为什么锁定文件时会发生 OverlappingFileLockException？

Question

我尝试锁定一个文件并使用以下代码对其进行写入：

public class TrainSetBuildTask implements Runnable {
    private String pathname;
    public TrainSetBuildTask(String pathname){
        this.pathname = pathname;
    }

    @Override
    public void run() {
          try {
              String content = "content\n";
             //write to a file
             FileOutputStream os = new FileOutputStream(new File(pathname), true);
             FileChannel channel = os.getChannel();
             FileLock lock = channel.tryLock();
             if (lock != null) {
                ByteBuffer bytes = ByteBuffer.wrap(content.getBytes()); 
                channel.write(bytes);
                lock.release();
             }
             channel.close();
             os.close();
          } catch (IOException e) {
              e.printStackTrace();
          }
     }
 }

并用类的实例新建两个线程：

    String pathname = "/home/sjtu123/test.arff";
    TrainSetBuildTask task1 = new TrainSetBuildTask(pathname);
    Thread t1 = new Thread(task1);
    TrainSetBuildTask task2 = new TrainSetBuildTask(pathname);
    Thread t2 = new Thread(task2);
    t1.start();
    t2.start();

然后我收到错误 OverlappingFileLockException。我想知道为什么会发生这种情况，因为我只是在每个线程中锁定文件一次？如何修复我的代码？

Answer 1

您不能同时锁定同一个文件多次。您要么需要使用 java 锁定对象来确保一次只有一个线程尝试锁定文件，要么协调线程等待直到没有其他线程锁定。

来自manual：

代表整个 Java 虚拟机持有文件锁。它们不适用于控制同一虚拟机内的多个线程对文件的访问。

Answer 2

文件被全局锁定，而不仅仅是每个线程或每个进程。

你需要像这样依次运行你的两个线程：

String pathname = "/home/sjtu123/test.arff";
TrainSetBuildTask task1 = new TrainSetBuildTask(pathname);
Thread t1 = new Thread(task1);
TrainSetBuildTask task2 = new TrainSetBuildTask(pathname);
Thread t2 = new Thread(task2);
t1.start();
t1.join(); // Wait for t1 to die.
t2.start();

cmets之后的新版本：

FileWriter pathname = new FileWriter("/home/sjtu123/test.arff");
TrainSetBuildTask task1 = new TrainSetBuildTask(pathname);
Thread t1 = new Thread(task1);
TrainSetBuildTask task2 = new TrainSetBuildTask(pathname);
Thread t2 = new Thread(task2);
t1.start();
t2.start();
t1.join();
j2.join();
synchronized (pathname){
    pathname.close();
}

和

public class TrainSetBuildTask implements Runnable {
    private FileWriter pathname;
    public TrainSetBuildTask(FileWriter pathname){
        this.pathname = pathname;
    }

    @Override
    public void run() {
          try {
             // Do work
             synchronized (pathname){
             // Write to file
             }
          } catch (IOException e) {
              e.printStackTrace();
          }
     }
 }