Direct file streaming from SSH using Java

Question

I am building an api with Java Spring Boot and what I want to do is use an endpoint to download a file. The problem is that the api accesses the file through ssh. I don't want the api to download the file and then return it, what I want is to make a direct stream from ssh to the response. Is this possible?

To connect by ssh and get the file I am using JSch. The files I want to download can be up to 2 GB in size. Thank you

Edit: I finally did this and it worked. Many thanks to @Martin Prikryl for his help.

@RequestMapping(value = "/endpoint-test/download-file-stream", method = RequestMethod.GET)
@ResponseStatus(HttpStatus.OK)
public static void  sendFileInResponse (HttpServletResponse response) throws Exception {
    String path_file = "/opt/dir/file_test.txt"; //this path is inside the remote server 
    ConexionJSch new_conection_to_filesystem = new ConexionJSch(); 
    ChannelSftp channelSftp = new_conection_to_filesystem.setupJsch();
    channelSftp.connect();
            
    SftpATTRS data = channelSftp.lstat(path_file);
    long fsize = data.getSize();
    try (InputStream inputStream = channelSftp.get(path_file)){
        response.setContentType("application/octet-stream");
        response.setContentLengthLong(fsize);
        response.setHeader("Content-Disposition", "inline;filename=test.bam");
        OutputStream outputStream = response.getOutputStream() 
        byte[] buff = new byte[2048];
        int length = 0;
        while ((length = inputStream.read(buff)) > 0) {
                outputStream.write(buff, 0, length);
                outputStream.flush();
        }
        inputStream.close();
        response.setHeader("Cache-Control", "private");
        response.setDateHeader("Expires", 0);    
    } finally{
        channelSftp.exit();
    }
}

public class ConexionJSch {
    private final String remoteHost;
    private final String username;
    private final String password;

    public ConexionJSch() {
        this.remoteHost = "xxx.xxx.xxx.xxx";
        this.username = "user";
        this.password = "pass";
    }
    
    public ChannelSftp setupJsch() throws JSchException {
        JSch jsch = new JSch();
        jsch.setKnownHosts("/home/ubuntu_user/.ssh/known_hosts");
        Session jschSession = jsch.getSession(username, remoteHost);
        jschSession.setPassword(password);
        jschSession.connect();
        return (ChannelSftp) jschSession.openChannel("sftp");
    }
}

Martin Prikryl · Accepted Answer · 2021-09-14T05:34:51.513

JSch has these two ChannelSftp.get overloads that will help you:

Based on your other question (How to return an InputStream as a file in Spring Boot?), in Spring Boot, you use HttpServletResponse API to stream the response (file). Combined with the ChannelSftp.get that accepts OutputStream, the code can be:

public static void sendFileInResponse(
        HttpServletResponse response, ChannelSftp channelSftp)
        throws IOException, SftpException {
    response.setContentType("your_content_type");
    response.setHeader("Content-Disposition", "inline;filename=your_file_name");
    channelSftp.get("/path/your_file_name", response.getOutputStream());
    outputStream.close();
}

This variant with OutputStream API is way shorter than your version with the channelSftp.get() InputStream API.

Though after reading bit on Spring Boot, it seems that a code like above is blocking and the download on the client side will effectively start only after the SFTP download finished. And consequently the web server will have to keep whole file in memory. It seems that more efficient solution is StreamingResponseBody.

Direct file streaming from SSH using Java

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