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j2ee,jsp,servlet文件下载server端

2017-07-01 08:25 351 查看
1.getOutputStream() has already been called for this response

报错的原因:

使用tomcat容器调用response.getOutputStream()方法的时候仅仅可以写二进制文件.

但调用requonse.getWriter()方法时，则仅仅可以在前台打印文本文件.

所以:

假设在jsp页面:

outPutStream.flush();
outPutStream.close();
outPutStream=null;
response.flushBuffer();
out.clear();
out = pageContext.pushBody();
这里out和pageContext是jsp的内置对象;

假设在后台:

假设使用response.getWrter,那么前面仅仅能用response.setContentType("text/html");

假设想用二进制流的方式输出,那么就不都够使用response.setContentType("text/html");

假设是想传输文件,推荐:

response.setContentType("application/x-msdownload;charset=utf-8");

同一时候为了防止乱码,response.setCharacterEncoding("utf-8");

filename = URLEncoder.encode(filename, "utf-8");

文件下载的代码:

public void getDownLoadBook() throws IOException{
response.setCharacterEncoding("utf-8");
String filepath=request.getSession().getServletContext().getRealPath("\\") + "upload\\books\\";
//通过前台bookId,从数据库中取出文件保存的信息
Book book=this.getBookService().getBook(bookId);
if(book==null)
{       response.setContentType("text/html");
PrintWriter writer=response.getWriter();
writer.write("<html><body>");
writer.write("此电子书不存在!");
writer.write("</body></html>");
writer.flush();
writer.close();
return;
}
String filename=book.getSaveName()+"";
java.io.File file = new java.io.File(filepath + filename);
if (!file.exists()) {
response.setContentType("text/html");
PrintWriter writer=response.getWriter();
writer.write("<html><body>");
writer.write("文件不存在!<br/>"+file.getAbsolutePath()+book.getOriginalName()+"不见了!");
writer.write("</body></html>");
writer.flush();
writer.close();
return;
}
response.setContentType("application/x-msdownload;charset=utf-8");
response.setCharacterEncoding("utf-8");
response.setContentLength((int) file.length());
filename=book.getOriginalName();
//编码文件名称,防止乱码
filename = URLEncoder.encode(filename, "utf-8");
response.setHeader("Content-Disposition", "attachment; filename=" +filename);
//得到输出流
javax.servlet.ServletOutputStream out = response.getOutputStream();
// 读取文件流
java.io.FileInputStream fileInputStream = new java.io.FileInputStream(file);
// 下载文件, 设置响应头和下载保存的文件名称
filename=book.getOriginalName();
if (filename != null && filename.length() > 0) {
if (fileInputStream != null) {
int filelen = fileInputStream.available();
文件太大时内存不能一次读出,要循环
int i=4096;
do{
byte a[] = new byte[4096];
i=fileInputStream.read(a);
out.write(a, 0, i);
}while(i>=4096);
}
fileInputStream.close();
out.flush();
out.close();
}
}
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