Java: opening and reading from a file without locking it

Question

I need to be able to mimic 'tail -f' with Java. I'm trying to read a log file as it's being written by another process, but when I open the file to read it, it locks the file and the other process can't write to it anymore. Any help would be greatly appreciated!

Here is the code that I'm using currently:

public void read(){
    Scanner fp = null;
    try{
        fp = new Scanner(new FileReader(this.filename));
        fp.useDelimiter("\n");
    }catch(java.io.FileNotFoundException e){
        System.out.println("java.io.FileNotFoundException e");
    }
    while(true){
        if(fp.hasNext()){
            this.parse(fp.next());
        }           
    }       
}

Answer 1

Rebuilding tail is tricky due to some special cases like file truncation and (intermediate) deletion. To open the file without locking use StandardOpenOption.READ with the new Java file API like so:

try (InputStream is = Files.newInputStream(path, StandardOpenOption.READ)) {
    InputStreamReader reader = new InputStreamReader(is, fileEncoding);
    BufferedReader lineReader = new BufferedReader(reader);
    // Process all lines.
    String line;
    while ((line = lineReader.readLine()) != null) {
        // Line content content is in variable line.
    }
}

For my attempt to create a tail in Java see:

• Method examineFile(…) in https://github.com/AugustusKling/yield/blob/master/src/main/java/yield/input/file/FileMonitor.java
• The above is used by https://github.com/AugustusKling/yield/blob/master/src/main/java/yield/input/file/FileInput.java to create a tail operation. The queue.feed(lineContent) passes line content for processing by listeners and would equal your this.parse(…).

Feel free to take inspiration from that code or simply copy the parts you require. Let me know if you find any issues that I'm not aware of.