極客書

ASP.NET - Multi Threading

線程被定義爲程序的執行路徑。每個線程定義一個唯一的控制流。如果應用程式涉及複雜且耗時的操作（如資料庫訪問或某些密集的I/O操作），那麼設置不同的執行路徑或線程（每個線程執行特定的作業）通常會很有幫助。

線程是輕量級進程。使用線程的一個常見例子是用現代作業系統實現並發編程。使用線程可以節省CPU周期的浪費，並提高應用程式的效率。

到目前爲止，我們編譯的程序中，一個線程作爲一個進程運行，這個進程是應用程式的運行實例。但是，這樣應用程式可以一次執行一個作業。爲了使它一次執行多個任務，可以將它劃分爲更小的線程。

在.Net中，線程是通過「System.threading」命名空間處理的。通過創建System.Threading.Thread類型的變量，可以創建一個新的開始使用的線程。它允許您在程序中創建和訪問單個線程。

Creating Thread

線程是通過創建線程對象創建的，爲其構造函數提供一個ThreadStart引用。

ThreadStart childthreat = new ThreadStart(childthreadcall);

Thread Life Cycle

線程的生命周期從System.Threading.thread類的對象創建時開始，到線程終止或完成執行時結束。

以下是線程生命周期中的各種狀態：

  • 未啓動狀態：創建線程實例但未調用Start方法時的情況。

  • 就緒狀態：線程準備好執行並等待CPU周期的情況。

  • 不可運行狀態：線程不可運行，當：

    • Sleep method has been called
    • Wait method has been called
    • Blocked by I/O operations

  • 死區狀態：線程已完成執行或已中止的情況。

Thread Priority

Thread類的Priority屬性指定一個線程相對於另一個線程的優先級。.Net運行時選擇優先級最高的就緒線程。

優先事項可分爲：

  • Above normal
  • Below normal
  • Highest
  • Lowest
  • Normal

創建線程後，將使用thread類的priority屬性設置其優先級。

NewThread.Priority = ThreadPriority.Highest;

Thread Properties & Methods

Thread類具有以下重要屬性：

Property Description
CurrentContext Gets the current context in which the thread is executing.
CurrentCulture Gets or sets the culture for the current thread.
CurrentPrinciple Gets or sets the thread's current principal for role-based security.
CurrentThread Gets the currently running thread.
CurrentUICulture Gets or sets the current culture used by the Resource Manager to look up culture-specific resources at run time.
ExecutionContext Gets an ExecutionContext object that contains information about the various contexts of the current thread.
IsAlive Gets a value indicating the execution status of the current thread.
IsBackground Gets or sets a value indicating whether or not a thread is a background thread.
IsThreadPoolThread Gets a value indicating whether or not a thread belongs to the managed thread pool.
ManagedThreadId Gets a unique identifier for the current managed thread.
Name Gets or sets the name of the thread.
Priority Gets or sets a value indicating the scheduling priority of a thread.
ThreadState Gets a value containing the states of the current thread.

Thread類有以下重要方法：

Methods Description
Abort Raises a ThreadAbortException in the thread on which it is invoked, to begin the process of terminating the thread. Calling this method usually terminates the thread.
AllocateDataSlot Allocates an unnamed data slot on all the threads. For better performance, use fields that are marked with the ThreadStaticAttribute attribute instead.
AllocateNamedDataSlot Allocates a named data slot on all threads. For better performance, use fields that are marked with the ThreadStaticAttribute attribute instead.
BeginCriticalRegion Notifies a host that execution is about to enter a region of code in which the effects of a thread abort or unhandled exception might endanger other tasks in the application domain.
BeginThreadAffinity Notifies a host that managed code is about to execute instructions that depend on the identity of the current physical operating system thread.
EndCriticalRegion Notifies a host that execution is about to enter a region of code in which the effects of a thread abort or unhandled exception are limited to the current task.
EndThreadAffinity Notifies a host that managed code has finished executing instructions that depend on the identity of the current physical operating system thread.
FreeNamedDataSlot Eliminates the association between a name and a slot, for all threads in the process. For better performance, use fields that are marked with the ThreadStaticAttribute attribute instead.
GetData Retrieves the value from the specified slot on the current thread, within the current thread's current domain. For better performance, use fields that are marked with the ThreadStaticAttribute attribute instead.
GetDomain Returns the current domain in which the current thread is running.
GetDomainID Returns a unique application domain identifier.
GetNamedDataSlot Looks up a named data slot. For better performance, use fields that are marked with the ThreadStaticAttribute attribute instead.
Interrupt Interrupts a thread that is in the WaitSleepJoin thread state.
Join Blocks the calling thread until a thread terminates, while continuing to perform standard COM and SendMessage pumping. This method has different overloaded forms.
MemoryBarrier Synchronizes memory access as follows: The processor executing the current thread cannot reorder instructions in such a way that memory accesses prior to the call to MemoryBarrier execute after memory accesses that follow the call to MemoryBarrier.
ResetAbort Cancels an Abort requested for the current thread.
SetData Sets the data in the specified slot on the currently running thread, for that thread's current domain. For better performance, use fields marked with the ThreadStaticAttribute attribute instead.
Start Starts a thread.
Sleep Makes the thread pause for a period of time.
SpinWait Causes a thread to wait the number of times defined by the iterations parameter.
VolatileRead() Reads the value of a field. The value is the latest written by any processor in a computer, regardless of the number of processors or the state of processor cache. This method has different overloaded forms.
VolatileWrite() Writes a value to a field immediately, so that the value is visible to all processors in the computer. This method has different overloaded forms.
Yield Causes the calling thread to yield execution to another thread that is ready to run on the current processor. The operating system selects the thread to yield to.

Example

下面的示例演示了Thread類的用法。頁面有一個標籤控制項，用於顯示來自子線程的消息。使用Response.Write（）方法直接顯示來自主程序的消息。因此它們出現在頁面的頂部。

源文件如下：

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="threaddemo._Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >

   <head runat="server">
      <title>
         Untitled Page
      </title>
   </head>
   
   <body>
      <form id="form1" runat="server">
         <div>
            <h3>Thread Example</h3>
         </div>
         
         <asp:Label ID="lblmessage" runat="server" Text="Label">
         </asp:Label>
      </form>
   </body>
   
</html>

代碼隱藏文件如下：

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;

using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;

using System.Xml.Linq;
using System.Threading;

namespace threaddemo
{
   public partial class _Default : System.Web.UI.Page
   {
      protected void Page_Load(object sender, EventArgs e)
      {
         ThreadStart childthreat = new ThreadStart(childthreadcall);
         Response.Write("Child Thread Started <br/>");
         Thread child = new Thread(childthreat);
         
         child.Start();
         
         Response.Write("Main sleeping  for 2 seconds.......<br/>");
         Thread.Sleep(2000);
         Response.Write("<br/>Main aborting child thread<br/>");
         
         child.Abort();
      }
      
      public void childthreadcall()
      {
         try{
            lblmessage.Text = "<br />Child thread started <br/>";
            lblmessage.Text += "Child Thread: Coiunting to 10";
            
            for( int i =0; i<10; i++)
            {
               Thread.Sleep(500);
               lblmessage.Text += "<br/> in Child thread </br>";
            }
            
            lblmessage.Text += "<br/> child thread finished";
            
         }catch(ThreadAbortException e){
         
            lblmessage.Text += "<br /> child thread - exception";
            
         }finally{
            lblmessage.Text += "<br /> child thread - unable to catch the  exception";
         }
      }
   }
}

Observe the following

  • 加載頁時，將使用childthreadcall（）方法的引用啓動新線程。主線程活動直接顯示在網頁上。

  • 第二個線程運行並向label控制項發送消息。

  • 主線程休眠2000毫秒，在此期間執行子線程。

  • 子線程一直運行到被主線程中止爲止。它引發ThreadAbortException並被終止。

  • 控制項返回到主線程。

執行時，程序發送以下消息：