SwingWorker (Java Platform SE 6)

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Standard Ed. 6

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javax.swing
Class SwingWorker<T,V>

java.lang.Object
 extended by javax.swing.SwingWorker<T,V>


Type Parameters:: T - the result type returned by this SwingWorker's doInBackground and get methods; V - the type used for carrying out intermediate results by this SwingWorker's publish and process methods

All Implemented Interfaces:: Runnable, Future<T>, RunnableFuture<T>

public abstract class SwingWorker<T,V>
extends Object
implements RunnableFuture<T>
extends Object
implements RunnableFuture<T>

An abstract class to perform lengthy GUI-interacting tasks in a dedicated thread.

When writing a multi-threaded application using Swing, there are two constraints to keep in mind: (refer to How to Use Threads 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.

Since:: 1.6

Nested Class Summary
`static class`	`SwingWorker.StateValue` Values for the `state` bound property.

Constructor Summary
`SwingWorker()` Constructs this `SwingWorker`.

Method Summary
`void`	`addPropertyChangeListener(PropertyChangeListener listener)` Adds a `PropertyChangeListener` to the listener list.
`boolean`	`cancel(boolean mayInterruptIfRunning)` Attempts to cancel execution of this task.
`protected abstract T`	`doInBackground()` Computes a result, or throws an exception if unable to do so.
`protected void`	`done()` Executed on the Event Dispatch Thread after the `doInBackground` method is finished.
`void`	`execute()` Schedules this `SwingWorker` for execution on a worker thread.
`void`	`firePropertyChange(String propertyName, Object oldValue, Object newValue)` Reports a bound property update to any registered listeners.
`T`	`get()` Waits if necessary for the computation to complete, and then retrieves its result.
`T`	`get(long timeout, TimeUnit unit)` Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
`int`	`getProgress()` Returns the `progress` bound property.
`PropertyChangeSupport`	`getPropertyChangeSupport()` Returns the `PropertyChangeSupport` for this `SwingWorker`.
`SwingWorker.StateValue`	`getState()` Returns the `SwingWorker` state bound property.
`boolean`	`isCancelled()` Returns `true` if this task was cancelled before it completed normally.
`boolean`	`isDone()` Returns `true` if this task completed.
`protected void`	`process(List<V> chunks)` Receives data chunks from the `publish` method asynchronously on the Event Dispatch Thread.
`protected void`	`publish(V... chunks)` Sends data chunks to the `process(java.util.List)` method.
`void`	`removePropertyChangeListener(PropertyChangeListener listener)` Removes a `PropertyChangeListener` from the listener list.
`void`	`run()` Sets this `Future` to the result of computation unless it has been cancelled.
`protected void`	`setProgress(int progress)` Sets the `progress` bound property.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

SwingWorker

public SwingWorker()

Constructs this SwingWorker.

Method Detail

doInBackground

protected abstract T doInBackground()
 throws Exception

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.

Returns:: the computed result
Throws:: Exception - if unable to compute a result

run

public final void run()

Sets this Future to the result of computation unless it has been cancelled.

Specified by:: run in interface Runnable
Specified by:: run in interface RunnableFuture<T>

See Also:: Thread.run()

publish

protected final void publish(V... chunks)

Sends data chunks to the process(java.util.List) 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");

might result in:

 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);
 }
 }
 }

Parameters:: chunks - intermediate results to process
See Also:: process(java.util.List)

process

protected void process(List<V> chunks)

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

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

Parameters:: chunks - intermediate results to process
See Also:: publish(V...)

done

protected void done()

Executed on the Event Dispatch Thread after the doInBackground method 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:: doInBackground(), isCancelled(), get()

setProgress

protected final void setProgress(int progress)

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);

might result in a single PropertyChangeListener notification with the value 3.

Parameters:: progress - the progress value to set
Throws:: IllegalArgumentException - is value not from 0 to 100

getProgress

public final int getProgress()

Returns the progress bound property.

Returns:: the progress bound property.

execute

public final void execute()

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.

cancel

public final boolean cancel(boolean mayInterruptIfRunning)

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.

Specified by:: cancel in interface Future<T>

Parameters:: mayInterruptIfRunning - true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete
Returns:: false if the task could not be cancelled, typically because it has already completed normally; true otherwise

isCancelled

public final boolean isCancelled()

Returns true if this task was cancelled before it completed normally.

Specified by:: isCancelled in interface Future<T>

Returns:: true if this task was cancelled before it completed

isDone

public final boolean isDone()

Returns true if this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will return true.

Specified by:: isDone in interface Future<T>

Returns:: true if this task completed

get

public final T get()
 throws InterruptedException,
 ExecutionException

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);

Specified by:: get in interface Future<T>

Returns:: the computed result
Throws:: InterruptedException - if the current thread was interrupted while waiting; ExecutionException - if the computation threw an exception

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:: get in interface Future<T>

Parameters:: timeout - the maximum time to wait; unit - the time unit of the timeout argument
Returns:: the computed result
Throws:: InterruptedException - if the current thread was interrupted while waiting; ExecutionException - if the computation threw an exception; TimeoutException - if the wait timed out

addPropertyChangeListener

public final void addPropertyChangeListener(PropertyChangeListener listener)

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().

Parameters:: listener - the PropertyChangeListener to be added

removePropertyChangeListener

public final void removePropertyChangeListener(PropertyChangeListener listener)

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().

Parameters:: listener - the PropertyChangeListener to be removed

firePropertyChange

public final void firePropertyChange(String propertyName,
 Object oldValue,
 Object newValue)

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().

Parameters:: propertyName - the programmatic name of the property that was changed; oldValue - the old value of the property; newValue - the new value of the property

getPropertyChangeSupport

public final PropertyChangeSupport 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.

Returns:: PropertyChangeSupport for this SwingWorker

getState

public final SwingWorker.StateValue getState()

Returns the SwingWorker state bound property.

Returns:: the current state

Overview

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javax.swing Class SwingWorker<T,V>

SwingWorker

doInBackground

run

publish

process

done

setProgress

getProgress

execute

cancel

isCancelled

isDone

get

get

addPropertyChangeListener

removePropertyChangeListener

firePropertyChange

getPropertyChangeSupport

getState

javax.swing
Class SwingWorker<T,V>