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.Foreach directive.2=org.apache.velocity.runtime.directive.Include directive.3=org.apache.velocity.runtime.directive.Parse directive.4=org.apache.velocity.runtime.directive.Macro directive.5=org.apache.velocity.runtime.directive.Literal directive.6=org.apache.velocity.runtime.directive.Evaluate directive.7=org.apache.velocity.runtime.directive.Break directive.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{
@Autowired
TemplateService templateService;
private static final VelocityEngine velocityEngine = new VelocityEngine();
@Override
public String getName() {
return "remoteVelocity";
}
@Override
public int getType() {
return LINE;
}
@Override
public 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 block
e.printStackTrace();
} catch (MethodInvocationException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ResourceNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.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