我想要一些通用的、可重复使用的东西getPosition()
方法将告诉我从流的起点读取的字节数。理想情况下,我希望它能够与所有输入流一起使用,这样当我从不同的来源获取它们时,我就不必包装它们中的每一个。
这样的野兽存在吗?如果没有,任何人都可以推荐现有的计数实现吗InputStream
?
您需要遵循中建立的装饰器模式java.io
来实施这一点。
让我们在这里尝试一下:
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;
}
}
输入流的目的是线程安全的,因此可以充分使用同步。我玩过volatile
and 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定。
PositionInputStream is = …
synchronized (is) {
is.read(buf);
pos = is.getPosition();
}
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