Creating new files concurrently

Question

To create a new, unique filename, I use the following code:

File file = new File(name);
synchronized (sync) {
    int cnt = 0;
    while (file.exists()) {
        file = new File(name + " (" + (cnt++) + ")");
    }
    file.createNewFile();
}

Next, I use the file and delete it. When I do this in a multithreaded situation, I sometimes get exceptions on the file.createNewFile():

java.io.IOException: Access is denied
    at java.io.WinNTFileSystem.createFileExclusively(Native Method)
    at java.io.File.createNewFile(File.java:1012)

The following code reproduces the problem (most of the times):

final int runs = 1000;
final int threads = 5;
final String name = "c:\\temp\\files\\file";
final byte[] bytes = getSomeBytes();
final Object sync = new Object();

ExecutorService exec = Executors.newFixedThreadPool(threads);
for (int thread = 0; thread < threads; thread++) {
    final String id = "Runnable " + thread;
    exec.execute(new Runnable() {
        public void run() {
            for (int i = 0; i < runs; i++) {
                try {
                    File file = new File(name);
                    synchronized (sync) {
                        int cnt = 0;
                        while (file.exists()) {
                            file = new File(name + " (" + (cnt++) + ")");
                        }
                        file.createNewFile();
                    }

                    Files.write(file.toPath(), bytes);
                    file.delete();
                } catch (Exception ex) {
                    System.err.println(id + ": exception after " + i
                            + " runs: " + ex.getMessage());
                    ex.printStackTrace();
                    return;
                }
            }
            System.out.println(id + " finished fine");
        }
    });
}
exec.shutdown();
while (!exec.awaitTermination(1, TimeUnit.SECONDS));

The method getSomeBytes() just generates an amount of bytes, the actual content is not important:

byte[] getSomeBytes() throws UnsupportedEncodingException,
        IOException {
    byte[] alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWYZ1234567890\r\n"
            .getBytes("UTF-8");
    try (ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
        for (int i = 0; i < 100000; i++) {
            baos.write(alphabet);
        }
        baos.flush();
        return baos.toByteArray();
    }
}

When I execute this code, it sometimes goes well but most of the times, it generates some exceptions like the output below for instance:

Runnable 1: exception after 235 runs: Access is denied
java.io.IOException: Access is denied
    at java.io.WinNTFileSystem.createFileExclusively(Native Method)
    at java.io.File.createNewFile(File.java:1012)
    at test.CreateFilesTest$1.run(CreateFilesTest.java:36)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
Runnable 4: exception after 316 runs: Access is denied
java.io.IOException: Access is denied
    at java.io.WinNTFileSystem.createFileExclusively(Native Method)
    at java.io.File.createNewFile(File.java:1012)
    at test.CreateFilesTest$1.run(CreateFilesTest.java:36)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
Runnable 2: exception after 327 runs: Access is denied
java.io.IOException: Access is denied
    at java.io.WinNTFileSystem.createFileExclusively(Native Method)
    at java.io.File.createNewFile(File.java:1012)
    at test.CreateFilesTest$1.run(CreateFilesTest.java:36)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
Runnable 3 finished fine
Runnable 0 finished fine

Any ideas? I have tested on a Windows 8 machine with java 1.7.0_45 and 1.8.0_31, both same results.

Not sure whether the problem is the same as in this question, but it can be. Using multiple threads in the same process seems to be part of the problem to my opinion but I can't be sure about that, it's faster reproduced however.

Answer 1

Seems that on Windows platform createNewFile may randomly fail if the file with the same name was just deleted even on single-threaded application. See this question for details. To fix your issue, you may try to ignore IOException from the createNewFile and continue. Something like this:

synchronized (sync) {
    int cnt = 0;
    while (true) {
        try {
            if(file.createNewFile())
                break;
        } catch (IOException e) {
            // continue;
        }
        file = new File(name + " (" + (cnt++) + ")");
    }
}

Note that you don't need to check file.exists() call as createNewFile() conveniently returns whether it created the file successfully.

Note that if you control all the temporary files you create and don't care about exact file name, usually there's no need to lock. You may just use global AtomicLong to get the next file name or append a thread ID to the file name.

Answer 2

Your loop isn't fail-safe. There's a timing-window problem. It should be more like this:

while (!file.createNewFile()) {
        file = new File(name + " (" + (cnt++) + ")");
    }