java - 如何延迟方法返回直到文件完全读取

Question

考虑以下代码示例，其中我删除了所有错误处理代码以保持可读性:

public class MyClass {

  protected Something myAttribute;
  protected boolean busy = true;

  public ReactiveLogger() {
    new Thread(new FileListener()).start();
  }

  private class FileListener implements Runnable {

    @Override
    public void run() {
      RandomAccessFile raf = new RandomAccessFile(LOG_FILE, "rw");
      while (true) {
        final String line = cutFirstLine(raf);
        if (line == null) {
          busy = false;
          Thread.sleep(1000);
        } else {
          busy = true;
          // handeLogMessage changes myAttribute
          handleLogMessage(line);
        }
      }
    }

    // definition of cutFirstLine etc.

  }

  // definition of handleLogMessage etc.

  public Something getSomething() {
    while (busy) {
      Thread.sleep(100);
    }
    return myAttribute;
  }

}

因此，我的类 MyClass 在后台(在另一个线程中)读取日志文件，并用从日志文件中读取的每一行更新属性 myAttribute。只要日志文件中有任何条目并且更新了我的属性，getter 函数 getMyAttribute() 的返回就应该延迟。一旦日志文件中不再有任何条目，getMyAttribute() 就应该返回 myAttribute。

尽管此代码示例可以按要求工作，但它似乎不是最优雅的解决方案。根本不清楚线程应该 hibernate 多长时间才能获得最佳结果。如果只是在 getter 函数中删除对 Thread.sleep 的调用，程序就会卡住。但如果我将 sleep 方法的值设置得太高，执行时间也会太长。

我怎样才能以更好的方式实现同​​样的目标？我已经看过 Java 多线程/并发编程资源，但似乎没有什么(如 Java 的 synchronized)适合这种情况。

Answer 1

多亏了这些评论，我再次查看了 synchronized/wait()/notify() 。所以这是另一个有效且更优雅的解决方案:

public class MyClass {

  protected Something myAttribute;
  protected boolean busy = true;
  protected final Object lock = new Object();

  public ReactiveLogger() {
    new Thread(new FileListener()).start();
  }

  private class FileListener implements Runnable {

    @Override
    public void run() {
      RandomAccessFile raf = new RandomAccessFile(LOG_FILE, "rw");
      while (true) {
        final String line = cutFirstLine(raf);
        if (line == null) {
          busy = false;
          synchronized (lock) {
            lock.notifyAll();
          }
          Thread.sleep(1000);
        } else {
          busy = true;
          // handeLogMessage changes myAttribute
          handleLogMessage(line);
        }
      }
    }

    // definition of cutFirstLine etc.

  }

  // definition of handleLogMessage etc.

  public Something getSomething() {
    synchronized (lock) {
      while (busy) {
        lock.wait();
      }
    }
    return myAttribute;
  }

}

但可能仍然有更好的解决方案。 Java 杰出人物 Josh Bloch 在他的《Effective Java 2nd Edition》一书中严格建议不要使用这些方法(来自 this answer):

Given the difficulty of using wait and notify correctly, you should use the higher-level concurrency utilities instead [...] using wait and notify directly is like programming in "concurrency assembly language", as compared to the higher-level language provided by java.util.concurrent. There is seldom, if ever, reason to use wait and notify in new code.