SSC - Concurrency and Multi-threading Concurrency in Swingszh/teaching/ssc/lecturenotes/... · 2014-11-18 · SSC - Communication and Networking Concurrency in Swing Responsive or

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 But let’s take a look at a BAD 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 NOTE: AWT (Abstract Window Toolkit) is the original Javaplatform-independent windowing, graphics, and user-interfacewidget toolkit



What threads you can create/control in Swing?

I Two threads created automatically by Java: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



How to write an unresponsive Swing?

I Do everything, esp. time consuming tasks in theEvent-Dispatching Thread

I Essentially add time consuming tasks in theactionPerformed method of the Swing component



Bad Swing programme:

Time-consuming tasks

Small tasks

Click button

Time

One single event dispatch thread



How to make Swing responsive?

I Responsive principle: Use threads other than event dispatchthread to execute time-consuming background tasks

I Such thread are called Worker threads, also known asbackground threads

However



Swing is not thread safe!I Why Swing is not thread safe?: invoking some Swing

components from multiple threads will cause threadinterference or memory consistency errors.

I Note: “It may seem strange that such an important part of theJava platform is not thread safe. It turns out that any attemptto create a thread-safe GUI library faces some fundamentalproblems”; also MultiThreaded toolkits: A failed dream?

I Safe principle: ONLY create and update Swing componentsfrom the Event Dispatch thread

I The same principle applies to those models used by Swingcomponents e.g. ListModel (JList), TableModel (JTable)

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

I Two ways to implement the safe principle:I SwingUtilities.invokeLater()

I SwingWorker

https://weblogs.java.net/blog/kgh/archive/2004/10/multithreaded_t.html



SwingUtilities.invokeLater()

I From the Safe principle, user interface updates must ONLYhappen in the event dispatch thread.

I Jobs carried out in other threads cannot update Swingcomponents

I Problem: in a time-consuming task, we need to update Swingcomponents to give user some feedback, e.g., a progress bar

I Solution: To use SwingUtilities.invokeLater in yourworker thread to post a ”job” to Swing, which it will then runon the event dispatch thread 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.



Good Swing programme:

Time-consuming tasks

Click button

Time

SwingWorker thread

Small tasks (Swing objects related)

Time

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: this method is essentiallyexecuted in a worker thread, where all background activitiesshould happen

I process() method: used to process intermediate results

generated in doInBackground

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

by doInBackground when it finishesI Question: where can I update Swing components?



How to use addPropertyChangeListener

I A bound property notifies listeners when its value changes

I Used for thread communicationI Two predefined bound properties in the SwingWorker class:

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 Swingszh/teaching/ssc/lecturenotes/... · 2014-11-18 · SSC - Communication and Networking Concurrency in Swing Responsive or