BufferedInputStream(缓冲输入流)详解_动力节点Java学院整理

BufferedInputStream(InputStream in)
BufferedInputStream(InputStream in, int size)

synchronized int   available()
void   close()
synchronized void   mark(int readlimit)
boolean   markSupported()
synchronized int   read()
synchronized int   read(byte[] buffer, int offset, int byteCount)
synchronized void   reset()
synchronized long   skip(long byteCount)

package java.io;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;

public class BufferedInputStream extends FilterInputStream {

  // 默认的缓冲大小是8192字节
  // BufferedInputStream 会根据“缓冲区大小”来逐次的填充缓冲区；
  // 即，BufferedInputStream填充缓冲区，用户读取缓冲区，读完之后，BufferedInputStream会再次填充缓冲区。如此循环，直到读完数据...
  private static int defaultBufferSize = 8192;

  // 缓冲数组
  protected volatile byte buf[];

  // 缓存数组的原子更新器。
  // 该成员变量与buf数组的volatile关键字共同组成了buf数组的原子更新功能实现，
  // 即，在多线程中操作BufferedInputStream对象时，buf和bufUpdater都具有原子性(不同的线程访问到的数据都是相同的)
  private static final
    AtomicReferenceFieldUpdater<BufferedInputStream, byte[]> bufUpdater =
    AtomicReferenceFieldUpdater.newUpdater
    (BufferedInputStream.class, byte[].class, "buf");

  // 当前缓冲区的有效字节数。
  // 注意，这里是指缓冲区的有效字节数，而不是输入流中的有效字节数。
  protected int count;

  // 当前缓冲区的位置索引
  // 注意，这里是指缓冲区的位置索引，而不是输入流中的位置索引。
  protected int pos;

  // 当前缓冲区的标记位置
  // markpos和reset()配合使用才有意义。操作步骤：
  // (01) 通过mark() 函数，保存pos的值到markpos中。
  // (02) 通过reset() 函数，会将pos的值重置为markpos。接着通过read()读取数据时，就会从mark()保存的位置开始读取。
  protected int markpos = -1;

  // marklimit是标记的最大值。
  // 关于marklimit的原理，我们在后面的fill()函数分析中会详细说明。这对理解BufferedInputStream相当重要。
  protected int marklimit;

  // 获取输入流
  private InputStream getInIfOpen() throws IOException {
    InputStream input = in;
    if (input == null)
      throw new IOException("Stream closed");
    return input;
  }

  // 获取缓冲
  private byte[] getBufIfOpen() throws IOException {
    byte[] buffer = buf;
    if (buffer == null)
      throw new IOException("Stream closed");
    return buffer;
  }

  // 构造函数：新建一个缓冲区大小为8192的BufferedInputStream
  public BufferedInputStream(InputStream in) {
    this(in, defaultBufferSize);
  }

  // 构造函数：新建指定缓冲区大小的BufferedInputStream
  public BufferedInputStream(InputStream in, int size) {
    super(in);
    if (size <= 0) {
      throw new IllegalArgumentException("Buffer size <= 0");
    }
    buf = new byte[size];
  }

  // 从“输入流”中读取数据，并填充到缓冲区中。
  // 后面会对该函数进行详细说明！
  private void fill() throws IOException {
    byte[] buffer = getBufIfOpen();
    if (markpos < 0)
      pos = 0;      /* no mark: throw away the buffer */
    else if (pos >= buffer.length) /* no room left in buffer */
      if (markpos > 0) { /* can throw away early part of the buffer */
        int sz = pos - markpos;
        System.arraycopy(buffer, markpos, buffer, 0, sz);
        pos = sz;
        markpos = 0;
      } else if (buffer.length >= marklimit) {
        markpos = -1;  /* buffer got too big, invalidate mark */
        pos = 0;    /* drop buffer contents */
      } else {      /* grow buffer */
        int nsz = pos * 2;
        if (nsz > marklimit)
          nsz = marklimit;
        byte nbuf[] = new byte[nsz];
        System.arraycopy(buffer, 0, nbuf, 0, pos);
        if (!bufUpdater.compareAndSet(this, buffer, nbuf)) {
          throw new IOException("Stream closed");
        }
        buffer = nbuf;
      }
    count = pos;
    int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
    if (n > 0)
      count = n + pos;
  }

  // 读取下一个字节
  public synchronized int read() throws IOException {
    // 若已经读完缓冲区中的数据，则调用fill()从输入流读取下一部分数据来填充缓冲区
    if (pos >= count) {
      fill();
      if (pos >= count)
        return -1;
    }
    // 从缓冲区中读取指定的字节
    return getBufIfOpen()[pos++] & 0xff;
  }

  // 将缓冲区中的数据写入到字节数组b中。off是字节数组b的起始位置，len是写入长度
  private int read1(byte[] b, int off, int len) throws IOException {
    int avail = count - pos;
    if (avail <= 0) {
      // 加速机制。
      // 如果读取的长度大于缓冲区的长度 并且没有markpos，
      // 则直接从原始输入流中进行读取，从而避免无谓的COPY（从原始输入流至缓冲区，读取缓冲区全部数据，清空缓冲区， 
      // 重新填入原始输入流数据）
      if (len >= getBufIfOpen().length && markpos < 0) {
        return getInIfOpen().read(b, off, len);
      }
      // 若已经读完缓冲区中的数据，则调用fill()从输入流读取下一部分数据来填充缓冲区
      fill();
      avail = count - pos;
      if (avail <= 0) return -1;
    }
    int cnt = (avail < len) ? avail : len;
    System.arraycopy(getBufIfOpen(), pos, b, off, cnt);
    pos += cnt;
    return cnt;
  }

  // 将缓冲区中的数据写入到字节数组b中。off是字节数组b的起始位置，len是写入长度
  public synchronized int read(byte b[], int off, int len)
    throws IOException
  {
    getBufIfOpen(); // Check for closed stream
    if ((off | len | (off + len) | (b.length - (off + len))) < 0) {
      throw new IndexOutOfBoundsException();
    } else if (len == 0) {
      return 0;
    }

    // 读取到指定长度的数据才返回
    int n = 0;
    for (;;) {
      int nread = read1(b, off + n, len - n);
      if (nread <= 0)
        return (n == 0) ? nread : n;
      n += nread;
      if (n >= len)
        return n;
      // if not closed but no bytes available, return
      InputStream input = in;
      if (input != null && input.available() <= 0)
        return n;
    }
  }

  // 忽略n个字节
  public synchronized long skip(long n) throws IOException {
    getBufIfOpen(); // Check for closed stream
    if (n <= 0) {
      return 0;
    }
    long avail = count - pos;

    if (avail <= 0) {
      // If no mark position set then don't keep in buffer
      if (markpos <0)
        return getInIfOpen().skip(n);

      // Fill in buffer to save bytes for reset
      fill();
      avail = count - pos;
      if (avail <= 0)
        return 0;
    }

    long skipped = (avail < n) ? avail : n;
    pos += skipped;
    return skipped;
  }

  // 下一个字节是否存可读
  public synchronized int available() throws IOException {
    int n = count - pos;
    int avail = getInIfOpen().available();
    return n > (Integer.MAX_VALUE - avail)
          ? Integer.MAX_VALUE
          : n + avail;
  }

  // 标记“缓冲区”中当前位置。
  // readlimit是marklimit，关于marklimit的作用，参考后面的说明。
  public synchronized void mark(int readlimit) {
    marklimit = readlimit;
    markpos = pos;
  }

  // 将“缓冲区”中当前位置重置到mark()所标记的位置
  public synchronized void reset() throws IOException {
    getBufIfOpen(); // Cause exception if closed
    if (markpos < 0)
      throw new IOException("Resetting to invalid mark");
    pos = markpos;
  }

  public boolean markSupported() {
    return true;
  }

  // 关闭输入流
  public void close() throws IOException {
    byte[] buffer;
    while ( (buffer = buf) != null) {
      if (bufUpdater.compareAndSet(this, buffer, null)) {
        InputStream input = in;
        in = null;
        if (input != null)
          input.close();
        return;
      }
      // Else retry in case a new buf was CASed in fill()
    }
  }
}

private void fill() throws IOException {
  byte[] buffer = getBufIfOpen();
  if (markpos < 0)
    pos = 0;
  else if (pos >= buffer.length) {
    if (markpos > 0) { /* can throw away early part of the buffer */
      int sz = pos - markpos;
      System.arraycopy(buffer, markpos, buffer, 0, sz);
      pos = sz;
      markpos = 0;
    } else if (buffer.length >= marklimit) {
      markpos = -1;  /* buffer got too big, invalidate mark */
      pos = 0;    /* drop buffer contents */
    } else {      /* grow buffer */
      int nsz = pos * 2;
      if (nsz > marklimit)
        nsz = marklimit;
      byte nbuf[] = new byte[nsz];
      System.arraycopy(buffer, 0, nbuf, 0, pos);
      if (!bufUpdater.compareAndSet(this, buffer, nbuf)) {
        // Can't replace buf if there was an async close.
        // Note: This would need to be changed if fill()
        // is ever made accessible to multiple threads.
        // But for now, the only way CAS can fail is via close.
        // assert buf == null;
        throw new IOException("Stream closed");
      }
      buffer = nbuf;
    }
  }

  count = pos;
  int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
  if (n > 0)
    count = n + pos;
}

private void fill() throws IOException {
  byte[] buffer = getBufIfOpen();
  if (markpos < 0)
    pos = 0;

  count = pos;
  int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
  if (n > 0)
    count = n + pos;
}

private void fill() throws IOException {
  byte[] buffer = getBufIfOpen();
  if (markpos >= 0 && pos >= buffer.length) {
    if (markpos > 0) {
      int sz = pos - markpos;
      System.arraycopy(buffer, markpos, buffer, 0, sz);
      pos = sz;
      markpos = 0;
    }
  }

  count = pos;
  int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
  if (n > 0)
    count = n + pos;
}

private void fill() throws IOException {
  byte[] buffer = getBufIfOpen();
  if (markpos >= 0 && pos >= buffer.length) {
    if ( (markpos <= 0) && (buffer.length >= marklimit) ) {
      markpos = -1;  /* buffer got too big, invalidate mark */
      pos = 0;    /* drop buffer contents */
    }
  }

  count = pos;
  int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
  if (n > 0)
    count = n + pos;
}

private void fill() throws IOException {
  byte[] buffer = getBufIfOpen();
  if (markpos >= 0 && pos >= buffer.length) {
    if ( (markpos <= 0) && (buffer.length < marklimit) ) {
      int nsz = pos * 2;
      if (nsz > marklimit)
        nsz = marklimit;
      byte nbuf[] = new byte[nsz];
      System.arraycopy(buffer, 0, nbuf, 0, pos);
      if (!bufUpdater.compareAndSet(this, buffer, nbuf)) {
        throw new IOException("Stream closed");
      }
      buffer = nbuf;
    }
  }

  count = pos;
  int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
  if (n > 0)
    count = n + pos;
}

int nsz = pos * 2;
if (nsz > marklimit)
  nsz = marklimit;
byte nbuf[] = new byte[nsz];

private void fill() throws IOException {
  byte[] buffer = getBufIfOpen();

  count = pos;
  int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
  if (n > 0)
    count = n + pos;
}

import java.io.BufferedInputStream;
import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.InputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.FileNotFoundException;
import java.lang.SecurityException;

/**
 * BufferedInputStream 测试程序
 *
 * @author skywang
 */
public class BufferedInputStreamTest {

  private static final int LEN = 5;

  public static void main(String[] args) {
    testBufferedInputStream() ;
  }

  /**
   * BufferedInputStream的API测试函数
   */
  private static void testBufferedInputStream() {

    // 创建BufferedInputStream字节流，内容是ArrayLetters数组
    try {
      File file = new File("bufferedinputstream.txt");
      InputStream in =
         new BufferedInputStream(
           new FileInputStream(file), 512);

      // 从字节流中读取5个字节。“abcde”，a对应0x61，b对应0x62，依次类推...
      for (int i=0; i<LEN; i++) {
        // 若能继续读取下一个字节，则读取下一个字节
        if (in.available() >= 0) {
          // 读取“字节流的下一个字节”
          int tmp = in.read();
          System.out.printf("%d : 0x%s\n", i, Integer.toHexString(tmp));
        }
      }

      // 若“该字节流”不支持标记功能，则直接退出
      if (!in.markSupported()) {
        System.out.println("make not supported!");
        return ;
      }
       
      // 标记“当前索引位置”，即标记第6个位置的元素--“f”
      // 1024对应marklimit
      in.mark(1024);

      // 跳过22个字节。
      in.skip(22);

      // 读取5个字节
      byte[] buf = new byte[LEN];
      in.read(buf, 0, LEN);
      // 将buf转换为String字符串。
      String str1 = new String(buf);
      System.out.printf("str1=%s\n", str1);

      // 重置“输入流的索引”为mark()所标记的位置，即重置到“f”处。
      in.reset();
      // 从“重置后的字节流”中读取5个字节到buf中。即读取“fghij”
      in.read(buf, 0, LEN);
      // 将buf转换为String字符串。
      String str2 = new String(buf);
      System.out.printf("str2=%s\n", str2);

      in.close();
    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (SecurityException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}