在写入文件时读取文件

Question

I've read some posts on stackoverflow about this topic but I'm still confused. When reading a file that is currently being written in Java, how do you keep track of how many lines have actually been written so that you don't get weird read results?

EDIT: sorry, I should have mentioned that the file writing it is in C++ and the one reading it is in Java so variables can't really be shared easily

Answer 1

When reading a file that is currently being written in Java, how do you keep track of how many lines have actually been written so that you don't get weird read results?

The problem is that you can never be sure that the current last character of the file is the end of a line. If it is a line terminator, you are OK. If BufferedReader.readLine() will interpret it as a complete line without a line terminator ... and weird results will ensue.

What you need to do is to implement your own line buffering. When you get an EOF you wait until the file grows some more and then resume reading the line.

Alternatively, if you are using Java 7 or later, the file watcher APIs allow you to watch for file writes without polling the file's size.

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By the way, there is an Apache commons class that is designed for doing this kind of thing:

http://commons.apache.org/io/api-2.0/org/apache/commons/io/input/Tailer.html

Answer 2

If I understand, the file is being written in C# in some process and another Java process wants to read it while it is being written.

Look at File Monitoring section on the tail command here . But I want to warn you that when I used the cygwin tail on Windows recently to follow log files that were rolling over, it sometimes failed under heavy load. Other implementations may be more robust.

Answer 3

To have a count of the number of lines, just keep a counter on the side that's doing the writing.

So, every time you write a line, increment a counter, and make that counter readable via a method, something like, public int getNumLinesWritten()

Answer 4

The obvious answer to me... Why not use a buffer? Use a string or whatever you need. (You could use a list/array of strings if you want, one for each line maybe?) Append to the string just as you would write to the file, then instead of reading from the file, read from that string. Would that work for you?