- 浏览: 3470244 次
- 性别:
- 来自: 北京
文章分类
最新评论
-
wanglf1207:
EJB的确是个不错的产品,只是因为用起来有点门槛,招来太多人吐 ...
weblogic-ejb-jar.xml的元素解析 -
qwfys200:
总结的不错。
Spring Web Flow 2.0 入门 -
u011577913:
u011577913 写道也能给我发一份翻译文档? 邮件437 ...
Hazelcast 参考文档-4 -
u011577913:
也能给我发一份翻译文档?
Hazelcast 参考文档-4 -
songzj001:
DbUnit入门实战
原文出处:http://blog.chenlb.com/2009/07/use-httpservletresponsewrapper-implement-etag-filter.html
最近对 http caching 感兴趣,决定一步步学习之。现先来了解 Etag。
什么是“ETag”?
HTTP协议规格说明定义ETag为“被请求变量的实体值” (参见 http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html —— 章节 14.19)。 另一种说法是,ETag是一个可以与Web资源关联的记号(token)。典型的Web资源可以一个Web页,但也可能是JSON或XML文档。服务器单 独负责判断记号是什么及其含义,并在HTTP响应头中将其传送到客户端。
如果http 请求头 If-None-Match 的内容,与服务器对资源算出来的 etag 相同,就返回 304 响应。
下面来动动手,实现一个 etag 过虑器。原理:用 HttpServletResponseWrapper 把正常的页面输出到一个 byte 数组里,然后计算 etag,etag 是否与请求头一致,再进一步处理。
代码实现:
- package com.chenlb.http;
- import java.io.ByteArrayOutputStream;
- import java.io.IOException;
- import java.io.PrintWriter;
- import java.util.Calendar;
- import java.util.Date;
- import java.util.zip.CRC32;
- import javax.servlet.Filter;
- import javax.servlet.FilterChain;
- import javax.servlet.FilterConfig;
- import javax.servlet.ServletException;
- import javax.servlet.ServletOutputStream;
- import javax.servlet.ServletRequest;
- import javax.servlet.ServletResponse;
- import javax.servlet.http.HttpServletRequest;
- import javax.servlet.http.HttpServletResponse;
- import javax.servlet.http.HttpServletResponseWrapper;
- public class EtagFilter implements Filter {
- public void destroy() {}
- public void doFilter(ServletRequest request, ServletResponse response,
- FilterChain chain) throws IOException, ServletException {
- HttpServletRequest servletRequest = (HttpServletRequest) request;
- HttpServletResponse servletResponse = (HttpServletResponse) response;
- ByteArrayOutputStream baos = new ByteArrayOutputStream();
- HttpServletResponseWrapper hsrw = new MyHttpResponseWrapper(servletResponse, baos);
- chain.doFilter(request, hsrw);
- hsrw.flushBuffer();
- byte [] bytes = baos.toByteArray();
- CRC32 crc = new CRC32();
- crc.update(bytes);
- String token = "w/\"" + crc.getValue() + '"' ;
- servletResponse.setHeader("ETag" , token);
- // always store the ETag in the header
- String previousToken = servletRequest.getHeader("If-None-Match" );
- if (previousToken != null && previousToken.equals(token)) {
- // compare previous token with current one
- System.out.println("ETag match: returning 304 Not Modified" );
- servletResponse.sendError(HttpServletResponse.SC_NOT_MODIFIED);
- // use the same date we sent when we created the ETag the first time through
- servletResponse.setHeader("Last-Modified" , servletRequest.getHeader( "If-Modified-Since" ));
- } else {
- // first time through - set last modified time to now
- Calendar cal = Calendar.getInstance();
- cal.set(Calendar.MILLISECOND, 0 );
- Date lastModified = cal.getTime();
- servletResponse.setDateHeader("Last-Modified" , lastModified.getTime());
- System.out.println("Writing body content" );
- servletResponse.setContentLength(bytes.length);
- ServletOutputStream sos = servletResponse.getOutputStream();
- sos.write(bytes);
- sos.flush();
- sos.close();
- }
- }
- public void init(FilterConfig config) throws ServletException {}
- private static class MyHttpResponseWrapper extends HttpServletResponseWrapper {
- ByteServletOutputStream servletOutputStream;
- PrintWriter printWriter;
- public MyHttpResponseWrapper(HttpServletResponse response, ByteArrayOutputStream buffer) {
- super (response);
- servletOutputStream = new ByteServletOutputStream(buffer);
- }
- public ServletOutputStream getOutputStream() throws IOException {
- return servletOutputStream;
- }
- public PrintWriter getWriter() throws IOException {
- if (printWriter == null ) {
- printWriter = new PrintWriter(servletOutputStream);
- }
- return printWriter;
- }
- public void flushBuffer() throws IOException {
- servletOutputStream.flush();
- if (printWriter != null ) {
- printWriter.flush();
- }
- }
- }
- private static class ByteServletOutputStream extends ServletOutputStream {
- ByteArrayOutputStream baos;
- public ByteServletOutputStream(ByteArrayOutputStream baos) {
- super ();
- this .baos = baos;
- }
- public void write( int b) throws IOException {
- baos.write(b);
- }
- }
- }
web.xml 配置:
- < filter >
- < filter-name > etag </ filter-name >
- < filter-class > com.chenlb.http.EtagFilter </ filter-class >
- </ filter >
- < filter-mapping >
- < filter-name > etag </ filter-name >
- < url-pattern > *.jsp </ url-pattern >
- </ filter-mapping >
测试环境是 tomcat 6.0.18。
用 httpwatch 可以观察效果。
第二次请求(刷新),返回 304 。说明有效了。
过虑器同时还加了 Last-Modified 是为了兼容不支持 Etag 头的客户端。
infoq 下载来的代码没试用通过,原因是没有 flush PrintWriter。虽然有 304,但返回的内容为空。
当然算 etag 可用其它算法,我这里用 crc32。infoq 例子中用 md5。
发表评论
-
字符串分割--java中String.split()用法
2013-03-06 14:25 74091在java.lang包中有String.sp ... -
说明SOA监管(SOA Governance)实例(收录备查)
2012-12-19 11:35 1709SOA 已经不是单纯技术问 ... -
Injecting Spring Beans into Java Servlets
2012-11-01 10:21 1902If you are working in a Java ... -
Fitnesse使用
2012-05-05 13:27 23419Fitnesse 的使用 一,介绍 Fitnesse是一种 ... -
Customizing the new FitNesse parser
2012-05-05 13:13 2087FitNesse began its life using ... -
Eclipse Indigo - Cannot install Android ADT Plugin
2012-02-29 01:17 3831When I try to install the And ... -
Eclipse Indigo - Cannot install Android ADT Plugin
2012-02-29 01:13 1935When I try to install the And ... -
java application中内嵌ActiveX控件
2011-11-14 15:57 5466我这里用的是SWT/JFace开发application,SW ... -
Google Java Developer Tools Downloads
2011-08-09 00:04 2303WindowBuilder Pro原来叫WindowB ... -
Jalita
2011-08-06 00:49 1517Jalita (Java light terminal ada ... -
【转】用Java写字符终端界面
2011-07-29 13:13 2076终端界面GUI开源项目charva。 这个框架让你可以用开发 ... -
[转]mybatis下的分页,支持所有的数据库
2011-07-21 13:21 14783大 家都知道,mybatis的自带分页方法只是逻 ... -
Java framework for text- & console-based forms?
2011-07-21 01:06 1667charva jcurses JNA , ... -
JNA(Java Native Access)学习入门
2011-07-21 01:04 22516Java Native Access 项目 在 ... -
JAVA上加密算法的实现用例
2011-06-25 12:38 4834来源:www.ibm.com ... -
如何将GlassFish作为Windows服务运行
2011-05-18 23:21 2313本文档来自GlassFish官方网站,详细介绍了将 G ... -
JAVA UDP打洞必备知识点---NAT
2011-05-05 12:56 8601一、引言 RFCl631 ... -
Keystore概念,Keytool工具使用
2011-04-28 16:20 2860近来由于项目需要做Single Sign On, 研究了一 ... -
使用IntrospectorCleanupListener 解决quartz引起的内存泄漏
2011-04-20 11:59 13280"在服务器运行过程中,Spring不停的运行的计划任 ... -
利用Eclipse Profile Plugin监控分析Tomcat性能
2011-04-18 16:14 3667目前新版本的Eclipse在启动应用服务器的时候有一个新的选 ...
相关推荐
源代码 博文链接:https://msj.iteye.com/blog/179663
在struts2中使用result里type="stream"的结果类型时,可以实现文件的下载管理,使用时也是比较顺畅,但是当在“下载提示窗口”中点击“取消按钮”时,总是报出“java.lang.IllegalStateException”异常,异常内容...
HttpServletResponseWrapper HttpSession HttpSessionActivationListener HttpSessionAttributeListener HttpSessionBindingEvent HttpSessionBindingListener HttpSessionContext HttpSessionEvent ...
addCookie(Cookie) - Method in class javax.servlet.http.HttpServletResponseWrapper The default behavior of this method is to call addCookie(Cookie cookie) on the wrapped response object. addCookie...
javax.servlet.http.HttpServletResponseWrapper javax.servlet.http.HttpSessionBindingListener javax.servlet.http.HttpSessionAttributeListener javax.servlet.http.HttpSessionActivationListener
javax.servlet.http.HttpServletResponseWrapper.class javax/servlet/resources/web-app_2_2.dtd javax/servlet/resources/j2ee_1_4.xsd javax/servlet/resources/j2ee_web_services_client_1_1.xsd javax/servlet/...