Interacting with Filesystems - Beginning Java 7 - page 781

}
};
EventQueue.invokeLater(r);
}
}
Listing 8-21 ' s FindAll class declares several class fields along with cre-
ateGUI() , findAll() , find() and main() classmethods.Becausemuchofthis
contenthaspreviouslybeendiscussed(in Chapter7 andearlierinthischapter),I'llfo-
cus on only a few items.
FindAll is a multithreaded application. As well as the main thread that executes
main() , FindAll 'sGUIrunsontheevent-dispatchthread(EDT)andcreatesawork-
erthreadtoexecutethe findAll() methodoffoftheEDT,tokeeptheGUIrespons-
ive.
At some point, threads must communicate with shared variables and this is where
lack-of-synchronizationproblemscanarise.I'veeliminatedtheseproblemsbycreating
a single volatile result field and using final local variables.
The result fieldis volatile sothattheEDTandworkerthreadcansee res-
ult 's String reference value on multicore or multiprocessor platforms where each
core/processorhasalocalcachedcopyofthisfield.If result wasn't volatile ,the
EDT might not see the reference to a new String object assigned to result when
findAll() findsamatch,andwouldprobablyappendacopyofthepreviouslyfound
match to the textarea. (This isn't a problem on single processor/single core platforms.)
Althoughthisrationalealsoholdsforthe startDir and srchText localvariables,
they're declared final instead of volatile . They need to be declared final
so that they can be accessed from the anonymous class that implements
java.lang.Runnable in the search button's action listener.
Ifyourecall, Chapter4 statesthat final fieldscanbesafelyaccessedwithoutsyn-
chronization.Asaresult, volatile isn'trequiredfora final field,andyoucannot
declareafieldtobe volatile and final atthesametime.(A final fieldcanbe
safely accessed but not necessarily the objects to which final reference fields refer.
Because String objectsareimmutable,therewouldbenoproblemifIcalled String
methods on startDir , srchText , and result .)
Thesearchbutton'sactionlistenerdeclaresarunnablewithinarunnable,andthecode
probably looks complicated. The following sequence of steps explains how this code
works: