Question

Here is a code snippet where I am opening a file for writing with two FileOutputStreams at the same time.

FileOutputStream fis = null;
File openFile = new File("myfile");
try {
    fis = new FileOutputStream(openFile);
    Toast toast = Toast.makeText(getApplicationContext(), "Opened", Toast.LENGTH_SHORT);
    toast.show();
}
catch (FileNotFoundException e1) {
    Toast toast = Toast.makeText(getApplicationContext(), "FileNotFound", Toast.LENGTH_SHORT);
    toast.show();
}

// now try to open it again
FileOutputStream fis2 = null;
try {
    fis2 = new FileOutputStream(openFile);
    Toast toast = Toast.makeText(getApplicationContext(), "Opened2", Toast.LENGTH_SHORT);
    toast.show();
}
catch (Exception e1) {
    Toast toast = Toast.makeText(getApplicationContext(), "Exception: " + e1.getMessage(), Toast.LENGTH_SHORT);
    toast.show();
}

I end up getting two Toast messages, “Opened” and “Opened2”.

I need to make sure I don’t open a file for reading/writing/deleting that is currently open for writing by another instance. How to I ensure I do not modify a file that is currently open for writing?

Answer 1

I need to make sure I don’t open a file for reading/writing/deleting
that is currently open for writing by another instance.

What do you mean by another instance?

• If this is another thread in your application use the synchronization for read/write methods.
• If this is another process created by yourself (application, service etc.) use exclusive FileLock to coordinate file read/write operations between processes.
• If this is another process you don’t control (i.e. created by someone else) you can check whether file is currently opened by inspecting /proc/…/fd/ tree, as lsof utility does. However this usually requires you to have root privileges.

Hope that give you a clue. Let me know if you need further assistance.