> Velocity is a Java-based template engine. It permits anyone to use a simple yet powerful template language to reference objects defined in Java code.
>
> When Velocity is used for web development, Web designers can work in parallel with Java programmers to develop web sites according to the Model-View-Controller (MVC) model, meaning that web page designers can focus solely on creating a site that looks good, and programmers can focus solely on writing top-notch code. Velocity separates Java code from the web pages, making the web site more maintainable over its lifespan and providing a viable alternative to Java Server Pages (JSPs) or PHP.
以上内容摘自[velocity的官方首页](http://velocity.apache.org/engine/devel/)。
以下通过示例来说明velocity的使用。
## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-06.md#项目结构)项目结构
在[http://velocity.apache.org/download.cgi](http://velocity.apache.org/download.cgi)中下载velocity-1.7、velocity-tools-2.0。
参考[00-03、从JSP开始](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-03.md)所述，创建项目`Project_0006_Velocity`，导入相关的jar，编写代码。
项目结构如下：
[](https://github.com/someus/another-tutorial-about-java-web/blob/master/img/00-06/01.png)
对于新增的jar，放到/WEB-INF/lib目录即可。但当多个webApp要使用时，放入CLASSPATH或Servlet容器（如Tomcat）的顶层lib是最好的选择.
## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-06.md#代码)代码
web.xml（在这一节，该文件可以忽略）：
~~~
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd">
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.jpg</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.png</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.js</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.css</url-pattern>
</servlet-mapping>
<session-config>
<session-timeout>
30
</session-timeout>
</session-config>
</web-app>
~~~
hello.vm:
~~~
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
#set( $this = "Velocity")
$this is great! <br/>
$name <br/>
hi , i am letian
<h1>你好</h1>
</body>
</html>
~~~
HelloServlet.java：
~~~
package me.letiantian.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.util.Properties;
import java.io.StringWriter;
import org.apache.velocity.app.Velocity;
import org.apache.velocity.app.VelocityEngine;
import org.apache.velocity.VelocityContext;
@WebServlet(name = "HelloServlet", urlPatterns = {"/hello"})
public class HelloServlet extends HttpServlet {
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
Properties properties=new Properties();
properties.setProperty("resource.loader", "webapp");
properties.setProperty("webapp.resource.loader.class", "org.apache.velocity.tools.view.servlet.WebappLoader");
properties.setProperty("webapp.resource.loader.path", "/WEB-INF/template");
properties.setProperty(Velocity.ENCODING_DEFAULT, "UTF-8");
properties.setProperty(Velocity.INPUT_ENCODING, "UTF-8");
properties.setProperty(Velocity.OUTPUT_ENCODING, "UTF-8");
VelocityEngine velocityEngine = new VelocityEngine(properties);
velocityEngine.setApplicationAttribute("javax.servlet.ServletContext", request.getServletContext());
VelocityContext context=new VelocityContext();
context.put("name", "user01");
StringWriter sw = new StringWriter();
velocityEngine.mergeTemplate("hello.vm", "utf-8", context, sw);
// velocityEngine.mergeTemplate("hello.vm", "utf-8", context, sw); //如果这行不注释，hello.vm的内容会出现两次
out.println(sw.toString());
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}
~~~
运行项目，用浏览器访问`http://127.0.0.1:8084/Project_0006_Velocity/hello`： [](https://github.com/someus/another-tutorial-about-java-web/blob/master/img/00-06/02.png)
## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-06.md#改进上面的代码)改进上面的代码
在WEB-INF目录下创建`velocity.properties`文件，其内容如下：
~~~
resource.loader=webapp
webapp.resource.loader.class=org.apache.velocity.tools.view.servlet.WebappLoader
webapp.resource.loader.path=/WEB-INF/template/
input.encoding=utf-8
output.encoding=utf-8
~~~
修改HelloServlet.java：
~~~
package me.letiantian.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.util.Properties;
import java.io.StringWriter;
import org.apache.velocity.app.VelocityEngine;
import org.apache.velocity.VelocityContext;
@WebServlet(name = "HelloServlet", urlPatterns = {"/hello"})
public class HelloServlet extends HttpServlet {
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
Properties properties=new Properties();
properties.load(getServletContext().getResourceAsStream("/WEB-INF/velocity.properties"));
VelocityEngine velocityEngine = new VelocityEngine(properties);
velocityEngine.setApplicationAttribute("javax.servlet.ServletContext", request.getServletContext());
VelocityContext context=new VelocityContext();
context.put("name", "user01");
StringWriter sw = new StringWriter();
velocityEngine.mergeTemplate("hello.vm", "utf-8", context, sw);
out.println(sw.toString());
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}
~~~
## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-06.md#资料)资料
[velocity模板加载](http://www.blogjava.net/sxyx2008/archive/2010/11/11/337799.html)
[velocity整合servlet](http://www.blogjava.net/sxyx2008/archive/2010/11/11/337819.html)
[Velocity三——基于Servlet+Velocity的web应用](http://bit1129.iteye.com/blog/2106142)
[使用 Velocity 实现客户端和服务器端模板](http://www.ibm.com/developerworks/cn/java/j-velocity/)
[Servlet 知识详解（一）之 —— ServletContext对象 和 ServletConfig对象 学习笔记](http://even2012.iteye.com/blog/1838063)
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