How to start concurrent threads that acquire a file lock and wait on each other

问题

I'm studying the FileLock class. What I want to do is start three Threads that will run at the same time and access a single file. While the file is locked by one thread, I want the other two threads to wait for their turn when the lock is released. However, when I run my code below, the threads don't even start all at the same time--they are started one after the other, as soon as each of their run() methods is finished. I don't understand.

public class Main {

    public static void main(String[] args) {
        Main m = new Main();
        SomeThread t1 = m.new SomeThread("t1");
        SomeThread t2 = m.new SomeThread("t2");
        SomeThread t3 = m.new SomeThread("t3");
        t1.run();
        t3.run();
        t2.run();
    }

    class SomeThread implements Runnable {
        String name;

        public SomeThread(String s) {
            name = s;
        }

        @Override
        public void run() {
            System.out.println(name + " started!");
            OtherClass.access(name);
        }
    }

    static class OtherClass {
        static File file = new File("testfile.txt");

        public static void access(String name) {
            FileChannel channel = null;
            FileLock lock = null;
            try {
                channel = new RandomAccessFile(file, "rw").getChannel();
                lock = channel.lock();
                System.out.println("locked by " + name);
                Thread.sleep(3000);
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } finally {
                if (lock != null) {
                    try {
                        lock.release();
                        System.out.println("released by " + name);
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                if (channel != null) {
                    try {
                        channel.close();
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            }
        }
    }
}

How can I achieve the scenario I'm trying to get at? And why aren't they starting at the same time? I thought the lock() method only makes the other threads accessing the same file wait until the lock is released.

回答1:

Threads are started with Thread.start, not Thread.run. run will just call the run method sequentially on the main thread.

You're not even creating threads actually:

public static void main(String[] args) {
    Main m = new Main();
    Thread t1 = new Thread(m.new SomeThread("t1"));
    Thread t2 = new Thread(m.new SomeThread("t2"));
    Thread t3 = new Thread(m.new SomeThread("t3"));
    t1.start();
    t2.start();
    t3.start();
}

回答2:

Forget it. It won't work. File locks are held on behalf of the entire process, not individual threads.

来源：https://stackoverflow.com/questions/12616975/how-to-start-concurrent-threads-that-acquire-a-file-lock-and-wait-on-each-other