Simple stream read/write question in java

Question

I'm trying to upload a file via URLConnection, but I need to read/write it as a binary file without any encoding changes.

So i've tried to read byte[] array from a FileInputStream, but now i have an issue. The PrintWriter object I use for outputing to the server does not allow me to do writer.write(content) (where content is of type byte[]). How can i fix this? Or is there another way to quickly copy binary data from a FileInputStream to a PrintWriter?

Thank you

Answer 1

I bet that this is an follow-up on this question: Upload files from Java client to a HTTP server

If you want to upload binary files as well using multipart/form-data, then you need to write them to the OutputStream instead. Here's the changed example of the code as I posted in your previous example, only the try block has been changed to keep a separate handle to the binary output stream, so that you can write any InputStreams to it without any encoding pains:

OutputStream output = null;
PrintWriter writer = null;
try {
    output = connection.getOutputStream();
    writer = new PrintWriter(new OutputStreamWriter(output, "UTF-8"), true); // true = Autoflush, important!

    writer.println("--" + boundary);
    writer.println("Content-Disposition: form-data; name=\"paramToSend\"");
    writer.println("Content-Type: text/plain; charset=UTF-8");
    writer.println();
    writer.println(paramToSend);

    writer.println("--" + boundary);
    writer.println("Content-Disposition: form-data; name=\"fileToUpload\"; filename=\"" + fileToUpload.getName() + "\"");
    writer.println("Content-Type: " + URLConnection.guessContentTypeFromName(fileToUpload.getName());
    writer.println("Content-Transfer-Encoding: binary");
    writer.println();
    InputStream input = null;
    try {
        input = new FileInputStream(fileToUpload);
        byte[] buffer = new byte[1024];
        for (int length = 0; (length = input.read(buffer)) > 0;) {
            output.write(buffer, 0, length);
        }
        output.flush();
    } finally {
        if (input != null) try { input.close(); } catch (IOException logOrIgnore) {}
    }
    writer.println();

    writer.println("--" + boundary + "--");
} finally {
    if (writer != null) writer.close();
}

Answer 2

Writer objects (including PrintWriter) are intended specifically for output of character data. It sounds like you want an OutputStream instead of a Writer here.

Where did your PrintWriter come from? If it was created by wrapping some kind of OutputStream with an OutputStreamWriter and then wrapping that with a PrintWriter, then you should just use the original write(byte[] b) method from the original OutputStream, rather than trying to use a Writer.

If you want to mix character output and byte output, you may need to use String.getBytes(). Check out this example:

OutputStream o = this.conn.getOutputStream(); // Based on your comment
String s = "Hello, world!";
byte[] b = ...;      // These are the raw bytes that you want to write
o.write(s.getBytes("UTF-8"));
o.write(b);

(Of course, this will only work if the system that is reading your output understands that you are writing a mixture of characters and raw bytes and knows how to handle the mixed data that you are sending it.)

Answer 3

You could use 'getOutputStream()' on your URLConnection. Where is the PrintWriter coming from?

Answer 4

You should not use a PrintWriter as that is designed for text representation, and you want binary. A plain OutputStream should do, as Writers all operate on chars, in essence, text.

What do you want to achieve?