如何查看输入流中的前两个字节?

对于一般的inputstream,我将它包装在一个bufferedinputstream中,并执行如下操作:

BufferedInputStream bis = new BufferedInputStream(inputStream);
bis.mark(2);
int byte1 = bis.read();
int byte2 = bis.read();
bis.reset();
// note: you must continue using the BufferedInputStream instead of the inputStream

您可能会发现pushbackinputstream非常有用:

http://docs.oracle.com/javase/6/docs/api/java/io/PushbackInputStream.html

使用BufferedInputStream时,请确保尚未对输入流进行缓冲,双缓冲将导致一些严重的难以找到的错误。 此外,您还需要以不同的方式处理读卡器,如果对读卡器进行缓冲,转换为流读卡器和缓冲将导致字节丢失。 另外,如果您使用的是读卡器,请记住您不是在读取字节,而是在默认编码中读取字符(除非设置了显式编码)。 您可能不知道的缓冲输入流示例是url url;url.openstream();

我没有此信息的任何引用,它来自调试代码。 我遇到的主要问题是从文件读取到压缩流的代码。 如果我记得正确,一旦你开始调试代码,有评论在Java源,某些事情不能正常工作。 我不记得使用BufferedReader和BufferedInputStream的信息在哪里 来自,但我认为即使在最简单的测试中,也会直接失败。 记住,要测试这一点,您需要标记的缓冲区大小(BufferedReader和BufferedInputStream的大小不同),当被读取的字节到达缓冲区的末尾时,就会出现问题。 注意,有一个源代码缓冲区大小,它可能与您在构造函数中设置的缓冲区大小不同。 我做这件事已经有一段时间了,所以我对细节的记忆可能有点模糊了。 测试是使用filterreader/filterinputstream完成的,将一个添加到直接流,一个添加到缓冲流以查看差异。

我在这里找到了一个peekableinputstream的实现:

http://www.heatonresearch.com/articles/147/page2.html

本文中所示的实现的思想是,它在内部保留一个“peeked”值数组。当调用read时,首先从被偷看的数组返回值,然后从输入流返回值。当您调用peek时,这些值将被读取并存储在“peek”数组中。

由于示例代码的许可证是lgpl,因此可以将其附加到此日志:

package com.heatonresearch.httprecipes.html;

import java.io.*;

/**
 * The Heaton Research Spider Copyright 2007 by Heaton
 * Research, Inc.
 * 
 * HTTP Programming Recipes for Java ISBN: 0-9773206-6-9
 * http://www.heatonresearch.com/articles/series/16/
 * 
 * PeekableInputStream: This is a special input stream that
 * allows the program to peek one or more characters ahead
 * in the file.
 * 
 * This class is released under the:
 * GNU Lesser General Public License (LGPL)
 * http://www.gnu.org/copyleft/lesser.html
 * 
 * @author Jeff Heaton
 * @version 1.1
 */
public class PeekableInputStream extends InputStream
{

  /**
   * The underlying stream.
   */
  private InputStream stream;

  /**
   * Bytes that have been peeked at.
   */
  private byte peekBytes[];

  /**
   * How many bytes have been peeked at.
   */
  private int peekLength;

  /**
   * The constructor accepts an InputStream to setup the
   * object.
   * 
   * @param is
   *          The InputStream to parse.
   */
  public PeekableInputStream(InputStream is)
  {
    this.stream = is;
    this.peekBytes = new byte[10];
    this.peekLength = 0;
  }

  /**
   * Peek at the next character from the stream.
   * 
   * @return The next character.
   * @throws IOException
   *           If an I/O exception occurs.
   */
  public int peek() throws IOException
  {
    return peek(0);
  }

  /**
   * Peek at a specified depth.
   * 
   * @param depth
   *          The depth to check.
   * @return The character peeked at.
   * @throws IOException
   *           If an I/O exception occurs.
   */
  public int peek(int depth) throws IOException
  {
    // does the size of the peek buffer need to be extended?
    if (this.peekBytes.length <= depth)
    {
      byte temp[] = new byte[depth + 10];
      for (int i = 0; i < this.peekBytes.length; i++)
      {
        temp[i] = this.peekBytes[i];
      }
      this.peekBytes = temp;
    }

    // does more data need to be read?
    if (depth >= this.peekLength)
    {
      int offset = this.peekLength;
      int length = (depth - this.peekLength) + 1;
      int lengthRead = this.stream.read(this.peekBytes, offset, length);

      if (lengthRead == -1)
      {
        return -1;
      }

      this.peekLength = depth + 1;
    }

    return this.peekBytes[depth];
  }

  /*
   * Read a single byte from the stream. @throws IOException
   * If an I/O exception occurs. @return The character that
   * was read from the stream.
   */
  @Override
  public int read() throws IOException
  {
    if (this.peekLength == 0)
    {
      return this.stream.read();
    }

    int result = this.peekBytes[0];
    this.peekLength--;
    for (int i = 0; i < this.peekLength; i++)
    {
      this.peekBytes[i] = this.peekBytes[i + 1];
    }

    return result;
  }

}