实现一个简单的文件下载 servlet [重复]

Question

我应该如何实现简单的文件下载servlet？

这个想法是，通过 GET 请求index.jsp?filename=file.txt，用户可以下载例如。 file.txt 来自文件 servlet，文件 servlet 会将该文件上传给用户。

我可以获取文件，但是如何实现文件下载？

Answer 1

假设您可以访问 servlet，如下所示

http://localhost:8080/myapp/download?id=7

我需要创建一个 servlet 并将其注册到 web.xml

web.xml

<servlet>
     <servlet-name>DownloadServlet</servlet-name>
     <servlet-class>com.myapp.servlet.DownloadServlet</servlet-class>
</servlet>
<servlet-mapping>
     <servlet-name>DownloadServlet</servlet-name>
     <url-pattern>/download</url-pattern>
</servlet-mapping>

下载Servlet.java

public class DownloadServlet extends HttpServlet {


    protected void doGet( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

         String id = request.getParameter("id");

         String fileName = "";
         String fileType = "";
         // Find this file id in database to get file name, and file type

         // You must tell the browser the file type you are going to send
         // for example application/pdf, text/plain, text/html, image/jpg
         response.setContentType(fileType);

         // Make sure to show the download dialog
         response.setHeader("Content-disposition","attachment; filename=yourcustomfilename.pdf");

         // Assume file name is retrieved from database
         // For example D:\\file\\test.pdf

         File my_file = new File(fileName);

         // This should send the file to browser
         OutputStream out = response.getOutputStream();
         FileInputStream in = new FileInputStream(my_file);
         byte[] buffer = new byte[4096];
         int length;
         while ((length = in.read(buffer)) > 0){
            out.write(buffer, 0, length);
         }
         in.close();
         out.flush();
    }
}

Answer 2

这取决于。如果所述文件通过您的 HTTP 服务器或 servlet 容器公开可用，您可以通过 response.sendRedirect() 简单地重定向到。

如果不是，则需要手动将其复制到响应输出流：

OutputStream out = response.getOutputStream();
FileInputStream in = new FileInputStream(my_file);
byte[] buffer = new byte[4096];
int length;
while ((length = in.read(buffer)) > 0){
    out.write(buffer, 0, length);
}
in.close();
out.flush();

当然，您需要处理适当的异常。

Answer 3

尝试资源

File file = new File("Foo.txt");
try (PrintStream ps = new PrintStream(file)) {
   ps.println("Bar");
}
response.setContentType("application/octet-stream");
response.setContentLength((int) file.length());
response.setHeader( "Content-Disposition",
         String.format("attachment; filename=\"%s\"", file.getName()));

OutputStream out = response.getOutputStream();
try (FileInputStream in = new FileInputStream(file)) {
    byte[] buffer = new byte[4096];
    int length;
    while ((length = in.read(buffer)) > 0) {
        out.write(buffer, 0, length);
    }
}
out.flush();

Answer 4

实现下载的最简单方法是将用户引导到文件位置，浏览器会自动为您完成。

您可以通过以下方式轻松实现：

HttpServletResponse.sendRedirect()

Answer 5

并发送一个大文件

byte[] pdfData = getPDFData();

String fileType = "";

res.setContentType("application/pdf");

httpRes.setContentType("application/.pdf");
httpRes.addHeader("Content-Disposition", "attachment; filename=IDCards.pdf");
httpRes.setStatus(HttpServletResponse.SC_OK);
OutputStream out = res.getOutputStream();
System.out.println(pdfData.length);
         
out.write(pdfData);
System.out.println("sendDone");
out.flush();