Question

我正在尝试创建一种方法，允许用户通过Servlet下载文件系统上的文件。现在，下载时显示正确的filename并下载文件，但文件内容始终为空。

我还尝试将文件内容打印到控制台但没有显示。

有人能说出我在这里做错了吗？

由于

String filePath = "uploads/" + request.getParameter( "filename" );
File downloadFile = new File( filePath );

String relativePath = getServletContext().getRealPath( "" );
System.out.println( "relativePath = " + relativePath );

ServletContext context = getServletContext();

String mimeType = context.getMimeType( filePath );
if( mimeType == null )
{
    mimeType = "application/octet-stream";
}
System.out.println( "MIME type: " + mimeType );

response.setContentType( mimeType );
response.setContentLength( (int) downloadFile.length() );

String headerKey = "Content-Disposition";
String headerValue = String.format( "attachment; filename=\"%s\"", downloadFile.getName() );
response.setHeader( headerKey, headerValue );
byte[] bytes = Files.readAllBytes( downloadFile.getAbsoluteFile().toPath() );
OutputStream outStream = response.getOutputStream();

System.out.println( bytes.toString() );

outStream.write( bytes );

outStream.flush();
outStream.close();

注解

@WebServlet(urlPatterns =
{ "/Routing/*" })
@MultipartConfig(location = "/tmp", fileSizeThreshold = 1024 * 1024,     maxFileSize = 1024 * 1024 * 5, maxRequestSize = 1024 * 1024 * 5 * 5)
public class Routing extends HttpServlet
{

Answer 1

我建议使用几种测试来隔离问题，因为这里仍有太多未知数。我在运行代码时没有遇到任何问题，因此很可能是servlet容器的配置问题，或者是关于文件系统的假设。

隔离问题的关键是从基础开始。尝试返回String而不是文件，以确保与服务器的通信实际上正常运行。如果您得到相同的响应，那么您知道问题不在于文件IO：

byte[] bytes = "This is a test.".getBytes();
int contentLength = bytes.length;
String mimeType = "text/plain";

response.setContentType( mimeType );
response.setContentLength( contentLength );
OutputStream outStream = response.getOutputStream();
outStream.write( bytes );
outStream.flush();
outStream.close();

如果上述测试有效，那么您就知道问题出在文件或用于读取文件的方法上。您的代码目前对正在请求的文件的所有内容做出了很多假设。执行一些严格的测试以确保文件可访问：

int fileSize = downloadFile.length();
System.out.println("File path: " + downloadFile.getAbsolutePath());
System.out.println("exists: " + downloadFile.exists());
System.out.println("canRead: " + downloadFile.canRead());
System.out.println("File size: " + fileSize);

最后检查文件系统报告的文件大小与读取的字节数：

byte[] bytes = Files.readAllBytes( downloadFile.getAbsoluteFile().toPath() );
int bytesRead = bytes.length;
System.out.println("bytes read: " + bytesRead);

这些测试的结果应该为您缩小问题范围。

Answer 2

此解决方案是BalusC File Servlet blog的修改后的解决方案。

我使用此解决方案的原因是因为它在写入数据之前reset() HttpServletResponse response。

@WebServlet(urlPatterns =
{ "/Routing/*" })
@MultipartConfig(location = "/tmp", fileSizeThreshold = 1024 * 1024,     maxFileSize = 1024 * 1024 * 5, maxRequestSize = 1024 * 1024 * 5 * 5)
public class FileServlet extends HttpServlet {

    // Constants ----------------------------------------------------------------------------------

    private static final int DEFAULT_BUFFER_SIZE = 10240; // 10KB.


    // Actions ------------------------------------------------------------------------------------

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException
    {
        // Get requested file by path info.
        String filePath = "uploads/" + request.getParameter( "filename" );

        // Check if file is actually supplied to the request URI.
        if (filePath == null) {
            // Do your thing if the file is not supplied to the request URI.
            // Throw an exception, or send 404, or show default/warning page, or just ignore it.
            response.sendError(HttpServletResponse.SC_NOT_FOUND); // 404.
            return;
        }

        // Decode the file name (might contain spaces and on) and prepare file object.
        File downloadFile = new File( filePath );

        // Check if file actually exists in filesystem.
        if (!downloadFile.exists()) {
            // Do your thing if the file appears to be non-existing.
            // Throw an exception, or send 404, or show default/warning page, or just ignore it.
            response.sendError(HttpServletResponse.SC_NOT_FOUND); // 404.
            return;
        }

        // Get content type by filename.
        String contentType = getServletContext().getMimeType(file.getName());

        // If content type is unknown, then set the default value.
        // For all content types, see: http://www.w3schools.com/media/media_mimeref.asp
        // To add new content types, add new mime-mapping entry in web.xml.
        if (contentType == null) {
            contentType = "application/octet-stream";
        }

        // Init servlet response.
        response.reset();
        response.setBufferSize(DEFAULT_BUFFER_SIZE);
        response.setContentType(contentType);
        response.setHeader("Content-Length", String.valueOf(file.length()));
        response.setHeader("Content-Disposition", "attachment; filename=\"" + file.getName() + "\"");

        // Prepare streams.
        BufferedInputStream input = null;
        BufferedOutputStream output = null;

        try {
            // Open streams.
            input = new BufferedInputStream(new FileInputStream(file), DEFAULT_BUFFER_SIZE);
            output = new BufferedOutputStream(response.getOutputStream(), DEFAULT_BUFFER_SIZE);

            // Write file contents to response.
            byte[] bytes = Files.readAllBytes( downloadFile.getAbsoluteFile().toPath() );
            output.write(buffer, 0, bytes.length);
        } finally {
            // Gently close streams.
            close(output);
            close(input);
        }
    }

    // Helpers (can be refactored to public utility class) ----------------------------------------

    private static void close(Closeable resource) {
        if (resource != null) {
            try {
                resource.close();
            } catch (IOException e) {
                // Do your thing with the exception. Print it, log it or mail it.
                e.printStackTrace();
            }
        }
    }
}

我希望这会有所帮助。

Answer 3

可能文件为空添加此行以确保文件不为空

System.out.println("File size: " + bytes.length);

Answer 4

添加 outStream.setContentLength（inStream.available）; 在outStream.write（bytes）之后;

从Java servlet下载文件为null

4 个答案: