java通过内存复制文件

public static long copyFile(String s1, String s2) throws Exception {
        File f1 = new File(s1);
        File f2 = new File(s2);
        long time = new java.util.Date().getTime();
        int length = 2097152;
        FileInputStream in = new FileInputStream(f1);
        FileOutputStream out = new FileOutputStream(f2);
        FileChannel inC = in.getChannel();
        FileChannel outC = out.getChannel();
        ByteBuffer b = null;
        while (true) {
            if (inC.position() == inC.size()) {
                inC.close();
                outC.close();
                return new java.util.Date().getTime() - time;
            }
            if ((inC.size() - inC.position()) < length) {
                length = (int) (inC.size() - inC.position());
            } else {
                length = 2097152;
            }
            b = ByteBuffer.allocateDirect(length);
            inC.read(b);
            b.flip();
            outC.write(b);
            outC.force(false);
        }
    }