《VB2008从入门到精通(PDF格式英文版)》第164章

when　running　a　word　processor；　you　don’t　want　the　spell　checker　slowing　down　your　typing。
The　operating　system　will　still　allocate　time　slices　in　this　case　and　use　preemptive　multitasking。　
Figure　13…3　illustrates　how　the　same　application　executes　on　a　multiple…core　micropro
cessor。　Did　you　notice　what　happened？　The　operating　system；　in　a　bid　to　make　more　efficient
use　of　the　microprocessor；　has　put　one　task　on　one　core　and　another　task　on　the　other　core。
Now　both　tasks　are　running　in　parallel。　And　now　it’s　possible　that　both　tasks　would　want　to
manipulate　the　same　piece　of　data　at　the　same　time。　In　a　single…core　microprocessor；　that　is
not　physically　possible。　However；　with　a　single　processor；　it　is　still　possible　for　one　task　to　be
interrupted　mid…flight　while　accessing　some　data；　and　for　the　other　task　to　step　in　and　use
inconsistent　data。　
Figure　13…3。　Multiple…core　microprocessor　running　two　tasks　
If　you　have　a　multitasking　application；　you　must　be　careful　how　you　manipulate　global
state　in　the　application。　If　you　are　not　careful；　you　could　corrupt　the　state　of　your　program。
■Note As　panies　like　Intel　and　AMD　introduce　microprocessors　with　more　and　more　cores；　it　is　your
job　as　a　software　developer　to　write　software　that　can　make　use　of　those　cores。　This　is　not　always　easy　and
requires　some　forethought；　since　you　need　to　work　out　the　logic。　For　example；　if　you　are　processing　data　that
is　sent　to　a　file；　you　can’t　read　the　file　before　the　data　has　been　processed。　
Using　Threads　
Tasks　can　run　in　two　ways　on　the　Windows　operating　system：　via　threads　and　via　processes。
A process　is　when　you　start　a　program；　causing　the　operating　system　to　instantiate　resources
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C　HA　P　TE　R 1　3 ■ L　E　AR　N　IN　G AB　O　U　T M　U　L　T　IT　HR　E　AD　IN　G　345　
and　time　slices　for　the　program。　A　thread　can　be　seen　as　a　lightweight　process　that　executes　in
the　context　of　a　process。
All　processes　will　start　a　single　thread。　The　thread　that　is　executed　as　part　of　the　process　is
the　main　thread；　and　when　it　exits；　so　does　your　application。　If　the　main　thread　creates　other
threads；　when　the　main　thread　exits；　so　do　the　created　background　threads。　From　an　architec
tural　perspective；　when　multitasking；　the　main　thread　will　execute　and　coordinate　the　threads
that　run　your　code。　Two　processes　cannot　reference　each　other。　This　is　so　that　if　one　process
crashes；　it　does　not　cause　the　other　process　to　crash。　Threads　executing　in　the　context　of　a
process　have　the　ability　to　bring　down　an　entire　process。
■Note You　could　run　multiple　processes　and　then　use　interprocess　munication　mechanisms　to　muni
cate。　However；　I　can’t　remend　that　solution；　since　you　can　get　the　same　effect　of　a　process　by　using　a
application　domain　（AppDomain）。　For　the　most　part；　you　don’t　need　to　concern　yourself　with　AppDomains。　You
will　use　threads；　since　they　are　lightweight　（pared　to　a　process）；　easy　to　manage；　and　easy　to　program。　
Creating　a　New　Thread　
You　can　create　a　new　thread　that　will　run　independently　of　its　originating　thread。　In　the　following
example；　the　main　thread　creates　two　other　threads。　Each　of　the　threads　outputs　some　text　to
the　console。　Note　that　the　type　Thread　and　other　related　types　used　in　this　chapter’s　examples
are　in　the　System。Threading　namespace。　
Imports　System。Threading　
Module　Module1　
Sub　Main（）　
SimpleThread（）　
End　Sub　
Sub　SimpleThreadTask1（）　
Console。WriteLine（〃hello　there〃）　
End　Sub　
Sub　SimpleThreadTask2（）　
Console。WriteLine（〃Well　then　goodbye〃）　
End　Sub　
Private　Sub　SimpleThread（）　
Dim　thread1　As　New　Thread（AddressOf　SimpleThreadTask1）　
Dim　thread2　As　New　Thread（AddressOf　SimpleThreadTask2）　
thread1。Start（）　
thread2。Start（）　
End　Sub　
End　Module　
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346 CH　AP　T　E　R 1　3 ■ L　E　A　R　N　I　N　G A　B　OU　T M　U　L　T　I　TH　R　E　A　DI　N　G　
To　create　a　thread；　you　instantiate　the　Thread　type　and　call　the　Start（）　method。　The　Thread
type　is　a　class　that　contains　all　of　the　functionality　necessary　to　start　and　control　a　multitasking
thread。　
When　running　a　thread；　the　Thread　type　needs　code　to　execute。　The　solution　used　by　Thread　is
a　delegate；　which　is　passed　to　Thread　via　the　constructor。　The　name　of　the　delegate　type　is
ThreadStart。　The　example　here　does　not　use　the　ThreadStart　type　explicitly；　because　both　threads
use　programming　constructs　that　do　not　need　a　delegate　declaration。　Here’s　the　explicit　syntax
for　pleteness：　
Dim　thread1　As　New　Thread（New　ThreadStart（AddressOf　SimpleThreadTask1））　
Dim　thread2　As　New　Thread（New　ThreadStart（AddressOf　SimpleThreadTask2））　
Running　the　thread　example；　you　may　see　this　output：　
well　then　goodbye　
hello　there　
Notice　how　hello　there　is　after　well　then　goodbye。　The　output　implies　that　the　second
thread　（thread2）　starts　before　the　first　thread　（thread1）。　However；　your　output　might　be　the
opposite；　which　demonstrates　the　true　nature　of　threading；　concurrency；　and　why　threading　is
so　difficult。　
Imagine　for　a　moment　that　the　sample　thread　code　were　not　multithreaded；　but　executed
in　a　serial　manner。　Calling　thread1。Start（）　and　then　thread2。Start（）　results　in　the　text　hello
there　being　first