1<web-app>
 2  <servlet>
 3    <servlet-name>velocity</servlet-name>
 4    <servlet-class>org.apache.velocity.tools.view.VelocityViewServlet</servlet-class>
 5  </servlet>
 6  <servlet-mapping>
 7    <servlet-name>velocity</servlet-name>
 8    <url-pattern>*.vm</url-pattern>
 9  </servlet-mapping>
10  <welcome-file-list>
11    <welcome-file>index.vm</welcome-file>
12  </welcome-file-list>
13</web-app>

二、可选配置tools.xml

<tools>
    <data type="boolean" key="xhtml" value="true"/>
    <data type="boolean" key="isSimple" value="true"/>
    <data type="number" key="version" value="3.3.0"/>
    <data key="foo">this is foo</data>
    <data key="bar">this is bar.</data>
    <toolbox scope="request">
        <tool key="toytool" class="ToyTool" restrictTo="index*"/>
    </toolbox>
    <toolbox scope="session">
        <tool key="map" class="java.util.HashMap"/>
    </toolbox>
</tools>

ServletFileUpload.isMultipartContent(request)

检测request中是否包含有multipart内容

2、如果有，生成DiskFileItemFactory工厂将进行相关的设置

         DiskFileItemFactory factory = new DiskFileItemFactory();
        // maximum size that will be stored in memory
        factory.setSizeThreshold(maxMemSize);
        // Location to save data that is larger than maxMemSize.
        factory.setRepository(new File("d:/"));

3、生成上传ServletFileUpload类，并将DiskFileFactory工厂传给它，并对ServletFileUpload进行配置

       // Create a new file upload handler
        ServletFileUpload upload = new ServletFileUpload(factory);
        // maximum file size to be uploaded.
        upload.setSizeMax(maxFileSize);

4、从request得到上传的文件列表

            // Parse the request to get file items.
            List fileItems = upload.parseRequest(request);

            // Process the uploaded file items
            Iterator i = fileItems.iterator();

5、处理文件：写入或者其他操作

while (i.hasNext()) {
                FileItem fi = (FileItem) i.next();
                if (!fi.isFormField()) {
                    // Get the uploaded file parameters
                    String fieldName = fi.getFieldName();
                    String fileName = fi.getName();
                    String contentType = fi.getContentType();
                    boolean isInMemory = fi.isInMemory();
                    long sizeInBytes = fi.getSize();
                    // Write the file
                    if (fileName.lastIndexOf("\\") >= 0) {
                        file = new File(
                                filePath
                                        + fileName.substring(fileName
                                                .lastIndexOf("\\")));
                    } else {
                        file = new File(
                                filePath
                                        + fileName.substring(fileName
                                                .lastIndexOf("\\") + 1));
                    }
                    fi.write(file);
                    out.println("Uploaded Filename: " + fileName + "<br>");
                }
            }

}

说明：

FileItem接口是对用户上传文件的封装

DiskFileItemFactory实现了FileItemFactory接口，主要方法有public FileItem createItem(String fieldName, String contentType, boolean isFormField, String fileName)

ServletFileUpload从FileUpload继承，而FileUpload又从FileUploadBase继承，功能：分析传入的request对象、得到文件列表FileItemIterator……