java - 给定 Java InputStream，如何确定流中的当前偏移量？

Question

我想要一个通用的、可重复使用的 getPosition() 方法，它会告诉我从流的起点读取的字节数。理想情况下，我希望它能与所有 InputStreams 一起使用，这样我就不必在从不同来源获取它们时包装它们中的每一个。

这样的野兽存在吗？如果没有，谁能推荐一个现有的计数 InputStream 实现？

Answer 1

您需要遵循 java.io 中建立的装饰器模式来实现它。

让我们在这里试一试:

import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;

public final class PositionInputStream
  extends FilterInputStream
{

  private long pos = 0;

  private long mark = 0;

  public PositionInputStream(InputStream in)
  {
    super(in);
  }

  /**
   * <p>Get the stream position.</p>
   *
   * <p>Eventually, the position will roll over to a negative number.
   * Reading 1 Tb per second, this would occur after approximately three 
   * months. Applications should account for this possibility in their 
   * design.</p>
   *
   * @return the current stream position.
   */
  public synchronized long getPosition()
  {
    return pos;
  }

  @Override
  public synchronized int read()
    throws IOException
  {
    int b = super.read();
    if (b >= 0)
      pos += 1;
    return b;
  }

  @Override
  public synchronized int read(byte[] b, int off, int len)
    throws IOException
  {
    int n = super.read(b, off, len);
    if (n > 0)
      pos += n;
    return n;
  }

  @Override
  public synchronized long skip(long skip)
    throws IOException
  {
    long n = super.skip(skip);
    if (n > 0)
      pos += n;
    return n;
  }

  @Override
  public synchronized void mark(int readlimit)
  {
    super.mark(readlimit);
    mark = pos;
  }

  @Override
  public synchronized void reset()
    throws IOException
  {
    /* A call to reset can still succeed if mark is not supported, but the 
     * resulting stream position is undefined, so it's not allowed here. */
    if (!markSupported())
      throw new IOException("Mark not supported.");
    super.reset();
    pos = mark;
  }

}

InputStreams 旨在实现线程安全，因此可以自由使用同步。我试过 volatile 和 AtomicLong 位置变量，但同步可能是最好的，因为它允许一个线程在流上操作并查询其位置而不放弃锁。

PositionInputStream is = …
synchronized (is) {
  is.read(buf);
  pos = is.getPosition();
}