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

Question

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 precompu

Answer 1

What you want is RandomAccessFileInputStream - implements InputStream interface with mark/reset, sometimes seek based on RandomAccessFiles. Some implementations exist which might do what you need.

One example complete with sources is in http://www.fuin.org/utils4j/index.html but you would find many others searching the internet and its is easy enough to code if none fits exactly.

Answer 2

Check out java.io.RandomAccessFile

Answer 3

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) {
        super(fis);
        myFileChannel = fis.getChannel();
    }

    @Override
    public boolean markSupported() {
        return true;
    }

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

    @Override
    public synchronized void reset() throws IOException {
        if (mark == -1) {
            throw new IOException("not marked");
        }
        myFileChannel.position(mark);
    }
}

Answer 4

java.nio.channels.FileChannel has a method position(long) to reset the position back to zero like fseek() in C.

Answer 5

BufferedInputStream supports mark by buffering the content in memory. It is best reserved for relatively small look-aheads of a predictable size.

Instead, RandomAccessFile can be used directly, or it could serve as the basis for a concrete InputStream, extended with a rewind() method.

Alternatively, a new FileInputStream can be opened for each pass.

Answer 6

If you get the associated FileChannel from the FileInputStream, you can use the position method to set the file pointer to anywhere in the file.

FileInputStream fis = new FileInputStream("/etc/hosts");
FileChannel     fc = fis.getChannel();


fc.position(100);// set the file pointer to byte position 100;