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下面這個Servlet用來計算大素數。因為計算非常大的數字（例如500位）可能要花不少時間，所以Servlet將立即返回已經找到的結果，同時在后臺繼續計算。后臺計算使用一個優先級較低的線程以避免過多地影響Web服務器的性能。如果計算還沒有完成，Servlet通過發送Refresh頭指示瀏覽器在幾秒之后繼續請求新的內容。
<BR> <BR>
注意，本例除了說明HTTP應答頭的用處之外，還顯示了Servlet的另外兩個很有價值的功能。首先，它表明Servlet能夠處理多個并發的連接，每個都有自己的線程。Servlet維護了一份已有素數計算請求的Vector表，通過查找素數個數（素數列表的長度）和數字個數（每個素數的長度）將當前請求和已有請求相匹配，把所有這些請求同步到這個列表上。第二，本例證明，在Servlet中維持請求之間的狀態信息是非常容易的。維持狀態信息在傳統的CGI編程中是一件很麻煩的事情。由于維持了狀態信息，瀏覽器能夠在刷新頁面時訪問到正在進行的計算過程，同時也使得Servlet能夠保存一個有關最近請求結果的列表，當一個新的請求指定了和最近請求相同的參數時可以立即返回結果。
<BR> <BR> PrimeNumbers.java
<BR> <BR>
注意，該Servlet要用到前面給出的ServletUtilities.java。另外還要用到：PrimeList.java，用于在后臺線程中創建一個素數的Vector；Primes.java，用于隨機生成BigInteger類型的大數字，檢查它們是否是素數。（此處略去PrimeList.java和Primes.java的代碼。）
<BR> package hall; <BR>
<BR> import java.io.*; <BR> import
javax.servlet.*; <BR> import javax.servlet.http.*;
<BR> import java.util.*; <BR>
<BR> public class PrimeNumbers extends HttpServlet {
<BR> private static Vector primeListVector = new
Vector(); <BR> private static int maxPrimeLists = 30;
<BR> <BR> public void
doGet(HttpServletRequest request, <BR>
HttpServletResponse response) <BR> throws
ServletException, IOException { <BR> int numPrimes =
ServletUtilities.getIntParameter(request, "numPrimes", 50);
<BR> int numDigits =
ServletUtilities.getIntParameter(request, "numDigits", 120);
<BR> PrimeList primeList =
findPrimeList(primeListVector, numPrimes, numDigits);
<BR> if (primeList == null) { <BR> primeList
= new PrimeList(numPrimes, numDigits, true); <BR>
synchronized(primeListVector) { <BR> if
(primeListVector.size() ＞= maxPrimeLists) <BR>
primeListVector.removeElementAt(0); <BR>
primeListVector.addElement(primeList); <BR> }
<BR> } <BR> Vector currentPrimes =
primeList.getPrimes(); <BR> int numCurrentPrimes =
currentPrimes.size(); <BR> int numPrimesRemaining =
(numPrimes - numCurrentPrimes); <BR> boolean
isLastResult = (numPrimesRemaining == 0); <BR> if
(!isLastResult) { <BR> response.setHeader("Refresh",
"5"); <BR> } <BR>
response.setContentType("text/html"); <BR> PrintWriter
out = response.getWriter(); <BR> String title = "Some "
+ numDigits + "-Digit Prime Numbers"; <BR>
out.println(ServletUtilities.headWithTitle(title) + <BR>
"＜BODY BGCOLOR=＼"#FDF5E6＼"＞＼n" + <BR> "＜H2
ALIGN=CENTER＞" + title + "＜/H2＞＼n" + <BR> "＜H3＞Primes
found with " + numDigits + <BR> " or more digits: " +
numCurrentPrimes + ".＜/H3＞"); <BR> if (isLastResult)
<BR> out.println("＜B＞Done searching.＜/B＞");
<BR> else <BR> out.println("＜B＞Still looking
for " + numPrimesRemaining + <BR> "
more＜BLINK＞...＜/BLINK＞＜/B＞"); <BR> out.println("＜OL＞");
<BR> for(int i=0; i＜numCurrentPrimes; i++) {
<BR> out.println(" ＜LI＞" + currentPrimes.elementAt(i));
<BR> } <BR> out.println("＜/OL＞");
<BR> out.println("＜/BODY＞＜/HTML＞"); <BR> }
<BR> <BR> public void
doPost(HttpServletRequest request, <BR>
HttpServletResponse response) <BR> throws
ServletException, IOException { <BR> doGet(request,
response); <BR> } <BR> <BR> //
檢查是否存在同類型請求（已經完成，或者正在計算）。 <BR> //
如存在，則返回現有結果而不是啟動新的后臺線程。 <BR> private PrimeList
findPrimeList(Vector primeListVector, <BR> int
numPrimes, <BR> int numDigits) { <BR>
synchronized(primeListVector) { <BR> for(int i=0;
i＜primeListVector.size(); i++) { <BR> PrimeList primes =
(PrimeList)primeListVector.elementAt(i); <BR> if
((numPrimes == primes.numPrimes()) && <BR>
(numDigits == primes.numDigits())) <BR> return(primes);
<BR> } <BR> return(null); <BR> }
<BR> } <BR> } <BR>
<BR> <BR> <BR>
<BR> PrimeNumbers.html <BR> ＜!DOCTYPE HTML
PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"＞
<BR> ＜HTML＞ <BR> ＜HEAD＞ <BR>
＜TITLE＞大素數計算＜/TITLE＞ <BR> ＜/HEAD＞
<BR> ＜CENTER＞ <BR> ＜BODY BGCOLOR="#FDF5E6"＞
<BR> ＜FORM ACTION="/servlet/hall.PrimeNumbers"＞
<BR> ＜B＞要計算幾個素數:＜/B＞ <BR> ＜INPUT TYPE="TEXT"
NAME="numPrimes" VALUE=25 SIZE=4＞＜BR＞ <BR>
＜B＞每個素數的位數:＜/B＞ <BR> ＜INPUT TYPE="TEXT" NAME="numDigits"
VALUE=150 SIZE=3＞＜BR＞ <BR> ＜INPUT TYPE="SUBMIT"
VALUE="開始計算"＞ <BR> ＜/FORM＞ <BR> ＜/CENTER＞
<BR> ＜/BODY＞ <BR> ＜/HTML＞ <BR>
<BR> <BR> <BR>
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<TD vAlign=top width="50%">前1篇 <A
href="http://www.aspcool.com/lanmu/dot.asp?ID=337&bbsuser=jsp"
target=_blank>七、HTTP應答狀態</A> <BR>前2篇 <A
href="http://www.aspcool.com/lanmu/dot.asp?ID=336&bbsuser=jsp"
target=_blank>六、訪問CGI變量</A> <BR>前3篇 <A
href="http://www.aspcool.com/lanmu/dot.asp?ID=335&bbsuser=jsp"
target=_blank>五、讀取HTTP請求頭</A> <BR>前4篇 <A
href="http://www.aspcool.com/lanmu/dot.asp?ID=334&bbsuser=jsp"
target=_blank>四、處理表單數據</A> <BR></TD>
<TD vAlign=top width="50%">后1篇 <A
href="http://www.aspcool.com/lanmu/dot.asp?ID=339&bbsuser=jsp"
target=_blank>九、處理Cookie</A> <BR>后2篇 <A
href="http://www.aspcool.com/lanmu/dot.asp?ID=340&bbsuser=jsp"
target=_blank>十、會話狀態</A> <BR>后3篇 <A
href="http://www.aspcool.com/lanmu/dot.asp?ID=341&bbsuser=jsp"
target=_blank>十一、JSP及語法概要</A> <BR></TD></TR></TBODY></TABLE><BR>
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