📄 perf4.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Writing Advanced Applications, Chapter 8: Caching Client/Server Applications</TITLE>
<META NAME="AUTHOR" CONTENT="Monica Pawlan and Calvin Austin">
<META NAME="KEYWORDS" CONTENT="programming, advanced, Java 2">
<META NAME="OWNER" CONTENT="Editorial/JDC">
<META NAME="revision" CONTENT="@(#)perf4.src 1.14 09/21/99 JDC">
</HEAD>
<!-- Start Body Insert-->
<BODY BGCOLOR="#ffffff">
<!-- End Body Insert-->
<!-- Start PageTop Insert -->
<TABLE BORDER="0" CELLSPACING="0" CELLPADDING="0" WIDTH="100%">
<TR ALIGN="CENTER" VALIGN="TOP">
<TD WIDTH="157" ALIGN="LEFT">
<IMG SRC="/images/pixel.gif" HEIGHT="40" WIDTH="40" ALT="">
<A HREF="http://java.sun.com/index.html"><IMG SRC="/images/javalogo52x88.gif" WIDTH="52" HEIGHT="88" ALT="Java Technology Home Page" BORDER="0"></A>
<BR>
<IMG SRC="/images/pixel.gif" WIDTH="157" HEIGHT="1" ALT=""></TD>
<TD>
<FORM NAME="seek1" METHOD="GET" ACTION="http://search.java.sun.com/query.html">
<TABLE BORDER="0" CELLPADDING="0" CELLSPACING="0" WIDTH="100%">
<TR>
<TD ALIGN="RIGHT">
<IMG SRC="/images/stripelt.gif" WIDTH="6" HEIGHT="14" ALT=""></TD>
<TD WIDTH="100%">
<TABLE BORDER="0" CELLPADDING="0" CELLSPACING="0" WIDTH="100%">
<TR>
<TD BGCOLOR="#CC9966" WIDTH="100%">
<IMG SRC="/images/pixel.gif" HEIGHT="2" WIDTH="1" ALT=""></TD>
</TR>
<TR>
<TD>
<IMG SRC="/images/pixel.gif" HEIGHT="2" WIDTH="1" ALT=""></TD>
</TR>
<TR>
<TD BGCOLOR="#CC9966">
<IMG SRC="/images/pixel.gif" HEIGHT="2" WIDTH="1" ALT=""></TD>
</TR>
<TR>
<TD>
<IMG SRC="/images/pixel.gif" HEIGHT="2" WIDTH="1" ALT=""></TD>
</TR>
<TR>
<TD BGCOLOR="#CC9966">
<IMG SRC="/images/pixel.gif" HEIGHT="2" WIDTH="1" ALT=""></TD>
</TR>
<TR>
<TD>
<IMG SRC="/images/pixel.gif" HEIGHT="2" WIDTH="1" ALT=""></TD>
</TR>
<TR>
<TD BGCOLOR="#CC9966">
<IMG SRC="/images/pixel.gif" HEIGHT="2" WIDTH="1" ALT=""></TD>
</TR>
</TABLE>
</TD>
<TD ALIGN="LEFT">
<IMG SRC="/images/stripert.gif" WIDTH="6" HEIGHT="14" ALT=""></TD>
<TD>
<TABLE BORDER="0" CELLPADDING="0" CELLSPACING="0">
<TR>
<TD VALIGN="CENTER">
<A HREF="http://java.sun.com/a-z/index.html"><IMG SRC="/images/azindex.gif" BORDER="0" WIDTH="72" HEIGHT="11" ALT="A-Z Index"></A></TD>
<TD VALIGN="CENTER">
<FONT FACE="Helvetica" SIZE="1">
<INPUT TYPE="text" SIZE="15" MAXLENGTH="128" NAME=qt></FONT></TD>
<TD VALIGN="CENTER">
<INPUT TYPE="image" SRC="/images/search.button.gif"
value="search" BORDER="0" WIDTH="55" HEIGHT="14" ALT="Search"></TD>
</TR>
</TABLE>
</TD>
</TR>
</TABLE>
</FORM>
<P>
<TABLE BORDER="0" WIDTH="100%" CELLPADDING="0" CELLSPACING="0">
<TR VALIGN="TOP">
<TD WIDTH="100%"><IMG SRC="/images/chiclet.row.gif" WIDTH="55"
HEIGHT="18" ALT=""></TD>
<TD ROWSPAN="4" ALIGN="RIGHT" WIDTH="152">
<A HREF="/developer/index.html"><IMG SRC="/images/developer.connection.header.gif" BORDER="0" HEIGHT="42" WIDTH="319" ALT="Java Developer Connection(SM)"></A></TD>
</TR>
<TR VALIGN="TOP">
<TD BGCOLOR="#FFFFFF" HEIGHT="1" WIDTH="100%">
<IMG SRC="/images/pixel.gif" HEIGHT="1" WIDTH="1" ALT=""></TD>
</TR>
<TR VALIGN="TOP">
<TD BGCOLOR="#CC9966" HEIGHT="1" WIDTH="100%">
<IMG SRC="/images/pixel.gif" HEIGHT="1" WIDTH="1" ALT=""></TD>
</TR>
<TR VALIGN="TOP">
<TD><A HREF="/developer/onlineTraining/"><IMG SRC="/images/online-training.gif" ALT="Online Training" BORDER=0></A></TD>
</TR>
</TABLE>
</TD>
</TR>
</TABLE>
<!-- End PageTop Insert -->
<!-- Start NavBar Insert -->
<TABLE BORDER="0" CELLSPACING="0" CELLPADDING="3" BGCOLOR="#FFFFFF" WIDTH="157" ALIGN="LEFT">
<!-- tab categories -->
<TR>
<TD><A HREF="http://java.sun.com/products/"><IMG SRC="/images/side.tab.products.gif" HEIGHT="15" WIDTH="130" BORDER="0" ALT="Downloads, APIs, Documentation"></A></TD>
</TR>
<TR>
<TD><A HREF="/developer/index.html"><IMG SRC="/images/side.tab.developer.gif" HEIGHT="15" WIDTH="130" BORDER="0" ALT="Java Developer Connection"></A></TD>
</TR>
<TR>
<TD><A HREF="/developer/infodocs/index.shtml"><IMG SRC="/images/side.tab.docs.gif" HEIGHT="15" WIDTH="130" BORDER="0" ALT="Tutorials, Tech Articles, Training"></A></TD>
</TR>
<TR>
<TD><A HREF="/developer/support/index.html"><IMG SRC="/images/side.tab.support.gif" HEIGHT="15" WIDTH="130" BORDER="0" ALT="Online Support"></A></TD>
</TR>
<TR>
<TD><A HREF="/developer/community/index.html"><IMG SRC="/images/side.tab.community.gif" HEIGHT="15" WIDTH="130" BORDER="0" ALT="Community Discussion"></A></TD>
</TR>
<TR>
<TD><A HREF="http://java.sun.com/industry/"><IMG SRC="/images/side.tab.news.gif" HEIGHT="15" WIDTH="130" BORDER="0" ALT="News & Events from Everywhere"></A></TD>
</TR>
<TR>
<TD><A HREF="http://java.sun.com/solutions"><IMG SRC="/images/side.tab.solutions.gif" HEIGHT="15" WIDTH="130" BORDER="0" ALT="Products from Everywhere"></A></TD>
</TR>
<TR>
<TD><A HREF="http://java.sun.com/casestudies"><IMG SRC="/images/side.tab.case.gif" HEIGHT="15" WIDTH="130" BORDER="0" ALT="How Java Technology is Used Worldwide"></A></TD>
</TR>
<TR><TD> </TD></TR>
<!-- End NavBar Insert -->
<!-- START SUB-NAV -->
<TR>
<TD><!-- INSERT SUB-NAV INFO -->
</TD>
</TR>
<!-- END SUB-NAV -->
</TABLE>
<TABLE BORDER="0" CELLPADDING="0" CELLSPACING="0">
<TR>
<TD>
<!-- Template Version 2.0 -->
<!-- ================== -->
<!-- Start Main Content -->
<!-- ================== -->
<TABLE BORDER="0" CELLPADDING="0" CELLSPACING="0">
<TR>
<TD VALIGN="TOP">
<FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A NAME="top"></A>
<DIV ALIGN="RIGHT">
<FONT SIZE="-1"><A HREF="/developer/onlineTraining/index.html">Training Index</A></FONT>
<H2>Writing Advanced Applications<BR>
<FONT SIZE="3">Chapter 8 Continued: Caching Client/Server Applications</FONT></H2>
<FONT SIZE="-1">[<A HREF="perf3.html"><<BACK</A>] [<A HREF="index.html#contents">CONTENTS</A>] [<A HREF="deploy.html">NEXT>></A>]</FONT></DIV>
<P>
Caching is one of the first techniques used to improve the
performance of web browsers and web servers. The browser
cache makes network lookup operations unnecessary because
a recent copy of the file is kept in the local cache, and the
web server cache reduces the cost of loading the file
from disk for each request. This section explains how you
can use caching in a similar way to improve performance in many
client/server applications written in the
Java<FONT SIZE="-2"><SUP>TM</SUP></FONT> programming language.
<P>
The <CODE>java.util.Collections</CODE> API available in the Java® 2
Software Development Kit (SDK) software makes implementing a cache simple.
This API provides the <CODE>HashMap</CODE> class, which works well for
caching one object, and the <CODE>LinkedList</CODE> class, which works well
in combination with the <CODE>HashMap</CODE> class for caching many objects.
<UL>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#one">Caching One Object</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#more">Caching Many Objects</A></FONT>
</UL>
<HR>
<A NAME="one"></A>
<H3>Caching One Object</H3>
A <CODE>HashMap</CODE> object stores data in key and value pairs.
When you put a data value in the <CODE>HashMap</CODE>, you assign it a
key and later use that key to retrieve the data.
<P>
A <CODE>HashMap</CODE> object is very similar to a <CODE>Hashtable</CODE>
and can be used to keep a temporary copy of previously generated results.
Objects kept in the <CODE>HashMap</CODE>
cache could, for example, be a list of completed auction results.
<P>
In this case, the results of a JDBC query might be requested hundreds of
times a second by persons wanting to know who was the highest bidder, but
the completed results lists only actually changes once a minute as each
auction completes. You can write your
program to retrieve unchanged objects from the results cache instead of
querying the database every time and gain a significant performance
improvement.
<P>
This <A HREF="./Code/DBCache.java">code example</A>
runs a database query once a minute, and returns
cached copies for requests that come between the queries.
</FONT>
<PRE>
import java.util.*;
import java.io.*;
class DBCacheRecord {
Object data;
long time;
public DBCacheRecord(Object results, long when) {
time=when;
data=results;
}
public Object getResults() {
return data;
}
public long getLastModified() {
return time;
}
}
public class DBCache {
Map cache;
public DBCache() {
cache = new HashMap();
}
public Object getDBData(String dbcommand) {
if(!cache.containsKey(dbcommand)) {
synchronized(cache) {
cache.put(dbcommand, readDBData(dbcommand));
}
} else {
if((new Date().getTime() ) -
((DBCacheRecord)cache.get(
dbcommand)).getLastModified()>=1000) {
synchronized(cache) {
cache.put(dbcommand, readDBData(dbcommand));
}
}
}
return ((DBCacheRecord)cache.get(
dbcommand)).getResults();
}
public Object readDBData(String dbcommand) {
/*Insert your JDBC code here For Example:
ResultSet results=stmt.executeQuery(dbcommand);
*/
String results="example results";
return(new DBCacheRecord(results,new
Date().getTime()));
}
public static void main(String args[]) {
DBCache d1=new DBCache();
for(int i=1;i<=20;i++) {
d1.getDBData(
"select count(*) from results where
TO_DATE(results.completed) <=SYSDATE");
}
}
}
</PRE>
<FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A NAME="more"></A>
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -