读取文件的第一个字节

Question

I need a very simple function that allows me to read the first 1k bytes of a file through FTP. I want to use it in MATLAB to read the first lines and, according to some parameters, to download only files I really need eventually. I found some examples online that unfortunately do not work. Here I'm proposing the sample code where I'm trying to download one single file (I'm using the Apache libraries).

FTPClient client = new FTPClient();
    FileOutputStream fos = null;

    try {
        client.connect("data.site.org");

        // filename to be downloaded. 
        String filename = "filename.Z";
        fos = new FileOutputStream(filename);

        // Download file from FTP server
        InputStream stream = client.retrieveFileStream("/pub/obs/2008/021/ab120210.08d.Z");
        byte[] b = new byte[1024];
        stream.read(b);
        fos.write(b);

    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        try {
            if (fos != null) {
                fos.close();
            }
            client.disconnect();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

the error is in stream which is returned empty. I know I'm passing the folder name in a wrong way, but I cannot understand how I have to do. I've tried in many way.

I've also tried with the URL's Java classes as:

    URL url;

    url = new URL("ftp://data.site.org/pub/obs/2008/021/ab120210.08d.Z");

    URLConnection con = url.openConnection();
    BufferedInputStream in =
            new BufferedInputStream(con.getInputStream());
    FileOutputStream out =
            new FileOutputStream("C:\\filename.Z");

    int i;
    byte[] bytesIn = new byte[1024];
    if ((i = in.read(bytesIn)) >= 0) {
        out.write(bytesIn);
    }
    out.close();
    in.close();

but it is giving an error when I'm closing the InputStream in!

I'm definitely stuck. Some comments about would be very useful!

Answer 1

Try this test

    InputStream is = new URL("ftp://test:test@ftp.secureftp-test.com/bookstore.xml").openStream();
    byte[] a = new byte[1000];
    int n = is.read(a);
    is.close();
    System.out.println(new String(a, 0, n));

it definitely works

Answer 2

From my experience when you read bytes from a stream acquired from ftpClient.retrieveFileStream , for the first run it is not guarantied that you get your byte buffer filled up. However, either you should read the return value of stream.read(b); surrounded with a cycle based on it or use an advanced library to fill up the 1024 length byte[] buffer:

InputStream stream = null;
try {
    // Download file from FTP server
    stream = client.retrieveFileStream("/pub/obs/2008/021/ab120210.08d.Z");
    byte[] b = new byte[1024];
    IOUtils.read(stream, b); // will call periodically stream.read() until it fills up your buffer or reaches end-of-file
    fos.write(b);

} catch (IOException e) {
    e.printStackTrace();
} finally {
    IOUtils.closeQuietly(inputStream);
}

Answer 3

I cannot understand why it doesn't work. I found this link where they used the Apache library to read 4096 bytes each time. I read the first 1024 bytes and it works eventually, the only thing is that if completePendingCommand() is used, the program is held for ever. Thus I've removed it and everything works fine.