http etag的普青跟文青用法

普通青年用etag：后置拦截

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
     HttpServletRequest servletRequest = (HttpServletRequest) req;
     HttpServletResponse servletResponse = (HttpServletResponse) res;

      ByteArrayOutputStream baos = new ByteArrayOutputStream();
     ETagResponseWrapper wrappedResponse = new ETagResponseWrapper(servletResponse, baos);
     chain.doFilter(servletRequest, wrappedResponse);

      byte[] bytes = baos.toByteArray();

      String token = '"' + ETagComputeUtils.getMd5Digest(bytes) + '"';//对已经生成的视图内容计算md5值，消耗CPU
     servletResponse.setHeader("ETag", token); // always store the ETag in the header

      String previousToken = servletRequest.getHeader("If-None-Match");
     if (previousToken != null && previousToken.equals(token)) { // 比较提交上来的且是上次从服务器获取的etags，相等则返回304，节省带宽
         logger.debug("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 {         // 赔了夫人又折兵，但是为了某些不支持1.1的浏览器还是要返回LM
         Calendar cal = Calendar.getInstance();
         cal.set(Calendar.MILLISECOND, 0);
         Date lastModified = cal.getTime();
         servletResponse.setDateHeader("Last-Modified", lastModified.getTime());

          logger.debug("Writing body content");
         servletResponse.setContentLength(bytes.length);
         ServletOutputStream sos = servletResponse.getOutputStream();
         sos.write(bytes);
         sos.flush();
         sos.close();
     }
 }

总结：简而言之就是牺牲性能换带宽，将已经经过底层操作（数据库操or外部接口之类的）获得的视图对象计算md5值，跟提交上来的etag进行比较，决定是发送304还是原本的web内容;缺点是对变更频率比较大的web内容没有良好的容忍性

文艺青年用etag：前置拦截

getToken方法从一个全局变量里取当前请求uri的计数器

public final boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws ServletException, IOException {
     String method = request.getMethod();
     if (!"GET".equals(method))
         return true;

      String previousToken = request.getHeader("If-None-Match");
     String token = getTokenFactory().getToken(request);//取得计数器

      // compare previous token with current one
     if ((token != null) && (previousToken != null && previousToken.equals('"' + token + '"'))) {//
         response.sendError(HttpServletResponse.SC_NOT_MODIFIED);
         // re-use original last modified timestamp
         response.setHeader("Last-Modified", request.getHeader("If-Modified-Since"))
         return false; // 不需要走处理链，直接返回304
     }

      // set header for the next time the client calls
     if (token != null) {
         response.setHeader("ETag", '"' + token + '"');

          // first time through - set last modified time to now
         Calendar cal = Calendar.getInstance();
         cal.set(Calendar.MILLISECOND, 0);
         Date lastModified = cal.getTime();
         response.setDateHeader("Last-Modified", lastModified.getTime());
     }

      return true;//没什么损失，继续走处理链
 }

全局变量是一个key-value的结构

public String getToken(HttpServletRequest request) {
     String view = request.getRequestURI();
     Integer count = counts.get(view);
     if (count == null) {
         return null;
     }

      return count.toString();
 }

其中这个K-V计数器的变更是绑定在底层数据库的写函数上的（以Hibernate 3 LocalSessionFactoryBean为例）

public class DeleteHandler extends DefaultDeleteEventListener {
     private ModifiedObjectTracker tracker;

      public void onDelete(DeleteEvent event) throws HibernateException {
         String viewname=eventname2view(event.getEntityName());//实体名转视图名字（比如说论坛某个帖子实质内容的ajax链接）
        Integer count=counts.get(viewname);//获取计数器并自增
        counts.put(view,count++);
     }

     public ModifiedObjectTracker getModifiedObjectTracker() {
         return tracker;
     }
      public void setModifiedObjectTracker(ModifiedObjectTracker tracker) {
         this.tracker = tracker;
     }
 }

总结：这么做不仅节省带宽，还节省CPU，更节省用户等待时间，有利于提高用户体验，可谓一举多得;需要注意的是，计数器那一块得设计好过期算法，那不然内存会不够你花