作家是一种原子方法吗？

Question

我想给文件写一些字符串.所以,我使用了BufferedWriter类.由于许多线程倾向于写入该文件,我想知道write和writeLine方法是否是原子的.

另外,我希望程序输出写入到多个文件,并且每个文件100线(比如file.txt0,file.txt1,...).例如

public class Main {
    static ExecutorService exec = Executors.newFixedThreadPool(5);
    BufferedWriter bw;
    public class myWriter implements Runnable {
        String str;
        myWriter (String str) {
            this.str = str;
        }
        public void run() {
            bw.write(str);
            bw.writeLine();
        }
    }
    public static void main(String[] args) {
        bw = new BufferedWriter(new  FileWriter("train.txt"));
        for (String arg: args)
            exec.execute(new myWriter(arg));
        exec.awaitTermination(100000, TimeUnit.MILLISECONDS);
    }
}

有人能帮我吗？如果它们不是原子的,我怎样才能使它们成为原子并避免碰撞？

Answer 1

不,那些不是原子的.

如果要将多个磁带写入同一文件,请使用FileLocks.

try {
    // Get a file channel for the file
    File file = new File("filename");
    FileChannel channel = new RandomAccessFile(file, "rw").getChannel();

    // Use the file channel to create a lock on the file.
    // This method blocks until it can retrieve the lock.
    FileLock lock = channel.lock();

    // Try acquiring the lock without blocking. This method returns
    // null or throws an exception if the file is already locked.
    try {
        lock = channel.tryLock();
    } catch (OverlappingFileLockException e) {
        // File is already locked in this thread or virtual machine
    }

    // Release the lock
    lock.release();

    // Close the file
    channel.close();
} catch (Exception e) {
}

Answer 2

下面的代码是来自jdk6的源代码,它是写入的实现BufferedWriter,因为synchronized在函数体中有一个,我认为write()in BufferedWriter是线程安全的.顺便说一句,write(String)通过调用实现write(String,int,int).

public void write(String s, int off, int len) throws IOException {

    synchronized (lock) {

        ensureOpen();

        int b = off, t = off + len;

        while (b < t) {

            int d = min(nChars - nextChar, t - b);
            s.getChars(b, b + d, cb, nextChar);
            b += d;
            nextChar += d;

            if (nextChar >= nChars)
                flushBuffer();
            }
        }
    }
}