老实说，从多个源向同一个writer写入的更好的近乎重复的方法是使用（阻塞）队列。这样，所有线程都可以将其消息放到队列中，而writer线程可以从队列中提取消息并将其写入文件。它通常更容易实现，也更高效
--编辑--
例子：

public class MyQueueableWriter implements Runnable {
  private BlockingQueue<Msg> q = new BlockingQueue();
  private FileOutputStream fis = ...;
  private volatile boolean running = true;

  public MyQueueableWriter(FileOutputStream fis) {
    this.fis = fis;
  }

  public void run() {
    try {
      while (running) {
        Message m = q.take();
        fis.write(...);
      }
      fis.close();
    } catch (IOException iox) {
      ...
    }
  }

  public void addMsg(Msg m) {
    q.put(m);
  }

  public void stop() {
    running = false;
  }
}

然后添加到队列：

public class EnqueueMsgRunnable implements Runnable {
  MyQueueableWriter q = ...;
  q.put(myMessage);
  q.put(myMessage2);
}

那就

for (int i =0; i < numSources; i++) {
  EnqueueMsgRunnable r = new EnqueueMsgRunnable(...);
  new Thread(r).start();
}