Java类如下

public static void downloadFile(String path,String fileName) {
try {
// 获得JSF上下文环境
            FacesContext context = FacesContext.getCurrentInstance();
// 获得ServletContext对象
            ServletContext servletContext = (ServletContext) context
                    .getExternalContext().getContext();
// 取得文件的绝对路径
            String realName = servletContext.getRealPath(path) + "/"
                    + fileName;
            HttpServletResponse httpServletResponse =

(HttpServletResponse) FacesContext
            .getCurrentInstance().getExternalContext().getResponse();
            downloadFile(httpServletResponse,realName,fileName);
        } catch (IOException e) {
            e.printStackTrace();
        }
        FacesContext.getCurrentInstance().responseComplete();
    }
public static void downloadFile(HttpServletResponse response,String  realName,String fileName)

throws IOException
    {      
        response.setHeader("Content-disposition",
                "attachment; filename=" + fileName);
        response.setContentType("application/x-download");      
//File exportFile = new File(realName);
        //response.setContentLength((int) exportFile.length());
        ServletOutputStream servletOutputStream = response.getOutputStream();
byte[] b = new byte[1024];
int i = 0;
        FileInputStream fis = new java.io.FileInputStream(realName);
while ((i = fis.read(b)) > 0) {
            servletOutputStream.write(b, 0, i);
        }
    }

使用方法

1、在backing bean的方法中调用函数1即可。如Abean中download方法调用了该方法，前台可以这样调用：

<h:commandButton value="download" action="#{aBean.download}"></h:commandButton>

或者

<h:commandLink value="download" action="#{fileUploadForm.download}"></h:commandLink>

2、jsp页面可以这样调用：

<%@ page contentType="text/html; charset=gb2312"%><%@page import="java.io.*"%><%
    String filename = "";
if (request.getParameter("filename") != null) {
        filename =     request.getParameter("filename");
    }
try {
        framework.util.FileUtils.downloadFile(response,getServletContext().getRealPath(filename),filename);
        } catch(final IOException e) {
            System.out.println ( "出现IOException." + e );
          } catch(final IllegalStateException e) {
            System.out.println ( "出现IllegalStateException." + e );
        } 
%>

于是jsf页面我们可以借助outputlink来调用该页面

<h:outputLink id="downloadfile"    value="#{page/FileDownload.jsp?filename=}">
<t:outputText value="下载文件" />
</h:outputLink>
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