Ricci, Alessandro

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Almae Matris Studiorum Campus

SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & Concurrency 1Sistemi Concorrenti e di Rete LSII Facoltà di Ingegneria - Cesenaa.a 2008/2009[module lab 2.2] GUI FRAMEWORKS & CONCURRENCYv1.0 BETASISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencyGUI FRAMEWORKS & CONCURRENCY• Once upon a time GUI applications were single-threaded...– GUI events processed by a “main event loop”• ..modern GUI Frameworks are not so different– creating a dedicated event dispatch thread (EDT) for handling GUI events– the thread fetches events off a queue and dispatches them to application-defined event handlers• Most of the modern GUI Frameworks are single-threaded– Java, QT, NextStep, Mac OS Cocoa, XWindow...• Multithreaded GUI as a “failed dream” [*]– many attempts, failed due to the generation of problems with race conditions and deadlock2[*] referred in this way  by Sun VP Graham Hamilton in his blog http://weblogs.java.net/blog/kgh/archive/2004/10SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencyMULTITHREADED GUI FRAMEWORKS: THE PROBLEM• Clash between input event processing and OO modelling of GUI components– that can easily lead to deadlocks and race conditions• Inconsistent lock ordering – in managing user-initiated actions and application-initiated actions • user actions “bubble up” from OS to the application– OS mouse click ->mouse click event of the GUI toolkit -> high-level event of the application listener...• application-actions “bubble down” from application to action– changing background color of a component at the application level ->  disptached to component class -> dispatched to OS for rendering,,,• most of the actions need to lock objects– model-view-control (MVC) pattern • the control calls into the model, which notifies the view that something has changed...• ...but the controller calls also the view, which may in turn call back into the model to query the model state3SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencySINGLE-THREADED GUI• Achieving thread-safety via thread-confinement– all GUI objects are accessed exclusively by the event thread• including visual components and data models – the application developer must make sure that these objects are properly confined• Sequential event processing– events like kind of task to be processed sequentially by the event thread• Problems and challenges– if one task takes long time to execute, other task must wait• blocking the overall GUI> ...so tasks that execute in the event-thread must return quickly– to initiate a long-term task a separate thread must be used• es: spell-checking a document, searching the file system> ...but typically a long-term task must provide a visual feedback for indicating progress or when it completes• and this code need to be executed by the event thread...4SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencyTHREAD CONFINEMENT IN SWING• All Swing components (such as JButton and JTable) and data models (e.g. Table Model and Tree Model) are confined to the event thread – any code that access these objects must run in the event thread• Exceptions– Swing methods that can be safely called from any thread• clearly identified in the Javadoc as thread-safe5SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencyTHREAD-SAFE SWING METHODS• Thread-Safe methods– SwingUtilities.isEventDispatchThread• to check if the current thread is the event thread– SwingUtilities.invokeLater• to schedule a Runnable for execution on the event thread – SwingUtilities.invokeAndWait• to schedule a Runnable task for execution on the event thread, blocking the current thread until it completes• cannot be called by the event thread– methods to enqueue and repaint or revalidation request on the event queue– methods for adding or removing listeners• can be called from any thread, but listeners will always be invoked in the event thread• The swing event thread can be thought as a single-thread Executor that processes tasks from the event queue– invokeLater and invokeAndWait used to submit new tasks to execute6SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencyA SIMPLE GUI EXECUTOR• Executor delegating tasks to SwingUtilities for executions7public class GuiExecutor extends AbstractExecutorService {    // Singletons have a private constructor and a public factory    private static final GuiExecutor instance = new GuiExecutor();    private GuiExecutor() { }    public static GuiExecutor instance() { return instance; }    public void execute(Runnable r) {        if (SwingUtilities.isEventDispatchThread())            r.run();        else            SwingUtilities.invokeLater(r);    }    // Plus trivial implementations of lifecycle methods}SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencySHORT-RUNNING GUI TASKS• Can be executed directly by the event thread• Simple example• The control never leaves the event thread– the event originates in the GUI toolkit, is delivered to the application, the application modifies the GUI in response to user’s action8final Random random = new Random();final JButton button = new JButton("Change Color");...button.addActionListener(new ActionListener() {    public void actionPerformed(ActionEvent e) {        button.setBackground(new Color(random.nextInt()));    }});SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencyLONG-RUNNING TASKS• Some of the processing must be offloaded to another thread– exploiting executors• Two main cases– long-term task without visual feedbacks• simple case, quite unfrequent– long-term task with visual feedbacks• complex case, most frequent9SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencyBINDING A TASK WITHOUT VISUAL FEEDBACKS• Exploiting a simple separated executor (or thread)• Example:10ExecutorService backgroundExec = Executors.newCachedThreadPool();...button.addActionListener(new ActionListener() {    public void actionPerformed(ActionEvent e) {        backgroundExec.execute(new Runnable() {            public void run() { doBigComputation(); }        });}});SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencyLONG-RUNNING TASK WITH USER FEEDBACKS• The long-running task must submit another task to run in the event thread whenever the user interface must be updated• Example:11button.addActionListener(new ActionListener() {    public void actionPerformed(ActionEvent e) {        button.setEnabled(false);        label.setText("busy");        backgroundExec.execute(new Runnable() {            public void run() {                try {                    doBigComputation();                } finally {                    GuiExecutor.instance().execute(new Runnable() {                        public void run() {                            button.setEnabled(true);                            label.setText("idle");                        }                    });                }            }        });    }});SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencySwingWorker• Java 6.0 provides auxiliary classes for making it easier to program complex long-term tasks that can interact with the GUI• SwingWorker class– provide a direct support for task cancellation, completion notification and progress indication12class SwingWorker<T,V> implements RunnableFuture<T> {  ...  // to be overridden  protected abstract  T doInBackground();   protected  void  done()  protected final void publish(V... chunks)  protected void process(List<V> chunks);    ...  // to be directly used  boolean cancel(boolean mayInterruptIfRunning);  protected  void setProgress(int progress);    ...  }SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencyTASK EXECUTION AND INTERFACE UPDATE• Support for asynchronous task execution & consequent interface update– doInBackground• encapsulate the computational body of the task to be executed asynchronously w.r.t. GUI activity, computing a result or throwing an exception if unable to do so• executed by some thread, not by the Swing EDT– done• encapsulate the action to do on the GUI when the task completed• executed by the Swing EDT• Support for asynchonous update of interfaces– publish(V... chunks)• used from inside doInBackground to deliver intermediate results for processing on the Event Dispatch Thread inside the process method– process(List<V> chunks);  • receives data chunks from the publish method asynchronously on the EDT13SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & ConcurrencyAN EXAMPLE: SWING WORKER TEST14class CounterTask extends SwingWorker<Integer, Integer> {   protected Integer doInBackground() throws Exception {     int i = 0;    int sum = 0;     int maxCount = 10;     while (!isCancelled() && i < maxCount) {       sum+=i;      i++;       publish(new Integer[] { i });       setProgress(100 * i / maxCount);       Thread.sleep(1000);     }     return sum;   }       protected void process(List<Integer> chunks) {     for (int i : chunks)       System.out.println("Step "+i);   }   protected void done() {     if (isCancelled()){       System.out.println("Task cancelled.");     } else {       System.out.println("Task completed.");     }  } } SISCO LS - II Facoltà Ingegneria - Cesena GUI Frameworks & Concurrency 15public class SwingWorkerTest {  public static void main(String[] args) {    JTextArea textArea = new JTextArea(10, 20);    JProgressBar progressBar = new JProgressBar(0, 100);    CounterTask task = new CounterTask();    JButton startButton = new JButton("Start");    startButton.addActionListener(new ActionListener() {       public void actionPerformed(ActionEvent e) { task.execute();}});    JButton cancelButton = new JButton("Cancel");    cancelButton.addActionListener(new ActionListener() {       public void actionPerformed(ActionEvent e) { task.cancel(true); }});    task.addPropertyChangeListener(new PropertyChangeListener() {       public void propertyChange(PropertyChangeEvent evt) { if ("progress".equals(evt.getPropertyName())) {   progressBar.setValue((Integer) evt.getNewValue()); }       }    });        JPanel buttonPanel = new JPanel();    buttonPanel.add(startButton);    buttonPanel.add(cancelButton);    JPanel cp = new JPanel();    LayoutManager layout = new BoxLayout(cp, BoxLayout.Y_AXIS);    cp.setLayout(layout);    cp.add(buttonPanel);    cp.add(new JScrollPane(textArea));    cp.add(progressBar);    JFrame frame = new JFrame("SwingWorker Test");    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);    frame.setContentPane(cp);    frame.pack();    frame.setVisible(true);  }}

module-lab-2.2-GUI Frameworks and Concurrency

http://campus.unibo.it/7730/1/module-lab-2.2-GUI_Frameworks_and_Concurrency.pdf

module-lab-2.2-GUI Frameworks and Concurrency

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