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 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(). :-(

Answer 1

Check out java.io.RandomAccessFile

Answer 2

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

Answer 3

RandomAccessFile is what you want:

• fseek() is translated to RandomAccessFile#seek
• ftell() is translated to RandomAccessFile#getFilePointer
• rewind() is seek(0)

Answer 4

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 it a "rewind" method.

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