Question

I’m writing an application (for educational purposes), which needs to use database management system (I wrote my own extremely primitive DBMS, it is part of the task). And I want to ensure that at any time my application is running contents of all tables are correct. For that purposes I wrote method, which looks through each file and make necessary checks. The problem is that I want to call this method only once, when application starts and deny access to files to ensure that nobody changed their contents while my program is working.

I use the following approach. When application starts, I initialize InputStreamReader and OutputStreamWriter, store them and close them only when my application is terminated.

Part of initialization method:

FileInputStream fis = new FileInputStream(file);
FileOutputStream fos = new FileOutputStream(file, true); 
InputStreamReader isr = new InputStreamReader(fis, "UTF-8");
OutputStreamWriter osw = new OutputStreamWriter(fos, "UTF-8");

this.tables.get(table).put("fis", fis);
this.tables.get(table).put("fos", fos);
this.tables.get(table).put("isr", isr);
this.tables.get(table).put("osw", osw);

Close method:

try {
    for(Map<String, Object> table_map: tables.values()) {
        OutputStreamWriter osw = (OutputStreamWriter)table_map.get("osw"); 
        InputStreamReader isr = (InputStreamReader)table_map.get("isr"); 

        if (osw != null)
            osw.close();
        if (isr != null)
            isr.close();
    }
}
catch (IOException e) {
    throw new DBException("Closing error");
}

Partly, this approach works, because when I try to modify any of these files using MS Notepad, I get the following error

“The process cannot access the file because it is being used by
another process”

That’s what I want to see. But if I use Notepad++, I can make any modifications when my application is running, that’s not what I expect to see. So what can I do to ensure that no other process can modify my files?

I tried to use FileLock, but it denies access only for my process, if I’m not mistaken.

Sorry for my poor English, hope you will understand my question anyway.

Answer 1

I’m not sure this is a problem worth solving. Whatever approach you take, someone with the correct privileges can probably undo your file protection and could make changes anyway.

It is best to focus on gracefully handling invalid data and otherwise trusting what is in the file. Adding some kind of integrity check (per row or table) will make it harder for someone to accidentally or maliciously change your data in a way that leaves it looking “valid”.