File cannot be deleted because the JVM holds it - a tricky one

Question

My post got a little too long, sorry. Here is a summary:

• File on disk cannot be deleted ("the JVM holds the file" error). both when deleting from the java code and when trying to manually delete the file from windows.
• All streams to that file are closed and set to null. All file objects set to null.
• The program does nothing at that point; but waiting 30 minutes allows me to deleted the file from windows. Weird. Is the file not used by java anymore? Plus, since nothing happens in the program, it indicates it cannot be some stream I forgot (plus, I triple checked nothing is open).
• Invoking System.gc() seemed to work when files were small. Did not help when they got to about 20MB.

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[EDIT2] - I tried writing some basic code to explain, but its tricky. I am sorry, I know it's difficult to answer like that. I can however write how I open and close streams, of course:

BufferedWriter bw = new BufferedWriter(new FileWriter(new File("C:\\folder\\myFile.txt")));
for(int i = 0; i < 10; i++)
{
    bw.write("line " + i);
    bw.newLine();
}
bw.close();
bw = null;

If I've used a file object:

File f = new File("C:\\folder\\myFile.txt");
// use it...
f = null;

Basic code, I know. But this is essentially what I do. I know for a fact I've closed all streams in this exact way. I know for a fact that nothing happens in the program in that 30-minutes interval in which I cannot delete the file, until I somehow magically can.

thank you for your input even without the coherent code. I appreciate that.

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Sorry for not providing any specific code here, since I can't pinpoint the problem (not exactly specific-code related). In any case, here is the thing:

I have written a program which reads, writes and modifies files on disk. For several reasons, the handling of the read/write is done in a different thread, which is constantly operating.

At some point, I am terminating the "read/write" thread, keeping only the main thread - it waits for input from a socket, totally unrelated to the file, and does nothing. Then, I try to delete the file (using either File.delete(), even tried nio.Files delete option).

The thing is - and it's very weird - sometimes it works, sometimes it doesn't. Even manually, going to the folder and trying to delete the file via windows, gives me the "The file is open by the JVM" message.

Now, I am well aware that keeping references from all kinds of streams to the file prevents me from deleting it. Well past that by now :) I have made sure that all streams are closed. I even set their values to null, including any "File" objects I have used (even though it shouldn't make any difference). All set to null, all closed. And the thread which generates all of them - the "read/write" thread - well, it's terminated since it got the the end of its run() method.

Usually, if I wait about 30 minutes, while the JVM still operates, I can delete the file manually from windows. The error magically disappears. When the JVM is closed, I can always delete the file right away.

I am lost here. Tried specifically invoking System.gc() before trying to delete the file, even called it like 10 times (not that it should matter). Sometimes it helped, but on other occasions, for example, when the file got larger (say 20MB), that didn't help.

What am I missing here? Obviously, this couldn't be my implicit fault (not closing some stream), since the read/write thread is dead, the main thread awaits something unrelated (so the program is at a "standstill"), I have explicitly closed all streams, even nullified the references (inStream = null), invoked the garbage collector.

What am I missing? Why is the file "deletable" after 30 minutes (nothing happens at that time - not something in my code). Am I missing some gentle reference/garbage collection thingy?

Answer 1

What you're doing just calls for problems. You say that "if an IOexception occurred, it is printed immediately" and it may be true, but given that something inexplicable happens, let's better doubt it.

I'd first ensure that everything gets always closed, and then I'd care about related logic (logging, exiting, ...).

Anyway, what you did is not how resources should be managed. The answer above is not exactly correct either. Anyway, try-with-resources is (besides @lombok.Cleanup) about the only way, clearly showing that nothing gets ever left open. Anything else is more complicated and more error-prone. I'd strongly recommend using it everywhere. This may be quite some work, but it also forces you to re-inspect all the critical code pieces.

Things like nullifying references and calling the GC should not help... and if they seem to do, it may be a chance.

Some ideas:

• Are you using memory mapped files?
• Are you sure System.exit is not disabled by a security manager?
• Are you running an antivirus? They love to scan files just after they get written.

Btw., locking files is one reason why the WOW never started for me. Sometimes the locks persisted long after the culprit was gone, at least according to tools I could use.

Answer 2

Are you closing your streams in a try...finally or try(A a = new A()) block? If not the streams may not be closed. I would strongly recommend using either Automatic Resource Block Management ( try(A a = new A()) ) or a try...finally block for all external resources.

try(BufferedWriter br = new BufferedWriter(new FileWriter(new File("C:\\folder\\myFile.txt")));
for(int i = 0; i < 10; i++)
{
  br.write("line " + i);
  br.newLine();
})