Check if a file is locked in Java

问题

My Java program wants to read a file which can be locked by another program writing into it. I need to check if the file is locked and if so wait until it is free. How do I achieve this?

The Java program is running on a Windows 2000 server.

回答1:

Under Windows with Sun's JVM, the FileLocks should work properly, although the JavaDocs leave the reliability rather vague (system dependent).

Nevertheless, if you only have to recognize in your Java program, that some other program is locking the file, you don't have to struggle with FileLocks, but can simply try to write to the file, which will fail if it is locked. You better try this on your actual system, but I see the following behaviour:

File f = new File("some-locked-file.txt");
System.out.println(f.canWrite()); // -> true
new FileOutputStream(f); // -> throws a FileNotFoundException

This is rather odd, but if you don't count platform independence too high and your system shows the same behaviour, you can put this together in a utility function.

With current Java versions, there is unfortunately no way to be informed about file state changes, so if you need to wait until the file can be written, you have to try every now and then to check if the other process has released its lock. I'm not sure, but with Java 7, it might be possible to use the new WatchService to be informed about such changes.

回答2:

Should work in Windows:

File file = new File("file.txt");
boolean fileIsNotLocked = file.renameTo(file);

回答3:

Use a FileLock in all the Java applications using that file and have them run within the same JVM. Otherwise, this can't be done reliably.

回答4:

If multiple processes (which can be a mix of Java and non-Java) might be using the file, use a FileLock. A key to using file locks successfully is to remember that they are only "advisory". The lock is guaranteed to be visible if you check for it, but it won't stop you from doing things to the file if you forget. All processes that access the file should be designed to use the locking protocol.

回答5:

You can try to get an exclusive lock on the file. As long as the exclusive lock cannot be obtained, another program has a lock (exclusive or shared) on the file.

回答6:

The best way is to use FileLock, but in my case (jdk 1.6) I tried with success:

public static boolean isFileUnlocked(File file) {
        try {
            FileInputStream in = new FileInputStream(file);
            if (in!=null) in.close();
            return true;
        } catch (FileNotFoundException e) {
            return false;
        } catch (Exception e) {
            e.printStackTrace();
        }

        return true;
    }

回答7:

Tested on windows only : you can check if the file is locked as following enhanced venergiac answer: check for (file.exist()) file exists but with FileNotFoundException means is locked! you will notice this message (The process cannot access the file because it is being used by another process)

        public static boolean isFilelocked(File file) {
                try {

                    FileInputStream in = new FileInputStream(file);
                    in.close();
                    return false;
                } catch (FileNotFoundException e) {
                    if(file.exist()){ 
                       return true;
                     }
                     return false;
                } catch (Exception e) {
                    e.printStackTrace();
                }

                return false;
            }

回答8:

Improved Amjad Abdul-Ghani answer, I found that no error was produced until attempting to read from the file

public static boolean isFilelocked(File file) {
     try {
         try (FileInputStream in = new FileInputStream(file)) {
             in.read();
             return false;
         }
     } catch (FileNotFoundException e) {
         return file.exists();
     } catch (IOException ioe) {
         return true;
     }
 }

回答9:

I tryed combination of answers (@vlad) on windows with access to Linux Smb share and it worked for me. The first part was enough for lock like Excel but not for some editors. I added second part (rename) for testing both situations.

public static boolean testLockFile(File p_fi) {
    boolean bLocked = false;
    try (RandomAccessFile fis = new RandomAccessFile(p_fi, "rw")) {
      FileLock lck = fis.getChannel().lock();
      lck.release();
    } catch (Exception ex) {
      bLocked = true;
    }
    if (bLocked)
      return bLocked;
    // try further with rename
    String parent = p_fi.getParent();
    String rnd = UUID.randomUUID().toString();
    File newName = new File(parent + "/" + rnd);
    if (p_fi.renameTo(newName)) {
      newName.renameTo(p_fi);
    } else
      bLocked = true;
    return bLocked;
  }

回答10:

For Windows, you can also use:

new RandomAccessFile(file, "rw")

If the file is exclusively locked (by MS Word for example), there will be exception: java.io.FileNotFoundException: <fileName> (The process cannot access the file because it is being used by another process). This way you do not need to open/close streams just for the check.

Note - if the file is not exclusively locked (say opened in Notepad++) there will be no exception.

来源：https://stackoverflow.com/questions/1500174/check-if-a-file-is-locked-in-java

标签

java

locking