How to read bytes from a file, whereas the result byte[] is exactly as long

I want the result byte[] to be exactly as long as the file content. How to achieve that开发者_C百科.

I am thinking of ArrayList<Byte>, but it doe not seem to be efficient.

Personally I'd go the Guava route:

File f = ...
byte[] content = Files.toByteArray(f);

Apache Commons IO has similar utility methods if you want.

If that's not what you want, it's not too hard to write that code yourself:

public static byte[] toByteArray(File f) throws IOException {
    if (f.length() > Integer.MAX_VALUE) {
        throw new IllegalArgumentException(f + " is too large!");
    }
    int length = (int) f.length();
    byte[] content = new byte[length];
    int off = 0;
    int read = 0;
    InputStream in = new FileInputStream(f);
    try {
        while (read != -1 && off < length) {
            read = in.read(content, off, (length - off));
            off += read;
        }
        if (off != length) {
            // file size has shrunken since check, handle appropriately
        } else if (in.read() != -1) {
            // file size has grown since check, handle appropriately
        }
        return content;
    } finally {
        in.close();
    }
}

I'm pretty sure File#length() doesn't iterate through the file. (Assuming this is what you meant by length()) Each OS provides efficient enough mechanisms to find file size without reading it all.

Allocate an adequate buffer (if necessary, resize it while reading) and keep track of how many bytes read. After finishing reading, create a new array with the exact length and copy the content of the reading buffer.

Small function that you can use :

// Returns the contents of the file in a byte array.
public static byte[] getBytesFromFile(File file) throws IOException {
    InputStream is = new FileInputStream(file);

    // Get the size of the file
    long length = file.length();

    // You cannot create an array using a long type.
    // It needs to be an int type.
    // Before converting to an int type, check
    // to ensure that file is not larger than Integer.MAX_VALUE.
    if (length > Integer.MAX_VALUE) {
        throw new RuntimeException(file.getName() + " is too large");
    }

    // Create the byte array to hold the data
    byte[] bytes = new byte[(int)length];

    // Read in the bytes
    int offset = 0;
    int numRead = 0;
    while (offset < bytes.length
           && (numRead=is.read(bytes, offset, bytes.length-offset)) >= 0) {
        offset += numRead;
    }

    // Ensure all the bytes have been read in
    if (offset < bytes.length) {
        throw new IOException("Could not completely read file "+file.getName());
    }

    // Close the input stream and return bytes
    is.close();
    return bytes;
}