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。
以下通过示例来说明velocity的使用。
项目结构
在http://velocity.apache.org/download.cgi中下载velocity-1.7、velocity-tools-2.0。
参考00-03、从JSP开始所述，创建项目Project_0006_Velocity，导入相关的jar，编写代码。
项目结构如下：
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.
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：
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：