeCode);
if (isOtherCode(code)) {
resultLabel.setText(code);
} else if (isUsernameCode(code)) {
// Cut userName from read code (if previously verified) and set label text.
resultLabel.setText(verify(code, true) ? code.split(":")[0] : null);
}
} else {
resultLabel.setText(null);
}
resultLabel.setVisible(true);
resultLabel.updateUI();
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
} catch (CancellationException e) {
return;
} catch (Exception e) {
e.printStackTrace();
}
}
As this SwingWorker doesn't have references to any panels, even though the done()-method is done in EDT, I need to somehow notify my mainClass that a code has been read successfully and that it can change the panl now according to the specific code.
Hope this clears things up a bit.

I think that you misunderstood, for what reasons SwingWorker is there, please read SwingWorker
tutorial, where implementations quite guarentee that output from methods:
done()
process()
publish()
setProgress()
should be done on EDT

The simple swing worker answer is to override the done() method. That is executed on the EDT - SwingWorker takes care of that for you.
You can do it yourself, using SwingUtilities.invokeLater.
For the way you pose your question, I suspect you don't have a full grasp of the threading issues and how you switch between threads. So a good review of the tutorial (if you haven't already) may be in order.

This is wrong:
protected byte[] doInBackground() throws Exception {
// ....
if (img != null) {
labelForStream.setIcon(new ImageIcon(resizedImg));
}
// ....
}
As this shows you making critical Swing calls from within the doInBackground method, something that should never be done. Instead consider publishing the Image or the ImageIcon, and setting the JLabel's Icon from the process method override.
As I note in my comment, sometimes its a good idea to use a PropertyChangeListener with your SwingWorker if it decreases code coupling. This is one reason that SwingWorker has its own PropertyChangeSupport and its own state enum.

JButton stays in pressed state

In my Java GUI app I have a JButton and when clicked it calls a function to connect to a database, then calls a function to clear a table in the DB, then calls a function that reads text from one file and loads variables, which calls a function that reads text from another file, compares the data from both and then calls a function to either update or insert data in the DB, all of that works fine.
However my question is related to the JButton, when its clicked I want to run a Indeterminate progress bar just so the user knows work is being done and then right before it leaves the the action listener setIndeterminate to false and set the value of the progress bar to 100(complete), but in my case when you click the button it stays in the clicked state and the progress bar freezes.
What should I implement to prevent this? threading possibly? but Im quite new to threading in java. here is my action listener:
private class buttonListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
if( e.getSource() == genButton )
{
progressBar.setIndeterminate(true);
progressBar.setString(null);
try
{
dbConnect(); //connects to DB
clearSchedules(); // deletes data in tables
readFile(); // reads first file and calls the other functions
dbClose();// closes the DB
progressBar.setIndeterminate(false);
progressBar.setValue(100);
}
catch (Exception e1){
System.err.println("Error: " + e1.getMessage());
}
}
}
}
On a side note, I would like to have the action bar actually move as the the program progresses but I wasnt sure how to monitor its progress.
Thanks, Beef.
UPDATE here is my example of SwingWorker and how I used it:
Declared globally
private functionWorker task;
private abstract class functionWorker extends SwingWorker {
public void execute() {
try {
dbConnect();
} catch (SQLException e) {
e.printStackTrace();
}
clearSchedules();
try {
readFile();
} catch (IOException e) {
e.printStackTrace();
}
dbClose();
}
}
Inside my actionPerformed method
if( e.getSource() == genButton )
{
progressBar.setIndeterminate(true);
progressBar.setString(null);
try
{
task.execute();
progressBar.setIndeterminate(false);
progressBar.setValue(100);
}
catch (Exception e1){
System.err.println("Error: " + e1.getMessage());
}
}

The problem is probably related to connecting to doing expensive operations in the UI thread (connecting to a database, reading from a file, calling other functions). Under no circumstances should you call code that uses excessive CPU time from the UI thread, as the entire interface can't proceed while it is executing your code, and it results in a 'dead' looking application, with components remaining in their state at the time before an expensive operation until completion. You should execute another thread, do the expensive work in that, and then use a SwingUtilities.invokeLater(Runnable doRun) with a passed runnable where you'd update the progress.
There may be synchronisation issues relating to the states of components, but you can fix these later.

Could I create the new thread when the action is performed and call the new functions in the thread, or should I do the threading within the actual function itself?
You can start a SwingWorker from your button's handler, as shown here. A related example implementing Runnable is seen here.

One method to handle progressbars are to extend SwingWorker in a class.
SwingWorker takes care of running background tasks for you and so you do not have to implement your own threading that can end up in unknown issues.
To begin with, your class that takes care of progress bar UI should implement PropertyChangeListener
And implement public void propertyChange(PropertyChangeEvent evt) { - to update the progressbar status based on a global variable.
The background task class should look like the following(this could be an inner class) :
class ProgressTask extends SwingWorker<Void, Void> {
#Override
public Void doInBackground() {
//handle your tasks here
//update global variable to indicate task status.
}
#Override
public void done() {
//re-enabled your button
}
}
on your button's event listener :
public void actionPerformed(ActionEvent evt) {
//disable your button
//Create new instance of "ProgressTask"
//make the task listen to progress changes by task.addPropertyChangeListener(this);
//calll task.execute();
}
I have tried to water down code example, you would have to read some tutorial to understand how all these pieces fit together. However, the main point is do not code your Threads, instead use SwingWorker

progressBar.setIndeterminate(true);
progressBar.setValue(0);
dbConnect(); //connects to DB
progressBar.setIndeterminate(true);
progressBar.setValue(10);
clearSchedules(); // deletes data in tables
progressBar.setIndeterminate(true);
progressBar.setValue(50);
readFile(); // reads first file and calls the other functions
progressBar.setIndeterminate(true);
progressBar.setValue(75);
dbClose();// closes the DB
progressBar.setIndeterminate(false);
progressBar.setValue(100);
You will need to tell the progress bar how much progress has been made because it does not know the percentage completed. Better yet, write a method that updates and repaints the progress bar rather than repeating the method calls here.
updateProgressBar(int progress, boolean isDeterminate, String msg){};
You will also need to make sure that your specific button is firing the action performed.
class IvjEventHandler implements java.awt.event.ActionListener {
public void actionPerformed(java.awt.event.ActionEvent e) {
if (e.getSource() == JMyPanel.this.getJButtonUpdate())
connEtoC1(e);
};
};
The connEtoC1(e); should execute a controller class or SwingWorker rather than firing from the GUI