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更新: 2008 年 7 月
Monitor 物件可公開同步處理程式碼區域之存取的能力,其方式是使用 Monitor.Enter、Monitor.TryEnter 和 Monitor.Exit 方法來鎖定及解除鎖定特定的物件。一旦您已鎖定程式碼區域,就可使用 Monitor.Wait、Monitor.Pulse 和 Monitor.PulseAll 方法。如果 Wait 被保留並等候通知,則會解除鎖定。當通知 Wait 時,則會返回並再次鎖定。Pulse 和 PulseAll 都會信號通知等候佇列中的下一執行緒繼續。
Visual Basic SyncLock 和 C# lock 陳述式 (Statement) 使用 Monitor.Enter 來取得鎖定,以及使用 Monitor.Exit 來釋放它。使用語言陳述式的好處在於,lock 或 SyncLock 區塊中的所有項目都會包含在 Try 陳述式中。Try 陳述式具有 Finally 區塊,可以確保鎖定已釋放。
Monitor 會鎖定物件 (也就是參考型別),而不會鎖定實值型別 (Value Type)。雖然實值型別可以傳遞到 Enter 和 Exit,但是每個呼叫都會個別地進行 Boxed 處理。因為每個呼叫會建立個別的物件,所以 Enter 永遠不會封鎖,並且不會真的同步處理應該要保護的程式碼。此外,傳遞給 Exit 的物件與傳遞給 Enter 的物件不同,因此 Monitor 擲回 SynchronizationLockException 時,會發出訊息「從程式碼的未同步化區塊呼叫物件同步化方法」。下列範例說明這些問題。
Private x As Integer
' The next line creates a generic object containing the value of
' x each time the code is executed, so that Enter never blocks.
Monitor.Enter(x)
Try
' Code that needs to be protected by the monitor.
Finally
' Always use Finally to ensure that you exit the Monitor.
' The following line creates another object containing
' the value of x, and throws SynchronizationLockException
' because the two objects do not match.
Monitor.Exit(x)
End Try
private int x;
// The next line creates a generic object containing the value of
// x each time the code is executed, so that Enter never blocks.
Monitor.Enter(x);
try {
// Code that needs to be protected by the monitor.
}
finally {
// Always use Finally to ensure that you exit the Monitor.
// The following line creates another object containing
// the value of x, and throws SynchronizationLockException
// because the two objects do not match.
Monitor.Exit(x);
}
雖然可以如下列範例所示,在呼叫 Enter 和 Exit 之前先對實值型別變數進行 box 處理,並且傳遞同一個 boxed 物件到這兩個方法,但是這樣做卻沒有好處。變數的變更並沒有反映在包裹的複本上,並且無法變更包裹複本的值。
Private o As Object = x
private Object o = x;
注意 Monitor 與 WaitHandle 物件之間的差異非常重要。Monitor 物件是純粹 Managed、完全可移植,並且就作業系統資源需求而言更具高效率。WaitHandle 物件表示作業系統可等候的物件,對 Managed 與 Unmanaged 程式碼之間的同步處理非常有用,並且公開某些進階的作業系統功能,如一次等候許多物件的功能。
下列程式碼範例示範組合使用 Monitor 類別 (使用 lock 和 SyncLock 編譯器陳述式實作)、Interlocked 類別和 AutoResetEvent 類別。
Imports System
Imports System.Threading
Imports Microsoft.VisualBasic
' Note: The class whose internal public member is the synchronizing method
' is not public; none of the client code takes a lock on the Resource object.
' The member of the nonpublic class takes the lock on itself. Written this
' way, malicious code cannot take a lock on a public object.
Class SyncResource
Public Sub Access(threadNum As Int32)
' Uses Monitor class to enforce synchronization.
SyncLock Me
' Synchronized: Despite the next conditional, each thread
' waits on its predecessor.
If threadNum Mod 2 = 0 Then
Thread.Sleep(2000)
End If
Console.WriteLine("Start Synched Resource access (Thread={0})", threadNum)
Thread.Sleep(200)
Console.WriteLine("Stop Synched Resource access (Thread={0})", threadNum)
End SyncLock
End Sub 'Access
End Class 'SyncResource
' Without the lock, the method is called in the order in which
' threads reach it.
Class UnSyncResource
Public Sub Access(threadNum As Int32)
' Does not use Monitor class to enforce synchronization.
' The next call throws the thread order.
If threadNum Mod 2 = 0 Then
Thread.Sleep(2000)
End If
Console.WriteLine("Start UnSynched Resource access (Thread={0})", threadNum)
Thread.Sleep(200)
Console.WriteLine("Stop UnSynched Resource access (Thread={0})", threadNum)
End Sub 'Access
End Class 'UnSyncResource
Public Class App
Private Shared numAsyncOps As Int32 = 5
Private Shared asyncOpsAreDone As New AutoResetEvent(False)
Private Shared SyncRes As New SyncResource()
Private Shared UnSyncRes As New UnSyncResource()
Private Shared threadNum As Int32
Public Shared Sub Main()
For threadNum = 0 To 4
ThreadPool.QueueUserWorkItem(AddressOf SyncUpdateResource, threadNum)
Next threadNum
' Wait until this WaitHandle is signaled.
asyncOpsAreDone.WaitOne()
Console.WriteLine(ControlChars.Tab + ControlChars.Lf + "All synchronized operations have completed." + ControlChars.Lf)
' Reset the thread count for unsynchronized calls.
numAsyncOps = 5
For threadNum = 0 To 4
ThreadPool.QueueUserWorkItem(AddressOf UnSyncUpdateResource, threadNum)
Next threadNum
' Wait until this WaitHandle is signaled.
asyncOpsAreDone.WaitOne()
Console.WriteLine(ControlChars.Tab + ControlChars.Cr + "All unsynchronized thread operations have completed.")
End Sub 'Main
' The callback method's signature MUST match that of
' a System.Threading.TimerCallback delegate
' (it takes an Object parameter and returns void).
Shared Sub SyncUpdateResource(state As Object)
' This calls the internal synchronized method, passing
' a thread number.
SyncRes.Access(CType(state, Int32))
' Count down the number of methods that the threads have called.
' This must be synchronized, however; you cannot know which thread
' will access the value **before** another thread's incremented
' value has been stored into the variable.
If Interlocked.Decrement(numAsyncOps) = 0 Then
asyncOpsAreDone.Set()
' Announce to Main that in fact all thread calls are done.
End If
End Sub 'SyncUpdateResource
' The callback method's signature MUST match that of
' a System.Threading.TimerCallback delegate
' (it takes an Object parameter and returns void).
Shared Sub UnSyncUpdateResource(state As [Object])
' This calls the unsynchronized method, passing
' a thread number.
UnSyncRes.Access(CType(state, Int32))
' Count down the number of methods that the threads have called.
' This must be synchronized, however; you cannot know which thread
' will access the value **before** another thread's incremented
' value has been stored into the variable.
If Interlocked.Decrement(numAsyncOps) = 0 Then
asyncOpsAreDone.Set()
' Announce to Main that in fact all thread calls are done.
End If
End Sub 'UnSyncUpdateResource
End Class 'App
using System;
using System.Threading;
// Note: The class whose internal public member is the synchronizing
// method is not public; none of the client code takes a lock on the
// Resource object.The member of the nonpublic class takes the lock on
// itself. Written this way, malicious code cannot take a lock on
// a public object.
class SyncResource {
public void Access(Int32 threadNum) {
// Uses Monitor class to enforce synchronization.
lock (this) {
// Synchronized: Despite the next conditional, each thread
// waits on its predecessor.
if (threadNum % 2 == 0)
Thread.Sleep(2000);
Console.WriteLine("Start Synched Resource access (Thread={0})", threadNum);
Thread.Sleep(200);
Console.WriteLine("Stop Synched Resource access (Thread={0})", threadNum);
}
}
}
// Without the lock, the method is called in the order in which threads reach it.
class UnSyncResource {
public void Access(Int32 threadNum) {
// Does not use Monitor class to enforce synchronization.
// The next call throws the thread order.
if (threadNum % 2 == 0)
Thread.Sleep(2000);
Console.WriteLine("Start UnSynched Resource access (Thread={0})", threadNum);
Thread.Sleep(200);
Console.WriteLine("Stop UnSynched Resource access (Thread={0})", threadNum);
}
}
public class App {
static Int32 numAsyncOps = 5;
static AutoResetEvent asyncOpsAreDone = new AutoResetEvent(false);
static SyncResource SyncRes = new SyncResource();
static UnSyncResource UnSyncRes = new UnSyncResource();
public static void Main() {
for (Int32 threadNum = 0; threadNum < 5; threadNum++) {
ThreadPool.QueueUserWorkItem(new WaitCallback(SyncUpdateResource), threadNum);
}
// Wait until this WaitHandle is signaled.
asyncOpsAreDone.WaitOne();
Console.WriteLine("\t\nAll synchronized operations have completed.\t\n");
// Reset the thread count for unsynchronized calls.
numAsyncOps = 5;
for (Int32 threadNum = 0; threadNum < 5; threadNum++) {
ThreadPool.QueueUserWorkItem(new WaitCallback(UnSyncUpdateResource), threadNum);
}
// Wait until this WaitHandle is signaled.
asyncOpsAreDone.WaitOne();
Console.WriteLine("\t\nAll unsynchronized thread operations have completed.");
}
// The callback method's signature MUST match that of a
// System.Threading.TimerCallback delegate (it takes an Object
// parameter and returns void).
static void SyncUpdateResource(Object state) {
// This calls the internal synchronized method, passing
// a thread number.
SyncRes.Access((Int32) state);
// Count down the number of methods that the threads have called.
// This must be synchronized, however; you cannot know which thread
// will access the value **before** another thread's incremented
// value has been stored into the variable.
if (Interlocked.Decrement(ref numAsyncOps) == 0)
asyncOpsAreDone.Set();
// Announce to Main that in fact all thread calls are done.
}
// The callback method's signature MUST match that of a
// System.Threading.TimerCallback delegate (it takes an Object
// parameter and returns void).
static void UnSyncUpdateResource(Object state) {
// This calls the unsynchronized method, passing a thread number.
UnSyncRes.Access((Int32) state);
// Count down the number of methods that the threads have called.
// This must be synchronized, however; you cannot know which thread
// will access the value **before** another thread's incremented
// value has been stored into the variable.
if (Interlocked.Decrement(ref numAsyncOps) == 0)
asyncOpsAreDone.Set();
// Announce to Main that in fact all thread calls are done.
}
}
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變更記錄
日期 |
記錄 |
原因 |
---|---|---|
2008 年 7 月 |
加入說明:SyncLock 和 lock 陳述式使用 Monitor.Enter 和 Exit。 |
客戶回函。 |