There's no guarantee that the content length you're provided is actually correct. Try something akin to the following:

ByteArrayOutputStream baos = new ByteArrayOutputStream();
InputStream is = null;
try {
  is = url.openStream ();
  byte[] byteChunk = new byte[4096]; // Or whatever size you want to read in at a time.
  int n;

  while ( (n = is.read(byteChunk)) > 0 ) {
    baos.write(byteChunk, 0, n);
  }
}
catch (IOException e) {
  System.err.printf ("Failed while reading bytes from %s: %s", url.toExternalForm(), e.getMessage());
  e.printStackTrace ();
  // Perform any other exception handling that's appropriate.
}
finally {
  if (is != null) { is.close(); }
}

You'll then have the image data in baos, from which you can get a byte array by calling baos.toByteArray().

This code is untested (I just wrote it in the answer box), but it's a reasonably close approximation to what I think you're after.

Your way of doing it seems like absolutely ok.

But if you saying:

"However, this method call tends to take a relatively long time to run"

You can have follow problems :

• Network, connection issue

• Are you sure that download each file in separate thread?

If you are using multithreading for that, be sure that VM args -XmsYYYYM and -XmxYYYYM configured well, because if not you can face problem , that your processor not using all cores. I have faced this problem some time ago.

As pskink said, your main problem is that you are wrapping your InputStream in a Reader. A Reader is meant to decode bytes into characters, which is, by defintion, what you want to avoid. You need to allocate a buffer and use the InputStream's read method to fill it.

Keep in mind that you have to know the amount of incoming bytes or read multiple times while processing the already received data.

Alternatively, you could use the read method to get an int, check whether the stream has ended, then cast that int to a byte and process one by one. But usually, you want to use a buffer, since that's faster.

Just in case these small changes make a difference, try this:

public static ByteBuffer getAsByteArray(URL url) throws IOException {
    URLConnection connection = url.openConnection();
    // Since you get a URLConnection, use it to get the InputStream
    InputStream in = connection.getInputStream();
    // Now that the InputStream is open, get the content length
    int contentLength = connection.getContentLength();

    // To avoid having to resize the array over and over and over as
    // bytes are written to the array, provide an accurate estimate of
    // the ultimate size of the byte array
    ByteArrayOutputStream tmpOut;
    if (contentLength != -1) {
        tmpOut = new ByteArrayOutputStream(contentLength);
    } else {
        tmpOut = new ByteArrayOutputStream(16384); // Pick some appropriate size
    }

    byte[] buf = new byte[512];
    while (true) {
        int len = in.read(buf);
        if (len == -1) {
            break;
        }
        tmpOut.write(buf, 0, len);
    }
    in.close();
    tmpOut.close(); // No effect, but good to do anyway to keep the metaphor alive

    byte[] array = tmpOut.toByteArray();

    //Lines below used to test if file is corrupt
    //FileOutputStream fos = new FileOutputStream("C:\\abc.pdf");
    //fos.write(array);
    //fos.close();

    return ByteBuffer.wrap(array);
}

You forgot to close fos which may result in that file being shorter if your application is still running or is abruptly terminated. Also, I added creating the ByteArrayOutputStream with the appropriate initial size. (Otherwise Java will have to repeatedly allocate a new array and copy, allocate a new array and copy, which is expensive.) Replace the value 16384 with a more appropriate value. 16k is probably small for a PDF, but I don't know how but the "average" size is that you expect to download.

Since you use toByteArray() twice (even though one is in diagnostic code), I assigned that to a variable. Finally, although it shouldn't make any difference, when you are wrapping the entire array in a ByteBuffer, you only need to supply the byte array itself. Supplying the offset 0 and the length is redundant.

Note that if you are downloading large PDF files this way, then ensure that your JVM is running with a large enough heap that you have enough room for several times the largest file size you expect to read. The method you're using keeps the whole file in memory, which is OK as long as you can afford that memory. :)

You can't do this out of the box with ftp.

If your file is on http, you could do something similar to:

URL url = new URL("http://q.com/test.mid");
InputStream is = url.openStream();
// Read from is

If you want to use a library for doing FTP, you should check out Apache Commons Net