一个文件下载的Servlet

把文件目录直接暴露给用户是很不安全的。所以要用Servlet来做，而且这样做，文件的存储方式就更丰富了，可以是从文件系统上取来的，也可以是数据库中经过计算生成的，或者从其它什么稀奇古怪的地方取来的。

public class DownloadServlet extends HttpServlet {
    private String contentType = "application/x-msdownload";
    private String enc = "utf-8";
    private String fileRoot = "";


    /**
     * 初始化contentType，enc，fileRoot
     */
    public void init(ServletConfig config) throws ServletException {
        String tempStr = config.getInitParameter("contentType");
        if (tempStr != null && !tempStr.equals("")) {
            contentType = tempStr;
        }
        tempStr = config.getInitParameter("enc");
        if (tempStr != null && !tempStr.equals("")) {
            enc = tempStr;
        }
        tempStr = config.getInitParameter("fileRoot");
        if (tempStr != null && !tempStr.equals("")) {
            fileRoot = tempStr;
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String filepath = request.getParameter("filepath");
        String fullFilePath = fileRoot + filepath;
        /*读取文件*/
        File file = new File(fullFilePath);
        /*如果文件存在*/
        if (file.exists()) {
            String filename = URLEncoder.encode(file.getName(), enc);
            response.reset();
            response.setContentType(contentType);
            response.addHeader("Content-Disposition", "attachment; filename=\"" + filename + "\"");
            int fileLength = (int) file.length();
            response.setContentLength(fileLength);
            /*如果文件长度大于0*/
            if (fileLength != 0) {
                /*创建输入流*/
                InputStream inStream = new FileInputStream(file);
                byte[] buf = new byte[4096];
                /*创建输出流*/
                ServletOutputStream servletOS = response.getOutputStream();
                int readLength;
                while (((readLength = inStream.read(buf)) != -1)) {
                    servletOS.write(buf, 0, readLength);
                }
                inStream.close();
                servletOS.flush();
                servletOS.close();
            }
        }
    }

web.xml

    <servlet>
        <servlet-name>downloadservlet-name>
        <servlet-class>org.mstar.servlet.DownloadServletservlet-class>
        <init-param>
            <param-name>fileRootparam-name>
            <param-value>d:/tempparam-value>
        init-param>
        <init-param>
            <param-name>contentTypeparam-name>
            <param-value>application/x-msdownloadparam-value>
        init-param>
        <init-param>
            <param-name>encparam-name>
            <param-value>utf-8param-value>
        init-param>
    servlet>
    <servlet-mapping>
        <servlet-name>downloadservlet-name>
        <url-pattern>/downurl-pattern>
    servlet-mapping>