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Java BufferedReader相關(guān)源碼實(shí)例分析

更新時(shí)間：2020年10月29日 10:59:01 作者：Y_wee

這篇文章主要介紹了Java BufferedReader相關(guān)源碼實(shí)例分析,文中通過示例代碼介紹的非常詳細(xì)，對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值,需要的朋友可以參考下

1、案例代碼

假設(shè)b.txt存儲(chǔ)了abcdegfhijk

 public static void main(String[] args) throws IOException {
    //字符緩沖流
    BufferedReader bufferedReader=new BufferedReader(new FileReader
        (new File("H:\\ioText\\b.txt")),8);
    //存儲(chǔ)讀取的數(shù)據(jù)
    char[] charsRead=new char[5];
    //讀取數(shù)據(jù)
    bufferedReader.read(charsRead);
    //遍歷并輸出charsRead
    for (char c:charsRead){
      System.out.println(c);
    }
  }

2、通過源碼（部分）分析案例

a、第一次讀取

public class BufferedReader extends Reader {
  private Reader in;//字符流
  private char cb[];//緩沖區(qū)
  private int nChars, nextChar;//nChars緩沖區(qū)可讀字符數(shù)，nextChar下一個(gè)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;//緩沖區(qū)默認(rèn)大小
  private static int defaultExpectedLineLength = 80;
  
  //案例調(diào)用的構(gòu)造方法
   public BufferedReader(Reader in, int sz) {
     //調(diào)用父類構(gòu)造
    super(in);
     //判斷緩沖區(qū)大小是否正常
    if (sz <= 0)
      throw new IllegalArgumentException("Buffer size <= 0");
     //用戶傳入的字符流
    this.in = in;
    //給緩沖區(qū)指定空間大?。ò咐付?）
    cb = new char[sz];
     //緩沖區(qū)可讀字符數(shù)和下一個(gè)字符位置初始化為0
    nextChar = nChars = 0;
  }
  
  //讀取數(shù)據(jù)
  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;
      }
      //調(diào)用read1方法進(jìn)行讀取（真正讀取數(shù)據(jù)的方法是read1方法）
      int n = read1(cbuf, off, len);
      if (n <= 0) return n;
      //將之前沒處理完的數(shù)據(jù)復(fù)制到自定以數(shù)組charsRead再次調(diào)用read1方法讀取
      while ((n < len) && in.ready()) {
        int n1 = read1(cbuf, off + n, len - n);
        if (n1 <= 0) break;
        n += n1;
      }
      return n;
    }
  }
  
  //cbuf用戶自定義數(shù)組（charsRead），off=0，len=5
   private int read1(char[] cbuf, int off, int len) throws IOException {
    if (nextChar >= nChars) {//第一次讀nextChar、nChars都為0，滿足條件
      if (len >= cb.length && markedChar <= UNMARKED && !skipLF) {
        return in.read(cbuf, off, len);
      }
      //刷新緩沖區(qū)，先往下找到fill方法源碼分析
      fill();
    }
    if (nextChar >= nChars) return -1;
    if (skipLF) {
      skipLF = false;
      if (cb[nextChar] == '\n') {
        nextChar++;
        if (nextChar >= nChars)
          fill();
        if (nextChar >= nChars)
          return -1;
      }
    }
    //執(zhí)行完fill方法到這里，（len=5，nChars - nextChar=8-0）->n=5
    int n = Math.min(len, nChars - nextChar);
    //將緩沖區(qū)cb從nextChar開始復(fù)制n=5個(gè)字符到自定義數(shù)組
    System.arraycopy(cb, nextChar, cbuf, off, n);
    //nextChar=5
    nextChar += n;
    //n=5
    return n;
  }
  
  //刷新緩沖區(qū)方法
  private void fill() throws IOException {
    int dst;
    if (markedChar <= UNMARKED) {//markedChar初始值為UNMARKED，滿足條件
      /* 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) {//滿足條件
      //nChars=8
      nChars = dst + n;
      //nextChar=0
      nextChar = dst;
    }
  }
  
}

第一次讀取后charsRead存儲(chǔ)了五個(gè)字符：abcde

b、第二次讀取

//cbuf用戶自定義數(shù)組（charsRead），off=0，len=5
   private int read1(char[] cbuf, int off, int len) throws IOException {
    if (nextChar >= nChars) {//第二次讀nextChar=5、nChars=8，不滿足條件
      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;
      }
    }
    //跳過if直接到這里，len=5，nChars - nextChar=8-5=3->n=3
    int n = Math.min(len, nChars - nextChar);
    //將緩沖區(qū)cb從nextChar=5開始復(fù)制n=3個(gè)字符到自定義數(shù)組
    System.arraycopy(cb, nextChar, cbuf, off, n);
    //nextChar=5+3=8
    nextChar += n;
    //n=8
    return n;
  }

第二次讀取只讀了三個(gè)字符把charsRead五個(gè)字符的前三個(gè)覆蓋：fghde

c、第三次讀取

//cbuf用戶自定義數(shù)組（charsRead），off=0，len=5
   private int read1(char[] cbuf, int off, int len) throws IOException {
    if (nextChar >= nChars) {//第三次讀nextChar=8、nChars=8，滿足條件
      if (len >= cb.length && markedChar <= UNMARKED && !skipLF) {
        return in.read(cbuf, off, len);
      }
      //刷新緩沖區(qū)，先往下找到fill方法源碼分析
      fill();
    }
    if (nextChar >= nChars) return -1;
    if (skipLF) {
      skipLF = false;
      if (cb[nextChar] == '\n') {
        nextChar++;
        if (nextChar >= nChars)
          fill();
        if (nextChar >= nChars)
          return -1;
      }
    }
     //執(zhí)行完fill方法到這里，（len=2，nChars - nextChar=8-0）->n=2
    int n = Math.min(len, nChars - nextChar);
    //將緩沖區(qū)cb從nextChar=0開始復(fù)制n=2個(gè)字符到自定義數(shù)組
    System.arraycopy(cb, nextChar, cbuf, off, n);
    //nextChar=5+3=8
    nextChar += n;
    //n=8
    return n;
  }
  
  //刷新緩沖區(qū)方法
  private void fill() throws IOException {
    int dst;
    if (markedChar <= UNMARKED) {//markedChar初始值為UNMARKED，滿足條件
      /* 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) {//滿足條件
      //nChars=8
      nChars = dst + n;
      //nextChar=0
      nextChar = dst;
    }
  }
  
}

第三次讀取了兩個(gè)字符到charsRead，把最后兩個(gè)字符覆蓋：fghijk

3、源碼執(zhí)行過程圖解