【導讀】本文對各種創(chuàng)造HTML數據流的方法進行了比較，其中有一些在特。定的情況下有更好的性能。ASP和VisualBasic編程設計的方法被合理采用了。開發(fā)者編寫ASP頁，其實是在創(chuàng)造一串格式化正文通過ASP提供給網絡客戶。測量性有很大影響。在幫助用戶協(xié)調他們Web應用性能的許多場合中，我已經。發(fā)現主要的獲勝因素是那個改變HTML數據流的方式。通的技術并且測試他們對簡單的ASP頁的性能的影響。很多ASP開發(fā)者已經遵循好的軟件設計原則并且模塊化他們的代碼。常采取用某一頁的部分離散的功能模塊組成新頁的形式?，F，通常是HTML代碼表，然后能通過各種各樣結合形式形成完整的一頁。不管現實操作執(zhí)行是從ASP的包含文件還是VisualBasicCOM零部件。考慮下列代碼碎片取自WriteHTML函數。這表示每當數據流的長度被改變的時候，數據表示就被破壞，包含。當然，在ASP和VisualBasic這都由你管理，因此實際上的花費不顯示在。實際上降低了數據流的大小通過改變數據串聯的優(yōu)先權的大小。ASP編譯器將考慮表達式的等式右邊并且從左到右評價它。

　　

【正文】 f the string concatenation operations by changing the order of precedence. In the original code sample the ASP plier will look at the expression to the right of the equals sign and just evaluate it from left to right. This results in 16 concatenation operations per iteration involving sHTML which is growing all the time. In the new version we are giving the piler a hint by changing the order in which it should carry out the operations. Now it will evaluate the expression from left to right but also inside out, . inside the parentheses first. This technique results in 15 concatenation operations per iteration involving smaller strings which are not growing and only one with the large, and growing, sHTML. Figure 1 shows an impression of the memory usage patterns of this optimization against the standard concatenation method. Figure 1 Comparison of memory usage pattern between standard and parenthesized concatenation Using parentheses can make quite a marked difference in performance and scalability in certain circumstances, as I will demonstrate later in this article. The StringBuilder We have seen the quick and easy solution to the string concatenation problem, and for many situations this may provide the best tradeoff between performance and effort to implement. If we want to get serious about improving the performance of building large strings, however, then we need to take the second alternative, which is to cut down the number of string allocation operations. In order to achieve this a StringBuilder is required. This is a class that maintains a configurable string buffer and manages insertions of new pieces of text into that buffer, causing string reallocation only when the length of the text exceeds the length of the string buffer. The Microsoft .NET framework provides such a class for free () that is remended for all string concatenation operations in that environment. In the ASP and classic Visual Basic world we do not have access to this class, so we need to build our own. Below is a sample StringBuilder class created using Visual Basic (errorhandling code has been omitted in the interest of brevity). Option Explicit 39。 default initial size of buffer and growth factor Private Const DEF_INITIALSIZE As Long = 1000 Private Const DEF_GROWTH As Long = 1000 39。 buffer size and growth Private m_nInitialSize As Long Private m_nGrowth As Long 39。 buffer and buffer counters Private m_sText As String Private m_nSize As Long Private m_nPos As Long Private Sub Class_Initialize() 39。 set defaults for size and growth m_nInitialSize = DEF_INITIALSIZE m_nGrowth = DEF_GROWTH 39。 initialize buffer InitBuffer End Sub 39。 set the initial size and growth amount Public Sub Init(ByVal InitialSize As Long, ByVal Growth As Long) If InitialSize 0 Then m_nInitialSize = InitialSize If Growth 0 Then m_nGrowth = Growth End Sub 39。 initialize the buffer Private Sub InitBuffer() m_nSize = 1 m_nPos = 1 End Sub 39。 grow the buffer Private Sub Grow(Optional MinimimGrowth As Long) 39。 initialize buffer if necessary If m_nSize = 1 Then m_nSize = m_nInitialSize m_sText = Space$(m_nInitialSize) Else 39。 just grow Dim nGrowth As Long nGrowth = IIf(m_nGrowth MinimimGrowth, m_nGrowth, MinimimGrowth) m_nSize = m_nSize + nGrowth m_sText = m_sText amp。 Space$(nGrowth) End If End Sub 39。 trim the buffer to the currently used size Private Sub Shrink() If m_nSize m_nPos Then m_nSize = m_nPos 1 m_sText = RTrim$(m_sText) End If End Sub 39。 add a single text string Private Sub AppendInternal(ByVal Text As String) If (m_nPos + Len(Text)) m_nSize Then Grow Len(Text) Mid$(m_sText, m_nPos, Len(Text)) = Text m_nPos = m_nPos + Len(Text) End Sub 39。 add a number of text strings Public Sub Append(ParamArray Text()) Dim nArg As Long For nArg = 0 To UBound(Text) AppendInternal CStr(Text(nArg)) Next nArg End Sub 39。 return the current string data and trim the buffer Public Function ToString() As String If m_nPos 0 Then Shrink ToString = m_sText Else ToString = End If End Function 39。 clear the buffer and reinit Public Sub Clear() InitBuffer End Sub The basic principle used in this class is that a variable (m_sText) is held at the class level that acts as a string buffer and this buffer is set to a certain size by filling it with space characters using the Space$ function. When more text needs to be concatenated with the existing text, the Mid$ function is used to insert the text at the correct position, after checking that our buffer is big enough to hold the new text. The ToString function returns the text currently stored in the buffer, also trimming the size of the buffer to the correct length for this text. The ASP code to use the StringBuilder would look like that shown below. Function WriteHTML( Data ) Dim oSB Dim nRep Set oSB = ( ) 39。 initialize the buffer with size and growth factor 15000, 7500 For nRep = 0 to 99 TRTD, (nRep + 1), /TDTD, _ Data( 0, nRep ), /TDTD, _ Data( 1, nRep ), /TDTD, _ Data( 2, nRep ), /TDTD, _ Data( 3, nRep ), /TDTD, _ Data( 4, nRep ), /TDTD, _ Data( 5, nRep ), /TD/TR Next WriteHTML = () Set oSB = Nothing End Function There is a definite overhead for using the StringBuilder because an instance