velocity自定义标签和指令

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velocity本身支持自定义标签和指令的扩展，

在 Velocity 模板语言的语法中，以美元符 $ 开头的为变量的声明或者引用，而以井号 # 开头的语句则为 Velocity 的指令（Directive）。

velocity支持的指令有：#set,#foreach,#if #else #end,#parse,#include,#evaluate,#define，#macro，

在velocity的jar包中的directive.properties中定义了这些实现：

directive.1=org.apache.velocity.runtime.directive.Foreachdirective.2=org.apache.velocity.runtime.directive.Includedirective.3=org.apache.velocity.runtime.directive.Parsedirective.4=org.apache.velocity.runtime.directive.Macrodirective.5=org.apache.velocity.runtime.directive.Literaldirective.6=org.apache.velocity.runtime.directive.Evaluatedirective.7=org.apache.velocity.runtime.directive.Breakdirective.8=org.apache.velocity.runtime.directive.Define

自定义标签和指定，比如我们定义了下面的remoteVelocity指令

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=gbk" /><title>click ok page</title></head><body>  This app runs well    #set($monkey= {"banana" : "good", "roast beef" : "bad"})  #remoteVelocity("namespace","velocityname",$monkey)</body></html>

要对这个指令的实现要继承Directive这个类，这个宏我们可以从其他服务获取vm的内容，动态渲染，这种方式可以统一管理公共模板，

import java.io.IOException;import java.io.Serializable;import java.io.StringWriter;import java.io.Writer;import java.util.HashMap;import java.util.Map;import org.apache.velocity.VelocityContext;import org.apache.velocity.app.VelocityEngine;import org.apache.velocity.context.InternalContextAdapter;import org.apache.velocity.exception.MethodInvocationException;import org.apache.velocity.exception.ParseErrorException;import org.apache.velocity.exception.ResourceNotFoundException;import org.apache.velocity.runtime.directive.Directive;import org.apache.velocity.runtime.parser.node.Node;import org.apache.velocity.runtime.parser.node.SimpleNode;import org.springframework.beans.factory.annotation.Autowired;import com.alibaba.citrus.service.template.TemplateService;import com.alibaba.click.util.HostUtil;public class RemoteVelocity extends Directive{@AutowiredTemplateService templateService;private static final VelocityEngine velocityEngine = new VelocityEngine();@Overridepublic String getName() {return "remoteVelocity";}@Overridepublic int getType() {return LINE;}@Overridepublic boolean render(InternalContextAdapter context, Writer writer,Node node) throws IOException, ResourceNotFoundException,ParseErrorException, MethodInvocationException {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);   SimpleNode sn_data = (SimpleNode) node.jjtGetChild(2); Object data = sn_data.value(context);   Map map = new HashMap();map.put("data", data);//String vel = HostUtil.getResponseText("http://127.0.0.1/index.html");String vel="#foreach($member in $data.entrySet())<li>$member.key - $member.value</li>#end ";writer.write(renderTemplate(map,vel));        return true;}public static String renderTemplate(Map params,String vimStr){        VelocityContext context = new VelocityContext(params);        StringWriter writer = new StringWriter();        try {velocityEngine.evaluate(context, writer, "", vimStr);} catch (ParseErrorException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (MethodInvocationException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ResourceNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();}//渲染模板        return writer.toString();}}

node.jjtGetChild(2) 这个方法可以获取对应指令的参数，下标从0开始，

在web工程的WEB-INF下面定义velocity.properties这个配置文件，用户扩展的指令最好放到这个文件里面，velocity的jar包里面提供了默认实现，我们可以覆盖重新定义自己的扩展，类就是对应自己的扩展类的类名

#自定义标签

userdirective=com.alibaba.click.test.RemoteVelocity

这样启动后就可以正常使用了。

Directive的三个方法：

getName：指令的名称getType:当前有LINE,BLOCK两个值，line行指令，不要end结束符，block块指令，需要end结束符public boolean render(InternalContextAdapter context, Writer writer,Node node) 具体处理过程