Why does getResourceAsStream() and reading file with FileInputStream return arrays of different length?

Question

I want to read files as byte arrays and realised that amount of read bytes varies depending on the used method. Here the relevant code:

    public byte[] readResource() {
    try (InputStream is = getClass().getClassLoader().getResourceAsStream(FILE_NAME)) {
        int available = is.available();
        byte[] result = new byte[available];
        is.read(result, 0, available);

        return result;
    } catch (Exception e) {
        log.error("Failed to load resource '{}'", FILE_NAME, e);
    }

    return new byte[0];
}

public byte[] readFile() {
    File file = new File(FILE_PATH + FILE_NAME);
    try (InputStream is = new FileInputStream(file)) {
        int available = is.available();
        byte[] result = new byte[available];
        is.read(result, 0, available);

        return result;
    } catch (Exception e) {
        log.error("Failed to load file '{}'", FILE_NAME, e);
    }

    return new byte[0];
}

Calling File.length() and reading with the FileInputStream returns the correct length of 21566 bytes for the given test file, though reading the file as a resources returns 21622 bytes.

Does anyone know why I get different results and how to fix it so that readResource() returns the correct result?

Answer 1

Why does getResourceAsStream() and reading file with FileInputStream return arrays of different length?

Because you're misusing the available() method in a way that is specifically warned against in the Javadoc :

"It is never correct to use the return value of this method to allocate a buffer intended to hold all data in this stream."

and

Does anyone know why I get different results and how to fix it so that readResource() returns the correct result?

Read in a loop until end of stream.

Answer 2

According to the the API docs of InputStream, InputStream.available() does not return the size of the resource - it returns

an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking

To get the size of a resource from a stream, you need to fully read the stream, and count the bytes read.

To read the stream and return the contents as a byte array, you could do something like this:

    try (   InputStream is = getClass().getClassLoader().getResourceAsStream(FILE_NAME);
            ByteArrayOutputStream bos = new ByteArrayOutputStream()) {

        byte[] buffer = new byte[4096];
        int bytesRead = 0;
        while ((bytesRead = is.read(buffer)) != -1) {
            bos.write(buffer, 0, bytesRead);
        }
        return bos.toByteArray();

    }