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How can I lock a file using java (if possible)
I have two process that invoke two Java programs that modifying the same text file. I notice the content of the text file is missing data. I suspect that when one java program obtain the write stream to the text file, I think it block the other java program from modifying it(like when you have a file open, you cant delete that file). Is there a way to work around this other than database? (not to say that db solution is not clean or elegant, just that we wrote lot of codes in manipulating this text file)
EDIT
It turns out that I made a mistake targeting the problem. The reason why, data in my text file is missing is because,
ProcessA: keep add rows of data to the text file
ProcessB: at the beginning, load all the rows of the text field into a List. It is then manipulate the contains of that list. At the end, ProcessB write the list back out, replacing the contain of the text file.
This work great in sequential process. But when running together, if ProcessA adding data to the file, during the time ProcessB manipulating the List, then when ProcessB write the List back out, whatever ProcessA just add, will be overrided. So my initial thought is before ProcessB write the List back out, sync the data between text file and the List. So when I write the List back out, it will contains everything. so here is my effort
public void synchronizeFile(){
try {
File file = new File("path/to/file/that/both/A/and/B/write/to");
FileChannel channel = new RandomAccessFile(file, "rw").getChannel();
FileLock lock = channel.lock(); //Lock the file. Block until release the lock
List<PackageLog> tempList = readAllLogs(file);
if(tempList.size() > logList.size()){
//data are in corrupted state. Synchronized them.
for(PackageLog pl : tempList){
if(!pl.equals(lookUp(pl.getPackageLabel().getPackageId(), pl.getPackageLabel().getTransactionId()))){
logList.add(pl);
}
}
}
lock.release(); //Release the file
channel.close();
} catch (IOException e) {
logger.error("IOException: ", e);
}
}
So logList is the current List that ProcessB want to write out. So before the write out, I read the file and store data into tempList, if tempList and logList are not the same, sync them. The problem is that at this point, both ProcessA and ProcessB currently access the file, so when I try to lock the file, and read from it List<PackageLog> tempList = readAllLogs(file);, I either get OverlappingFileLockException, or java.io.IOException: The process cannot access the file because another process has locked a portion of the file. Please please help me fix this problem 🙁
EDIT2: My understand of Lock
public static void main(String[] args){
File file = new File("C:\\dev\\harry\\data.txt");
FileReader fileReader = null;
BufferedReader bufferedReader = null;
FileChannel channel = null;
FileLock lock = null;
try{
channel = new RandomAccessFile(file, "rw").getChannel();
lock = channel.lock();
fileReader = new FileReader(file);
bufferedReader = new BufferedReader(fileReader);
String data;
while((data = bufferedReader.readLine()) != null){
System.out.println(data);
}
}catch(IOException e){
e.printStackTrace();
}finally{
try {
lock.release();
channel.close();
if(bufferedReader != null) bufferedReader.close();
if(fileReader != null) fileReader.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
and I got this error IOException: The process cannot access the file because another process has locked a portion of the file
So, you could use the method that Vineet Reynolds suggests in his comment.
If the two processes are actually just separate Threads within the same application, then you could set a flag somewhere to indicate that the file is open.
If it’s two separate applications/processes altogether, the underlying filesystem should lock the files. When you get an I/O error from your output stream, you should be able to wrap a try/catch block around that, and then set your app up to retry later, or whatever the desired behavior is for your particular application.
Files aren’t really designed to be written to simultaneously by multiple applications. If you can maybe describe why you want to write to a file simultaneously from multiple processes, there may be other solutions that could be suggested.
Updates after your recent edits:
Ok, so you need at least 3 files to do what you’re talking about. You definitely cannot try to read/write data to a single file concurrently. Your three files are:
ProcessB’s loop:
ProcessA’s loop: