Class SwingWorker<T,V>
- Type Parameters:
T- the result type returned by thisSwingWorker'sdoInBackgroundandgetmethodsV- the type used for carrying out intermediate results by thisSwingWorker'spublishandprocessmethods
- All Implemented Interfaces:
Runnable,Future<T>,RunnableFuture<T>
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 :
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Current thread: The
execute()method is called on this thread. It schedulesSwingWorkerfor the execution on a worker thread and returns immediately. One can wait for theSwingWorkerto complete using thegetmethods. -
Worker thread: The
doInBackground()method is called on this thread. This is where all background activities should happen. To notifyPropertyChangeListenersabout bound properties changes use thefirePropertyChangeandgetPropertyChangeSupport()methods. By default there are two bound properties available:stateandprogress. -
Event Dispatch Thread: All Swing related activities occur on this thread.
SwingWorkerinvokes theprocessanddone()methods and notifies anyPropertyChangeListenerson 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.
- Since:
- 1.6
-
Nested Class Summary
Nested ClassesModifier and TypeClassDescriptionstatic enumValues for thestatebound property.Nested classes/interfaces declared in interface java.util.concurrent.Future
Future.State -
Constructor Summary
Constructors -
Method Summary
Modifier and TypeMethodDescriptionfinal voidAdds aPropertyChangeListenerto the listener list.final booleancancel(boolean mayInterruptIfRunning) Attempts to cancel execution of this task.protected abstract TComputes a result, or throws an exception if unable to do so.protected voiddone()Executed on the Event Dispatch Thread after thedoInBackgroundmethod is finished.final voidexecute()Schedules thisSwingWorkerfor execution on a worker thread.final voidfirePropertyChange(String propertyName, Object oldValue, Object newValue) Reports a bound property update to any registered listeners.final Tget()Waits if necessary for the computation to complete, and then retrieves its result.final TWaits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.final intReturns theprogressbound property.final PropertyChangeSupportReturns thePropertyChangeSupportfor thisSwingWorker.final SwingWorker.StateValuegetState()Returns theSwingWorkerstate bound property.final booleanReturnstrueif this task was cancelled before it completed normally.final booleanisDone()Returnstrueif this task completed.protected voidReceives data chunks from thepublishmethod asynchronously on the Event Dispatch Thread.protected final voidSends data chunks to theprocess(java.util.List<V>)method.final voidRemoves aPropertyChangeListenerfrom the listener list.final voidrun()Sets thisFutureto the result of computation unless it has been cancelled.protected final voidsetProgress(int progress) Sets theprogressbound property.Methods declared in class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, waitMethods declared in interface java.util.concurrent.Future
exceptionNow, resultNow, state
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Constructor Details
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SwingWorker
public SwingWorker()Constructs thisSwingWorker.
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Method Details
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doInBackground
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run
public final void run()Sets thisFutureto the result of computation unless it has been cancelled.- Specified by:
runin interfaceRunnable- Specified by:
runin interfaceRunnableFuture<T>
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publish
Sends data chunks to theprocess(java.util.List<V>)method. This method is to be used from inside thedoInBackgroundmethod to deliver intermediate results for processing on the Event Dispatch Thread inside theprocessmethod.Because the
processmethod is invoked asynchronously on the Event Dispatch Thread multiple invocations to thepublishmethod might occur before theprocessmethod 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");might result in:process("1", "2", "3", "4", "5", "6")Sample Usage. This code snippet loads some tabular data and updates
DefaultTableModelwith it. Note that it safe to mutate the tableModel from inside theprocessmethod 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; }@Overrideprotected DefaultTableModel doInBackground() throws Exception { for (Object[] row = loadData(); ! isCancelled() && row != null; row = loadData()) { publish((Object[]) row); } return tableModel; }@Overrideprotected void process(List<Object[]> chunks) { for (Object[] row : chunks) { tableModel.addRow(row); } } }- Parameters:
chunks- intermediate results to process- See Also:
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process
Receives data chunks from thepublishmethod asynchronously on the Event Dispatch Thread.Please refer to the
publish(V...)method for more details.- Parameters:
chunks- intermediate results to process- See Also:
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done
protected void done()Executed on the Event Dispatch Thread after thedoInBackgroundmethod is finished. The default implementation does nothing. Subclasses may override this method to perform completion actions on the Event Dispatch Thread. Note that you can query status inside the implementation of this method to determine the result of this task or whether this task has been cancelled.- See Also:
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setProgress
protected final void setProgress(int progress) Sets theprogressbound property. The value should be from 0 to 100.Because
PropertyChangeListeners are notified asynchronously on the Event Dispatch Thread multiple invocations to thesetProgressmethod might occur before anyPropertyChangeListenersare invoked. For performance purposes all these invocations are coalesced into one invocation with the last invocation argument only.For example, the following invocations:
setProgress(1); setProgress(2); setProgress(3);
might result in a singlePropertyChangeListenernotification with the value3.- Parameters:
progress- the progress value to set- Throws:
IllegalArgumentException- is value not from 0 to 100
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getProgress
public final int getProgress()Returns theprogressbound property.- Returns:
- the progress bound property.
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execute
public final void execute()Schedules thisSwingWorkerfor execution on a worker thread. There are a number of worker threads available. In the event all worker threads are busy handling otherSwingWorkersthisSwingWorkeris placed in a waiting queue.Note:
SwingWorkeris only designed to be executed once. Executing aSwingWorkermore than once will not result in invoking thedoInBackgroundmethod twice. -
cancel
public final boolean cancel(boolean mayInterruptIfRunning) Attempts to cancel execution of this task. This method has no effect if the task is already completed or cancelled, or could not be cancelled for some other reason. Otherwise, if this task has not started whencancelis called, this task should never run. If the task has already started, then themayInterruptIfRunningparameter determines whether the thread executing this task (when known by the implementation) is interrupted in an attempt to stop the task.The return value from this method does not necessarily indicate whether the task is now cancelled; use
Future.isCancelled().- Specified by:
cancelin interfaceFuture<T>- Parameters:
mayInterruptIfRunning-trueif the thread executing this task should be interrupted (if the thread is known to the implementation); otherwise, in-progress tasks are allowed to complete- Returns:
falseif the task could not be cancelled, typically because it has already completed;trueotherwise. If two or more threads cause a task to be cancelled, then at least one of them returnstrue. Implementations may provide stronger guarantees.
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isCancelled
public final boolean isCancelled()Returnstrueif this task was cancelled before it completed normally.- Specified by:
isCancelledin interfaceFuture<T>- Returns:
trueif this task was cancelled before it completed
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isDone
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get
Waits if necessary for the computation to complete, and then retrieves its result.Note: calling
geton the Event Dispatch Thread blocks all events, including repaints, from being processed until thisSwingWorkeris complete.When you want the
SwingWorkerto block on the Event Dispatch Thread we recommend that you use a modal dialog.For example:
class SwingWorkerCompletionWaiter implements 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);- Specified by:
getin interfaceFuture<T>- Returns:
- the computed result
- Throws:
CancellationException- if the computation was cancelledInterruptedException- if the current thread was interrupted while waitingExecutionException- if the computation threw an exception
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get
public final T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException 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.- Specified by:
getin interfaceFuture<T>- Parameters:
timeout- the maximum time to waitunit- the time unit of the timeout argument- Returns:
- the computed result
- Throws:
CancellationException- if the computation was cancelledInterruptedException- if the current thread was interrupted while waitingExecutionException- if the computation threw an exceptionTimeoutException- if the wait timed out
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addPropertyChangeListener
Adds aPropertyChangeListenerto 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. Iflistenerisnull, no exception is thrown and no action is taken.Note: This is merely a convenience wrapper. All work is delegated to
PropertyChangeSupportfromgetPropertyChangeSupport().- Parameters:
listener- thePropertyChangeListenerto be added
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removePropertyChangeListener
Removes aPropertyChangeListenerfrom the listener list. This removes aPropertyChangeListenerthat was registered for all properties. Iflistenerwas added more than once to the same event source, it will be notified one less time after being removed. Iflistenerisnull, 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
PropertyChangeSupportfromgetPropertyChangeSupport().- Parameters:
listener- thePropertyChangeListenerto be removed
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firePropertyChange
Reports a bound property update to any registered listeners. No event is fired ifoldandneware equal and non-null.This
SwingWorkerwill be the source for any generated events.When called off the Event Dispatch Thread
PropertyChangeListenersare notified asynchronously on the Event Dispatch Thread.Note: This is merely a convenience wrapper. All work is delegated to
PropertyChangeSupportfromgetPropertyChangeSupport().- Parameters:
propertyName- the programmatic name of the property that was changedoldValue- the old value of the propertynewValue- the new value of the property
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getPropertyChangeSupport
Returns thePropertyChangeSupportfor thisSwingWorker. This method is used when flexible access to bound properties support is needed.This
SwingWorkerwill be the source for any generated events.Note: The returned
PropertyChangeSupportnotifies anyPropertyChangeListeners asynchronously on the Event Dispatch Thread in the event thatfirePropertyChangeorfireIndexedPropertyChangeare called off the Event Dispatch Thread.- Returns:
PropertyChangeSupportfor thisSwingWorker
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getState
Returns theSwingWorkerstate bound property.- Returns:
- the current state
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