Question

I want to read the first x bytes from a java.net.URLConnection (although I’m not forced to use this class – other suggestions welcome).

My code looks like this:

val head = new Array[Byte](2000)  
new BufferedInputStream(connection.getInputStream).read(head)
IOUtils.toString(new ByteArrayInputStream(head), charset)

It works, but does this code load only the first 2000 bytes from the network?

Next trial

As ‘JB Nizet’ said it is not useful to use a buffered input stream, so I tried it with an InputStreamReader:

val head = new Array[Char](2000)  
new InputStreamReader(connection.getInputStream, charset).read(head)
new String(head)

This code may be better, but the load times are about the same. So does this procedure limit the transferred bytes ?

Answer 1

You can use read(Reader, char[]) from Apache Commons IO. Just pass a 2000-character buffer to it and it will fill it with as many characters as possible, up to 2000.

Be sure you understand the objections in the other answers/comments, in particular:

• Don’t use Buffered... wrappers, it goes against your intentions.
• If you read textual data, then use a Reader to read 2000 characters instead of InputStream reading 2000 bytes. The proper procedure would be to determine the character encoding from the headers of a response (Content-Type) and set that encoding into InputStreamReader.
• Calling plain read(char[]) on a Reader will not fully fill the array you give to it. It can read as little as one character no matter how big the array is!
• Don’t forget to close the reader afterwards.

Other than that, I’d strongly recommend you to use Apache HttpClient in favor of java.net.URLConnection. It’s much more flexible.

---

Edit: To understand the difference between Reader.read and IOUtils.read, it’s worth examining the source of the latter:

public static int read(Reader input, char[] buffer,
                       int offset, int length)
    throws IOException
{
    if (length < 0) {
        throw new IllegalArgumentException("Length must not be negative: " + length);
    }
    int remaining = length;
    while (remaining > 0) {
        int location = length - remaining;
        int count = input.read(buffer, offset + location, remaining);
        if (EOF == count) { // EOF
            break;
        }
        remaining -= count;
    }
    return length - remaining;
}

Since Reader.read can read less characters than a given length (we only know it’s at least 1 and at most the length), we need to iterate calling it until we get the amount we want.