Swing Worker Threads 2 Handout

8/2/2019 Swing Worker Threads 2 Handout

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Swing and Threads SwingUtilities SwingWorker MySwingWorker IOC

Threads - 2

Radu NicolescuDepartment of Computer ScienceUniversity of Auckland

1 October 2010

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1 Swing and Threads

2 SwingUtilities

3 SwingWorker

4 MySwingWorker

5 IOC

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Swing specific threads

Initial thread: the thread that starts the main()

Event-dispatch thread (EDT, aka GUI thread): executesdrawing and event handling code - should not run

time-consuming tasks Worker (background) threads: for time consuming tasks -

must not interact directly with the GUI

Two high-level solutions for the proper interaction with the

GUI thread Using SwingUtilities

Using SwingWorker

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SwingUtilities.invokeLater(runnable-task)

sometimes invoked by the initial thread, to properly create andstart the GUI, if other statements follow (in the initial thread)

typically invoked from a worker thread

schedules a given task to be asynchronously executed by the

even-dispatch thread and returns immediately (withoutwaiting the actual execution)

S w i n g U t i l i t i e s . i n v o k e L a t e r ( new R u n n ab l e ( ) ) {

p u b l i c v oi d run ( ) {// this code will be executed by the event-dispatch thread,// e.g., to update the GUI

}} ) ;

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S i d Th d S i U ili i S i W k M S i W k IOC


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SwingUtilities.invokeAndWait(runnable-task)

invokeAndWait() is similar to invokeLater(), but synchronous,i.e., the calling thread waits until the event-dispatch threadcompletes the given task

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S i d Th d S i Utiliti S i W k M S i W k IOC


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SwingWorker

SwingWorker is a class with two faceshttp://www.google.com/images?q=janus+images

A SwingWorker encapsulates:

one method, doInBackground(), which defines the task thatneeds to run by an automatically assigned background thread(no need to create this)

two methods, process() and done(), which will beautomatically executed by the GUI thread (at proper times)

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http://www.google.com/images?q=janus+imageshttp://www.google.com/images?q=janus+images


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SwingWorker

the doInBackground() method will return a final result, and,optionally, publishes, item by item, a sequence of interimobjects

the GUI process() and done() methods are provided to displaythe final result of doInBackground() and the sequence of itspublished interim objects (if any)

To effectively use SwingWorker, you need to subclass it, andoverride the above three methods

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SwingWorker

Additionally, a SwingWorker can use two methods that helptransfer objects from the background thread to the GUI thread(with no further user intervention):

one method, publish(), which transfers interim objects fromdoInBackground() to process()

one methods, get(), typically run in done(), which picks upthe final object created by doInBackground()

Attention, these two methods are final, i.e. not overridable

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SwingWorker

SwingWorker is a generic predefined type, SwingWorker,where:

T is the type of the final result of doInBackground() andpicked by get()

V is the type of the interim objects published bydoInBackground() further transferred to process()

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SwingWorker birds eye view

c l a s s MySwingWorker e x t e n d s SwingWorker {

@ O v e r r i d ep u b l i c T d o I n B a c k g r o u n d ( ) // background thread

// optionally, calls s u p e r . p u b l i s h (V) , repeatedly

// finally, returns a T

@ O v e r r i d ep r ot e c te d v o id p r o c e s s ( L i s t ) // GUI thread

// gets lists of Vs, published by d o I n B a c k g r o u n d ( )

@ O v e r r i d ep r ot e c te d v o id done ( ) // GUI thread

// typically, calls s u p e r . Get ( )// to pick the final T, returned by d o I n B a c k g r o u n d ( )

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MySwingWorker example

As an example, consider a class MySwingWorker, where:

T = Double, V = Integer

doInBackground() generates a sequence of Integers, say 0, 1,2, . . . , 20

doInBackground() publishes these Integers as interim values,to process()

finally, doInBackground() returns an average Double, here

10.0 the GUI thread displays all interim and final values

the GUI must remain responsive at all times, even if thebackground process takes a long time to complete

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MySwingWorker example the GUI

Controls: button BoredButton, button SwingWorker, textareaTextBox

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MySwingWorker example the GUI

button WorkerButton: starts the background execution of aMySwingWorker, disables the button and returns immediately(to keep the GUI responsive)

button BoredButton: prints Wait!, to indicate that the GUI

is NOT frozen (even if MySwingWorker is executing in thebackground)

textarea TextBox: displays results from MySwingWorker(interim and final) and texts printed via BoredButton

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MySwingWorker example overall structure

c l a s s MySwingWorkere x t e n d s SwingWorker {

@ O v e r r i d e

p u b l i c D o ub l e d o I n B a c k g r o u n d ( ) { . . . }

@ O v e r r i d ep r ot ec te d v o id p r o c e s s ( L i s t ) { . . . }

@ O v e r r i d ep r ot ec te d v o id done ( ) { . . . }

}

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MySwingWorker example doInBackground()

p u b l i c D o ub l e d o I n B a c k g r o u n d ( ) {Double avg = 0 . 0 ;i n t k = 0 ;f o r ( i n t i =0; i


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MySwingWorker example doInBackground()

p u b l i c D o ub l e d o I n B a c k g r o u n d ( ) {. . .

// this artificial sleep favours that published numbers// are grouped together into lists of three

t r y {i f ( i %3==0) Thread . s l e e p ( 1 0 0 0 ) ;

} catch ( I n t e r r u p t e d E x c e p t i o n e x ) {;

}. . .

}

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MySwingWorker example process()

@ O v e r r i d ep r ot ec t ed v oi d p r o c e s s ( L i s t ){//GUI thread

T ra ce Bo x . a pp en d ( . . . p r o c e s s : ) ;

f o r ( I n t e g e r i : numbers ) {TraceBox . append ( i+ ) ;

}

TraceBox . append ( \ r \n ) ;}

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MySwingWorker example done()

@ O v e r r i d e

p r ot ec t ed v oi d done ( ) { // GUI threadDouble avg = 0 . 0 ;

t r y {

a v g =t h i s

. g e t ( ) ;} catch ( I n t e r r u p t e d E x c e p t i o n e x ) {;

} catch ( E x e c u t i o n E x c e p t i o n e x ) {;

}

TraceB ox . append ( . . . done : avg=+avg+\ r \n ) ;W o r k er B u t to n . s e t E n a b l e d ( t r u e ) ;

}

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MySwingWorker example WorkerButton handler

A click on WorkerButton starts the background execution of aMySwingWorker, disables the button and returns immediately (tokeep the GUI responsive):

p u b l i c v oi d W o r k e r B u t t o n C l i c k ( ) { // GUI threadMySwingWorker msw = new MySwingWorker ( ) ;msw . e x e c u t e ( ) ;W o r k er B u t to n . s e t E n a b l e d ( f a l s e ) ;

}

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MySwingWorker example BoredButton handler

A click on BoredButton prints something, to indicate that the GUIis NOT frozen (even if MySwingWorker is executing in thebackground):

p u b l i c v oi d B o r e d B u t t o n C l i c k ( ) { // GUI threadTraceBox . append ( w a i t !\ r \n ) ;

}

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Inversion of Control (IOC)

There is a bit of magic, in SwingWorker, this class with twofaces

you define the methods process() and done(),

but you do not invoke them (not directly)

the infrastructure will schedule their execution, in the GUIthread (as appropriate)

This is a nice example of an interesting design pattern, called IOC(Inversion of Control)

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Inversion of Control (IOC)

Normally

either we call methods already created by others (librarymethods)

or we call methods created by ourselves

In the IOC pattern, the library calls our methods

we create methods that we do not ourselves call (not directly)

the existing infrastructure (library) will automatically call ourmethods, as appropriate

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SwingWorker get() exceptions

get() is blocking call the caller waits until doInBackground()terminates

you need not worry about this, if you call get() in the done()method

however, in general, a waiting get() caller can be interrupted,i.e. can receive an InterruptedException

a waiting get() caller can receive an indication thatdoInBackground() terminated abnormally, via anExecutionException

the ExecutionException can be further investigated (but wedont follow this issue here)

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Swing Worker Threads 2 Handout

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