VB2008从入门到精通(PDF格式英文版)-第128章
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Pulse（） method triggers a signal that awakens the first thread； but the first thread will execute
only after the second thread has released control of the lock。
The big advantages of a Monitor in parison to SyncLock are that the Monitor can be
used anywhere in the code and that the Monitor will release a lock while waiting for an answer。
You would use SyncLock when you want to control access to a block of code。 If the access goes
beyond method boundaries； then it is preferable to use Monitor。 It is not that you could not use
a SyncLock that spanned boundaries； but if you need to add code that could produce a dead
lock； it is easier to control that code using a Monitor。
Now that we’ve covered the fundamentals of multithreading； the next sections focus on
more advanced threading architectures—reader/writer and producer/consumer—and using
asynchronous calls。
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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 359
Implementing a Reader/Writer
Threaded Architecture
The reader/writer threaded architecture is based on the idea that if one thread is reading and
another thread would like to read as well； why not let it？ However； if one thread wants to write；
then only that thread can write。 In other words； multiple threads are allowed to read data
simultaneously； but to write； a thread must have an exclusive lock。
implements a System。Threading。ReaderWriterLock class； which contains the reader/
writer functionality。 This class is like a Monitor； in that it gives you the control to manage how
data is accessed but does not determine what you are accessing。 The ReaderWriterLock type
has a number of methods and properties。 The most important ones are listed in Table 13…1。
Table 13…1。 Important ReaderWriterLock Methods
Method Description
AcquireReaderLock（） Acquires a reader lock。 Multiple threads can acquire a
reader lock。
AcquireWriterLock（） Acquires a writer lock。 Only a single thread can acquire
a writer lock。
DowngradeFromWriterLock（） Converts a writer lock into a reader lock。 Using this
method avoids the need to call ReleaseWriterLock（）
and AcquireReaderLock（）。
UpgradeToWriterLock（） Converts a reader lock into a writer lock。 Using this
method avoids the need to call ReleaseReaderLock（）
and AcquireWriterLock（）。
ReleaseLock（） Releases all locks； regardless how many times you have
called to acquire the reader or writer locks。
ReleaseReaderLock（） Decrements the reader lock a single count。 To pletely
release a reader lock； you need to make sure the number
of times you called ReleaseReaderLock（） is equal to the
number of times you called AcquireReaderLock（）。
ReleaseWriterLock（） Decrements the writer lock a single count。 To pletely
release a reader lock； you need to make sure the number
of times you called ReleaseWriterLock（） is equal to the
number of times you called AcquireWriterLock（）。
Let’s look at a collection example that has four threads： three readers and one writer。 The
example uses Thread。Sleep（） strategically； so that you can see how a reader thread and writer
thread interact with each other。
Imports System。Threading
Friend Module TestMoreTopics
Private rwlock As New ReaderWriterLock（）
Private elements As New List（Of Integer）（）
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360 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
Sub Task1（）
Thread。Sleep（1000）
Console。WriteLine（〃Thread 1 waiting for read lock〃）
rwlock。AcquireReaderLock（…1）
Console。WriteLine（〃Thread 1 has read lock〃）
Dim item As Integer
For Each item In elements
Console。WriteLine（〃Thread 1 Item （〃 & item & 〃）〃）
Thread。Sleep（1000）
Next
Console。WriteLine（〃Thread 1 releasing read lock〃）
rwlock。ReleaseLock（）
End Sub
Sub Task2（）
Thread。Sleep（1250）
Console。WriteLine（〃Thread 2 waiting for read lock〃）
rwlock。AcquireReaderLock（…1）
Console。WriteLine（〃Thread 2 has read lock〃）
Dim item As Integer
For Each item In elements
Console。WriteLine（〃Thread 2 Item （〃 & item & 〃）〃）
Thread。Sleep（1000）
Next
Console。WriteLine（〃Thread 2 releasing read lock〃）
rwlock。ReleaseLock（）
End Sub
Sub Task3（）
Thread。Sleep（1750）
Console。WriteLine（〃Thread 3 waiting for read lock〃）
rwlock。AcquireReaderLock（…1）
Console。WriteLine（〃Thread 3 has read lock〃）
Dim item As Integer
For Each item In elements
Console。WriteLine（〃Thread 3 Item （〃 & item & 〃）〃）
Thread。Sleep（1000）
Next
Console。WriteLine（〃Thread 3 releasing read lock〃）
rwlock。ReleaseLock（）
End Sub
Sub Task4（）
Thread。Sleep（1500）
Console。WriteLine（〃Thread 4 waiting for write lock〃）
rwlock。AcquireWriterLock（…1）
Console。WriteLine（〃Thread 4 has write Lock〃）
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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 361
elements。Add（30）
Console。WriteLine（〃Thread 4 releasing write lock〃）
rwlock。ReleaseLock（）
End Sub
Private Sub ReaderWriter（）
elements。Add（10）
elements。Add（20）
Dim thread1 As New Thread（AddressOf Task1）
Dim thread2 As New Thread（AddressOf Task2）
Dim thread3 As New Thread（AddressOf Task3）
Dim thread4 As New Thread（AddressOf Task4）
thread1。Start（）
thread2。Start（）
thread3。Start（）
thread4。Start（）
End Sub
Public Sub RunAll（）
TestMoreTopics。ReaderWriter（）
End Sub
End Module
The bolded code contains all of the references to the reader/writer class imple
mentation。 Unlike the keyword SyncLock or the type Monitor； the ReaderWriterLock type is
instantiated and the instance is shared between threads。
The code to acquire a reader lock or writer lock has a parameter of value …1； which means
to wait until the lock is acquired。 A positive value means to wait for a number of milliseconds； and
if the lock has not been acquired； then return from the method call。 If you do use a timeout； before
you attemp
