An abstract class to perform lengthy GUI-interaction tasks in a background thread. Several background threads can be used to execute such tasks. However, the exact strategy of choosing a thread for any particular
SwingWorker
is unspecified and should not be relied on.
When writing a multi-threaded application using Swing, there are two constraints to keep in mind: (refer to Concurrency in Swing for more details):
These constraints mean that a GUI application with time intensive computing needs at least two threads: 1) a thread to perform the lengthy task and 2) the Event Dispatch Thread (EDT) for all GUI-related activities. This involves inter-thread communication which can be tricky to implement.
SwingWorker is designed for situations where you need to have a long running task run in a background thread and provide updates to the UI either when done, or while processing. Subclasses of SwingWorker must implement the doInBackground() method to perform the background computation.
Workflow
There are three threads involved in the life cycle of a SwingWorker :
Current thread: The execute() method is called on this thread. It schedules SwingWorker for the execution on a worker thread and returns immediately. One can wait for the SwingWorker to complete using the get methods.
Worker thread: The doInBackground() method is called on this thread. This is where all background activities should happen. To notify PropertyChangeListeners about bound properties changes use the firePropertyChange and getPropertyChangeSupport() methods. By default there are two bound properties available: state and progress.
Event Dispatch Thread: All Swing related activities occur on this thread. SwingWorker invokes the process and done() methods and notifies any PropertyChangeListeners on this thread.
Often, the Current thread is the Event Dispatch Thread.
Before the doInBackground method is invoked on a worker thread, SwingWorker notifies any PropertyChangeListeners about the state property change to StateValue.STARTED. After the doInBackground method is finished the done method is executed. Then SwingWorker notifies any PropertyChangeListeners about the state property change to StateValue.DONE.
SwingWorker is only designed to be executed once. Executing a SwingWorker more than once will not result in invoking the doInBackground method twice.
Sample Usage
The following example illustrates the simplest use case. Some processing is done in the background and when done you update a Swing component.
Say we want to find the "Meaning of Life" and display the result in a JLabel.
final JLabel label;
class MeaningOfLifeFinder extends SwingWorker<String, Object> {
@Override
public String doInBackground() {
return findTheMeaningOfLife();
}
@Override
protected void done() {
try {
label.setText(get());
} catch (Exception ignore) {
}
}
}
(new MeaningOfLifeFinder()).execute();
The next example is useful in situations where you wish to process data as it is ready on the Event Dispatch Thread.
Now we want to find the first N prime numbers and display the results in a JTextArea. While this is computing, we want to update our progress in a JProgressBar. Finally, we also want to print the prime numbers to System.out.
class PrimeNumbersTask extends
SwingWorker<List<Integer>, Integer> {
PrimeNumbersTask(JTextArea textArea, int numbersToFind) {
//initialize
}
@Override
public List<Integer> doInBackground() {
while (! enough && ! isCancelled()) {
number = nextPrimeNumber();
publish(number);
setProgress(100 * numbers.size() / numbersToFind);
}
}
return numbers;
}
@Override
protected void process(List<Integer> chunks) {
for (int number : chunks) {
textArea.append(number + "\n");
}
}
}
JTextArea textArea = new JTextArea();
final JProgressBar progressBar = new JProgressBar(0, 100);
PrimeNumbersTask task = new PrimeNumbersTask(textArea, N);
task.addPropertyChangeListener(
new PropertyChangeListener() {
public void propertyChange(PropertyChangeEvent evt) {
if ("progress".equals(evt.getPropertyName())) {
progressBar.setValue((Integer)evt.getNewValue());
}
}
});
task.execute();
System.out.println(task.get()); //prints all prime numbers we have got
Because SwingWorker implements Runnable, a SwingWorker can be submitted to an Executor for execution.
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