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):

• Time-consuming tasks should not be run on the Event Dispatch Thread. Otherwise the application becomes unresponsive.
• Swing components should be accessed on the Event Dispatch Thread only.

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.

Computes a result, or throws an exception if unable to do so.

Note that this method is executed only once.

Note: this method is executed in a background thread.

Sends data chunks to the process(java.util.List<V>) method. This method is to be used from inside the doInBackground method to deliver intermediate results for processing on the Event Dispatch Thread inside the process method.

Because the process method is invoked asynchronously on the Event Dispatch Thread multiple invocations to the publish method might occur before the process method is executed. For performance purposes all these invocations are coalesced into one invocation with concatenated arguments.

For example:

publish("1");
 publish("2", "3");
 publish("4", "5", "6");

process("1", "2", "3", "4", "5", "6")

Sample Usage. This code snippet loads some tabular data and updates DefaultTableModel with it. Note that it safe to mutate the tableModel from inside the process method because it is invoked on the Event Dispatch Thread.

class TableSwingWorker extends
         SwingWorker<DefaultTableModel, Object[]> {
     private final DefaultTableModel tableModel;

     public TableSwingWorker(DefaultTableModel tableModel) {
         this.tableModel = tableModel;
     }

      @Override
     protected DefaultTableModel doInBackground() throws Exception {
         for (Object[] row = loadData();
                  ! isCancelled() && row != null;
                  row = loadData()) {
             publish((Object[]) row);
         }
         return tableModel;
     }

      @Override
     protected void process(List<Object[]> chunks) {
         for (Object[] row : chunks) {
             tableModel.addRow(row);
         }
     }
 }

Receives data chunks from the publish method asynchronously on the Event Dispatch Thread.

Please refer to the publish(V...) method for more details.

Sets the progress bound property. The value should be from 0 to 100.

Because PropertyChangeListeners are notified asynchronously on the Event Dispatch Thread multiple invocations to the setProgress method might occur before any PropertyChangeListeners are invoked. For performance purposes all these invocations are coalesced into one invocation with the last invocation argument only.

For example, the following invokations:

setProgress(1);
 setProgress(2);
 setProgress(3);

Schedules this SwingWorker for execution on a worker thread. There are a number of worker threads available. In the event all worker threads are busy handling other SwingWorkers this SwingWorker is placed in a waiting queue.

Note: SwingWorker is only designed to be executed once. Executing a SwingWorker more than once will not result in invoking the doInBackground method twice.

Attempts to cancel execution of this task. This attempt will fail if the task has already completed, has already been cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

After this method returns, subsequent calls to Future.isDone() will always return true. Subsequent calls to Future.isCancelled() will always return true if this method returned true.

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.

When you want the SwingWorker to block on the Event Dispatch Thread we recommend that you use a modal dialog.

For example:

class SwingWorkerCompletionWaiter extends PropertyChangeListener {
     private JDialog dialog;

     public SwingWorkerCompletionWaiter(JDialog dialog) {
         this.dialog = dialog;
     }

     public void propertyChange(PropertyChangeEvent event) {
         if ("state".equals(event.getPropertyName())
                 && SwingWorker.StateValue.DONE == event.getNewValue()) {
             dialog.setVisible(false);
             dialog.dispose();
         }
     }
 }
 JDialog dialog = new JDialog(owner, true);
 swingWorker.addPropertyChangeListener(
     new SwingWorkerCompletionWaiter(dialog));
 swingWorker.execute();
 //the dialog will be visible until the SwingWorker is done
 dialog.setVisible(true);

Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.

Please refer to get() for more details.

Adds a PropertyChangeListener to the listener list. The listener is registered for all properties. The same listener object may be added more than once, and will be called as many times as it is added. If listener is null, no exception is thrown and no action is taken.

Note: This is merely a convenience wrapper. All work is delegated to PropertyChangeSupport from getPropertyChangeSupport().

Removes a PropertyChangeListener from the listener list. This removes a PropertyChangeListener that was registered for all properties. If listener was added more than once to the same event source, it will be notified one less time after being removed. If listener is null, or was never added, no exception is thrown and no action is taken.

Note: This is merely a convenience wrapper. All work is delegated to PropertyChangeSupport from getPropertyChangeSupport().

Reports a bound property update to any registered listeners. No event is fired if old and new are equal and non-null.

This SwingWorker will be the source for any generated events.

When called off the Event Dispatch Thread PropertyChangeListeners are notified asynchronously on the Event Dispatch Thread.

Note: This is merely a convenience wrapper. All work is delegated to PropertyChangeSupport from getPropertyChangeSupport().

Returns the PropertyChangeSupport for this SwingWorker. This method is used when flexible access to bound properties support is needed.

This SwingWorker will be the source for any generated events.

Note: The returned PropertyChangeSupport notifies any PropertyChangeListeners asynchronously on the Event Dispatch Thread in the event that firePropertyChange or fireIndexedPropertyChange are called off the Event Dispatch Thread.