如何通过偏移读取更改文本文件

Question

For example, there is one text file.

text files change.

test.txt = abcde

1min later

test.txt = cdefg
(a,b be deleted / f,g add)

in this situation, i read all the past file to e .

so i want to read the f of the current file. (This is an example and no one knows what words will come.)

The way I search for the last word I read and make an offset is impossible because there is a possibility of duplication.

I would appreciate your help :)

Answer 1

The answer will depend on filesystem semantics, and use of buffers. Java runs on top of an operating system, which provides an abstraction called "files", which java exposes through its own APIs (with FileInputStream and so on).

If you are using a BufferedReader , that reader has a buffer, where it stores an in-memory copy of whatever it reads from the actual file (as reported by the underlying OS). Therefore, if you change things on the file outside Java, there is no guarantee that your reader will return the latest, fresh version -- instead of whatever it read into the buffer a while ago.

The safe way to get fresh contents is to:

1. close the file
2. open it again
3. seek to the position you want to read, and read the contents.

Even then, you are not fully guaranteed to get really fresh contents, because it depends on the OS's way of handling writes (which in turn may depend on what device the files live on: network shares are not the same as spinning disks...). There may be a delay between one application thinking that it has written things to disk (or worse: writing it to an internal buffer that has not yet been flushed to disk), and the actual file being completely written out and ready to be read by others. Note the emphasis on completely : you can get partially-updated versions of files if you read them while they are still being written elsewhere.

This is one of the reasons that people use databases: they guarantee that you never get partially-updated versions (by using transactions and atomic updates). There are better ways than files to communicate two applications on the same machine; for example, consider using sockets.