Java BufferedReader相关源码实例分析

1、案例代码

假设b.txt存储了abcdegfhijk

 public static void main(String[] args) throws IOException {
    //字符缓冲流
    BufferedReader bufferedReader=new BufferedReader(new FileReader
        (new File("H:\\ioText\\b.txt")),8);
    //存储读取的数据
    char[] charsRead=new char[5];
    //读取数据
    bufferedReader.read(charsRead);
    //遍历并输出charsRead
    for (char c:charsRead){
      System.out.println(c);
    }
  }

2、通过源码（部分）分析案例

a、第一次读取

public class BufferedReader extends Reader {
  private Reader in;//字符流
  private char cb[];//缓冲区
  private int nChars, nextChar;//nChars缓冲区可读字符数，nextChar下一个字符位置
  private static final int INVALIDATED = -2;
  private static final int UNMARKED = -1;
  private int markedChar = UNMARKED;
  private int readAheadLimit = 0; 
  private boolean skipLF = false;
  private boolean markedSkipLF = false;
  private static int defaultCharBufferSize = 8192;//缓冲区默认大小
  private static int defaultExpectedLineLength = 80;
  
  //案例调用的构造方法
   public BufferedReader(Reader in, int sz) {
     //调用父类构造
    super(in);
     //判断缓冲区大小是否正常
    if (sz <= 0)
      throw new IllegalArgumentException("Buffer size <= 0");
     //用户传入的字符流
    this.in = in;
    //给缓冲区指定空间大小（案例指定为8）
    cb = new char[sz];
     //缓冲区可读字符数和下一个字符位置初始化为0
    nextChar = nChars = 0;
  }
  
  //读取数据
  public int read(char cbuf[], int off, int len) throws IOException {
    synchronized (lock) {
      ensureOpen();
      if ((off < 0) || (off > cbuf.length) || (len < 0) ||
        ((off + len) > cbuf.length) || ((off + len) < 0)) {
        throw new IndexOutOfBoundsException();
      } else if (len == 0) {
        return 0;
      }
      //调用read1方法进行读取（真正读取数据的方法是read1方法）
      int n = read1(cbuf, off, len);
      if (n <= 0) return n;
      //将之前没处理完的数据复制到自定以数组charsRead再次调用read1方法读取
      while ((n < len) && in.ready()) {
        int n1 = read1(cbuf, off + n, len - n);
        if (n1 <= 0) break;
        n += n1;
      }
      return n;
    }
  }
  
  //cbuf用户自定义数组（charsRead），off=0，len=5
   private int read1(char[] cbuf, int off, int len) throws IOException {
    if (nextChar >= nChars) {//第一次读nextChar、nChars都为0，满足条件
      if (len >= cb.length && markedChar <= UNMARKED && !skipLF) {
        return in.read(cbuf, off, len);
      }
      //刷新缓冲区，先往下找到fill方法源码分析
      fill();
    }
    if (nextChar >= nChars) return -1;
    if (skipLF) {
      skipLF = false;
      if (cb[nextChar] == '\n') {
        nextChar++;
        if (nextChar >= nChars)
          fill();
        if (nextChar >= nChars)
          return -1;
      }
    }
    //执行完fill方法到这里，（len=5，nChars - nextChar=8-0）->n=5
    int n = Math.min(len, nChars - nextChar);
    //将缓冲区cb从nextChar开始复制n=5个字符到自定义数组
    System.arraycopy(cb, nextChar, cbuf, off, n);
    //nextChar=5
    nextChar += n;
    //n=5
    return n;
  }
  
  //刷新缓冲区方法
  private void fill() throws IOException {
    int dst;
    if (markedChar <= UNMARKED) {//markedChar初始值为UNMARKED，满足条件
      /* No mark */
      dst = 0;//初始化dst
    } else {
      /* Marked */
      int delta = nextChar - markedChar;
      if (delta >= readAheadLimit) {
        /* Gone past read-ahead limit: Invalidate mark */
        markedChar = INVALIDATED;
        readAheadLimit = 0;
        dst = 0;
      } else {
        if (readAheadLimit <= cb.length) {
          /* Shuffle in the current buffer */
          System.arraycopy(cb, markedChar, cb, 0, delta);
          markedChar = 0;
          dst = delta;
        } else {
          /* Reallocate buffer to accommodate read-ahead limit */
          char ncb[] = new char[readAheadLimit];
          System.arraycopy(cb, markedChar, ncb, 0, delta);
          cb = ncb;
          markedChar = 0;
          dst = delta;
        }
        nextChar = nChars = delta;
      }
    }
​
    int n;
    do {
      //dst=0，cb.length - dst=8-0->n=8
      n = in.read(cb, dst, cb.length - dst);
    } while (n == 0);
    if (n > 0) {//满足条件
      //nChars=8
      nChars = dst + n;
      //nextChar=0
      nextChar = dst;
    }
  }
  
}

第一次读取后charsRead存储了五个字符：abcde

b、第二次读取

//cbuf用户自定义数组（charsRead），off=0，len=5
   private int read1(char[] cbuf, int off, int len) throws IOException {
    if (nextChar >= nChars) {//第二次读nextChar=5、nChars=8，不满足条件
      if (len >= cb.length && markedChar <= UNMARKED && !skipLF) {
        return in.read(cbuf, off, len);
      }
      fill();
    }
    if (nextChar >= nChars) return -1;
    if (skipLF) {
      skipLF = false;
      if (cb[nextChar] == '\n') {
        nextChar++;
        if (nextChar >= nChars)
          fill();
        if (nextChar >= nChars)
          return -1;
      }
    }
    //跳过if直接到这里，len=5，nChars - nextChar=8-5=3->n=3
    int n = Math.min(len, nChars - nextChar);
    //将缓冲区cb从nextChar=5开始复制n=3个字符到自定义数组
    System.arraycopy(cb, nextChar, cbuf, off, n);
    //nextChar=5+3=8
    nextChar += n;
    //n=8
    return n;
  }

第二次读取只读了三个字符把charsRead五个字符的前三个覆盖：fghde

c、第三次读取

//cbuf用户自定义数组（charsRead），off=0，len=5
   private int read1(char[] cbuf, int off, int len) throws IOException {
    if (nextChar >= nChars) {//第三次读nextChar=8、nChars=8，满足条件
      if (len >= cb.length && markedChar <= UNMARKED && !skipLF) {
        return in.read(cbuf, off, len);
      }
      //刷新缓冲区，先往下找到fill方法源码分析
      fill();
    }
    if (nextChar >= nChars) return -1;
    if (skipLF) {
      skipLF = false;
      if (cb[nextChar] == '\n') {
        nextChar++;
        if (nextChar >= nChars)
          fill();
        if (nextChar >= nChars)
          return -1;
      }
    }
     //执行完fill方法到这里，（len=2，nChars - nextChar=8-0）->n=2
    int n = Math.min(len, nChars - nextChar);
    //将缓冲区cb从nextChar=0开始复制n=2个字符到自定义数组
    System.arraycopy(cb, nextChar, cbuf, off, n);
    //nextChar=5+3=8
    nextChar += n;
    //n=8
    return n;
  }
  
  //刷新缓冲区方法
  private void fill() throws IOException {
    int dst;
    if (markedChar <= UNMARKED) {//markedChar初始值为UNMARKED，满足条件
      /* No mark */
      dst = 0;//初始化dst
    } else {
      /* Marked */
      int delta = nextChar - markedChar;
      if (delta >= readAheadLimit) {
        /* Gone past read-ahead limit: Invalidate mark */
        markedChar = INVALIDATED;
        readAheadLimit = 0;
        dst = 0;
      } else {
        if (readAheadLimit <= cb.length) {
          /* Shuffle in the current buffer */
          System.arraycopy(cb, markedChar, cb, 0, delta);
          markedChar = 0;
          dst = delta;
        } else {
          /* Reallocate buffer to accommodate read-ahead limit */
          char ncb[] = new char[readAheadLimit];
          System.arraycopy(cb, markedChar, ncb, 0, delta);
          cb = ncb;
          markedChar = 0;
          dst = delta;
        }
        nextChar = nChars = delta;
      }
    }
​
    int n;
    do {
      //dst=0，cb.length - dst=8-0->n=8
      n = in.read(cb, dst, cb.length - dst);
    } while (n == 0);
    if (n > 0) {//满足条件
      //nChars=8
      nChars = dst + n;
      //nextChar=0
      nextChar = dst;
    }
  }
  
}

第三次读取了两个字符到charsRead，把最后两个字符覆盖：fghijk

3、源码执行过程图解

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