编写自定义的 Velocity 指令 #cache，,使用方法#cache(＂

编写自定义的 Velocity 指令 #cache，,使用方法#cache("

使用方法

#cache("News","home")## 读取数据库中最新新闻并显示<ul>#foreach($news in $NewsTool.ListTopNews(10))<li><span class='date'>$date.format("yyyy-MM-dd",${news.pub_time})</span><span class='title'>${news.title}</span></li>#end</ul>#end

/** * Velocity模板上用于控制缓存的指令 * @author Winter Lau * @date 2009-3-16 下午04:40:19 */public class CacheDirective extends Directive {    final static Hashtable<String,String> body_tpls = new Hashtable<String, String>();    @Override    public String getName() { return "cache"; } //指定指令的名称    @Override    public int getType() { return BLOCK; } //指定指令类型为块指令    /* (non-Javadoc)    * @see org.apache.velocity.runtime.directive.Directive#render()    */    @Override    public boolean render(InternalContextAdapter context, Writer writer, Node node)        throws IOException, ResourceNotFoundException, ParseErrorException,        MethodInvocationException     {        //获得缓存信息        SimpleNode sn_region = (SimpleNode) node.jjtGetChild(0);        String region = (String)sn_region.value(context);        SimpleNode sn_key = (SimpleNode) node.jjtGetChild(1);        Serializable key = (Serializable)sn_key.value(context);        Node body = node.jjtGetChild(2);        //检查内容是否有变化        String tpl_key = key+"@"+region;        String body_tpl = body.literal();        String old_body_tpl = body_tpls.get(tpl_key);        String cache_html = CacheHelper.get(String.class, region, key);        if(cache_html == null || !StringUtils.equals(body_tpl, old_body_tpl)){            StringWriter sw = new StringWriter();            body.render(context, sw);            cache_html = sw.toString();            CacheHelper.set(region, key, cache_html);            body_tpls.put(tpl_key, body_tpl);        }        writer.write(cache_html);        return true;    }}//该片段来自于http://byrx.net