Velocity使用总结

一、载入Velocity依赖包

<dependency>
    <groupId>org.apache.velocity</groupId>
    <artifactId>velocity</artifactId>
    <version>1.7</version>
</dependency>

二、Velocity类

package com.qunar.piao.data.integration.common;

import java.io.ByteArrayInputStream;
import java.io.CharArrayWriter;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.InputStream;
import java.io.Reader;
import java.io.Writer;
import java.util.HashMap;
import java.util.Map;

import org.apache.velocity.VelocityContext;
import org.apache.velocity.app.Velocity;
import org.apache.velocity.context.Context;

/**
 * Velocity引擎帮助类
 * 
 */
public class VelocityHelper {
    /** 单态实例 */
    private static final VelocityHelper instance = new VelocityHelper();

    /** 私有构造函数 */
    private VelocityHelper() {
        // 初始化velocity的信息 主要设置一些Velocity的默认属性

        // 初始化
        try {
            Velocity.init();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * <pre>
     * 取得实例
     * </pre>
     */
    public static VelocityHelper getInstance() {
        return instance;
    }

    /**
     * <pre>
     * 渲染：从reader到writer
     * </pre>
     * 
     * @param context
     * @param writer
     * @param reader
     * @return
     */
    public boolean evaluate(Context context, Writer writer, Reader reader) {
        try {
            return Velocity.evaluate(context, writer, "", reader);
        } catch (Exception e) {
            throw new RuntimeException("velocity evaluate error! detail [" + e.getMessage() + "]");
        }
    }

    /**
     * <pre>
     * 通过Map过滤一个输入流
     * </pre>
     * 
     * @param map
     * @param reader
     * @return
     */
    @SuppressWarnings("unchecked")
    public InputStream evaluate(Map map, Reader reader) {
        try {
            // 把产生的输出流(字符流)，转换成输入流(字节流)
            byte[] dataBytes = this.evaluateToWriter(map, reader).toString().getBytes();
            return new ByteArrayInputStream(dataBytes);
        } catch (Exception e) {
            throw new RuntimeException("velocity evaluate error! detial [" + e.getMessage() + "]");
        }
    }

    /**
     * <pre>
     * 通过Map过滤一个输入流
     * </pre>
     * 
     * @param map
     * @param reader
     * @return
     */
    @SuppressWarnings("unchecked")
    public Writer evaluateToWriter(Map map, Reader reader) {
        try {
            VelocityContext context = convertVelocityContext(map);
            CharArrayWriter writer = new CharArrayWriter();
            // 开始评估
            this.evaluate(context, writer, reader);

            return writer;
        } catch (Exception e) {
            throw new RuntimeException("velocity evaluate error! detail [" + e.getMessage() + "]");
        }
    }

    /**
     * <pre>
     * 取得Velocity系统属性
     * </pre>
     * 
     * @param key
     * @return
     */
    public Object getProperty(String key) {
        return Velocity.getProperty(key);
    }

    /**
     * <pre>
     * 把Map转换成Context
     * </pre>
     */
    private VelocityContext convertVelocityContext(Map<String, Object> map) {
        VelocityContext context = new VelocityContext();
        if (map == null) {
            return context;
        }
        for (Map.Entry<String, Object> entry : map.entrySet()) {
            context.put(entry.getKey(), entry.getValue());
        }
        return context;
    }

    /**
     * @param args
     * @throws FileNotFoundException
     */
    public static void main(String[] args) throws FileNotFoundException {
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("date", "2011-11-21");
        map.put("weather", "晴朗 李秋");

        Reader reader = new FileReader("D:\index.vm");
        Writer writer = VelocityHelper.getInstance().evaluateToWriter(map, reader);

        // 今天是：2011-11-21，天气：晴朗 李秋！
        System.out.println(writer.toString());
    }
}

三、在D盘添加模版文件

今天是：${date}，天气：${weather}！

四、运行VelocityHelper

今天是：2011-11-21，天气：晴朗 李秋！

五、Velocity语法总结

5.1 设置变量

#set( $test = $test2 + 3)

#set( $test.a = "abc")

#set( $test.b = 1)

5.2 循环

#foreach($conversionRate in $conversionRateList)
    $velocityCount //第几个，从1开始
    #foreach($member in $conversionRate.entrySet())
         $velocityCount //第几个，从1开始
         $member.value //值
    #end
#end

5.3 if判断

#if ($foo < 10)
    Go North
#elseif ($foo== 10)
    Go East
#else
    Go West
#end  

5.4 字符串截取

$member.value.substring(0,1)

5.5 字符串长度

$!{str.length()}

参考：

http://velocity.apache.org/tools/releases/1.3/generic/

http://www.360doc.com/content/11/1203/22/834950_169480722.shtml