SSC - Concurrency and Multi-threading Concurrency in Swing · SSC - Communication and Networking Concurrency in Swing SwingUtilities.invokeLater() I User interface updates must ONLY

SSC - Communication and Networking

SSC - Concurrency and Multi-threadingConcurrency in Swing

Shan He

School for Computational ScienceUniversity of Birmingham

Module 06-19321: SSC


Outline

Outline of Topics

Concurrency in Swing



Responsive or Unresponsive UI?

I Screen liveliness: users expect a program is always responsive,no matter what it’s doing

I Let’s take a look at an example

I Swing is a single-threaded programming model

I Event-dispatch thread: handles all interaction events.I How the Swing works:

I Step 1: Swing generates event objects that contain relevantevent information, e.g. event source and event ID.

I Step 2: placed event objects to a single event queue orderedby their entry time.

I Step 3: event-dispatch thread, regularly checks and takesevent objects from the queue one by one and sends them tothe interested parties

I Step 4: the interested parties react to the event notification



How the Swing works:

Event nEvent n-1Event n-2

…

Event 2Event 1

Event queue

Paint

List Selection Listener

Event listener

Event dispatch thread

Others Notifies interested parties

Update UI

Queue up events



Let’s trace the threads

I We can tracing the threads executed in a Swing applicationI Five thread involved:

I Main thread by the main() methodI “AWT-Windows” (daemon thread): listen to UI events from

AWT windowsI “AWT-Shutdown”: Handling the exit of AWT windows, e.g.,

terminates the event dispatch thread and exitsI “AWT-EventQueue-0”: Event-Dispatching Thread, which is

the one and only thread responsible for handling all the eventsI “DestroyJavaVM”: Handling the exit of the main thread after

the main() method completes

I AWT: Abstract Window Toolkit the original Javaplatform-independent windowing, graphics, and user-interfacewidget toolkit



What threads you can create/control in Swing?

I Two thread:I Main thread by the main() methodI “AWT-EventQueue-0”: Event-Dispatching Thread, which is

the one and only thread responsible for handling all the events

I Plus other threads you created in Swing: called worker thread



Bad Swing programme:

Time-consuming tasks

Small tasks

Click button

Time

One single event dispatch thread



How to make Swing responsive and safe?I Responsive principle: Use threads other than event dispatch

thread to execute time-consuming background tasksI Such thread are called Worker threads, also known as

background threads

I Safe principle: To update user interface, invoke all Swingcomponent methods from the event dispatch thread

I The code ignore the above safe principle might work, but mightproduce unpredictable errors that are difficult to reproduce.

I Most Swing object methods are not “thread safe”: threadinterference or memory consistency errors.

I Never call methods such as Thread.sleep(), Object.wait(),Condition.await() inside an event handler.

I Two ways of doing this:I SwingUtilities.invokeLater()

I SwingWorker



Good Swing programme:

Time-consuming tasks

Click button

Time

SwingWorker thread

Small tasks (Swing objects related)

Time

Event dispatch thread



SwingUtilities.invokeLater()

I User interface updates must ONLY happen on the eventdispatch thread.

I Jobs carried out in other threads cannot update Swingcomponents

I Problem: in a time-consuming task, we need to update Swingcomponents

I Solution: To use SwingUtilities.invokeLater to posta ”job” to Swing, which it will then run on the event dispatchthread at its next convenience.



How SwingUtilities.invokeLater() works

EventDispatchThread

WorkerThread

SwingUtilities.invokeLater(new Runnable() {public void run() {InvokeLaterSwingExample.this.tfCount.setText("Count is " + count);}

});



SwingUtilities.invokeLater()

I From Oracle:“Causes runnable to have its run method calledin the dispatch thread of the system EventQueue. This willhappen after all pending events are processed. ”

I After calling invokeLater() in the worker thread, thecode hands over its run method to even dispatch thread andcontinues to run

I Java example: Update JTextField in a worker thread



SwingWorker class

I javax.swing.SwingWorker : An abstract class to performlengthy GUI-interaction tasks in a background thread.

I Simplifies complicated thread communications by a numberfeatures:

I SwingWorkerdone() method: automatically invoked on the

event dispatch thread when the background task is finished.I implements java.util.concurrent.Future to:

I allow the background task to provide a return value to otherthreads,

I cancel the background taskI discover whether the background task has finished or been

cancelled.

I SwingWorker.publish() : provides intermediate resultsI Defines bound properties by background task: changes to

these properties trigger events, causing event-handlingmethods to be invoked on the event dispatch thread.



How to use SwingWorkerI Class SwingWorker<T,V>

I Type Parameters:I T - the result type returned by this SwingWorker’s

doInBackground and get methods

I V - the type used for carrying out intermediate results by thisSwingWorker’s publish and process methods

I doInBackground() method: where all backgroundactivities should happen

I process() method: used to process intermediate results in

doInBackground

I done() method: used to process the final results returned

by doInBackground when it finishesI Question: Can you update Swing components in the above

three methods?



How to use addPropertyChangeListener

I A bound property notifies listeners when its value changes

I Used for thread communicationI Two predefined bound properties: progress and state

.I progress : int value ∈ [0, 100]

I state : A constant indicates where the SwingWorker object

is in its lifecycle, can be DONE , PENDING and STARTED

I Use setProgress() to change progress

I Java example: to update progress bar



With SwingWorker

Documents

SSC - Concurrency and Multi-threading Concurrency in Swing · SSC - Communication and Networking Concurrency in Swing SwingUtilities.invokeLater() I User interface updates must ONLY