Java file input with rewind()/reset() capability


I need to write a function that takes in some kind of input stream thing (e.g. an InputStream or a FileChannel) in order to read a large file in two passes: once to precompute some capacities, and second to do the "real" work. I do not want the whole file loaded into memory at once (unless it is small).

Is there an appropriate Java class that provides this capability? FileInputStream itself does not support mark()/reset(). BufferedInputStream does, I think, but I'm not clear whether it has to store the whole file to do this.

C is so simple, you just use fseek(), ftell(), and rewind(). 🙁

Best Answer

I think the answers referencing a FileChannel are on the mark .

Here's a sample implementation of an input stream that encapsulates this functionality. It uses delegation, so it's not a true FileInputStream, but it is an InputStream, which is usually sufficient. One could similarly extend FileInputStream if that's a requirement.

Not tested, use at your own risk :)

public class MarkableFileInputStream extends FilterInputStream {
    private FileChannel myFileChannel;
    private long mark = -1;

    public MarkableFileInputStream(FileInputStream fis) {
        myFileChannel = fis.getChannel();

    public boolean markSupported() {
        return true;

    public synchronized void mark(int readlimit) {
        try {
            mark = myFileChannel.position();
        } catch (IOException ex) {
            mark = -1;

    public synchronized void reset() throws IOException {
        if (mark == -1) {
            throw new IOException("not marked");