深入研究BufferedReader底層源碼

• 2019 年 10 月 15 日
• 筆記

1 概述

最近研究JDK IO流這一塊源碼，發現真的比較簡單，而且還有很多意外發現，如果大家對JDK源碼感興趣，不妨從IO流這一塊入手，說不定你會愛上JDK源碼。今天我所分享的就是BufferedReader

2 BufferedReader源碼分析

public class BufferedReader extends Reader {        // 真正幹活的流，這裡使用到了裝飾著模式      private Reader in;        //快取字元數組      private char cb[];        /*        nchars cb[]數組目前存儲的字元總數        nextChar cb[]數組下一個字元      */      private int nChars, nextChar;          /*        INVALIDATED代表無效 當markedChar=INVALIDATED代表無效時 調用reset()方法會拋出異常      */      private static final int INVALIDATED = -2;        // 該流中沒有任何標記      private static final int UNMARKED = -1;        // mark標記      private int markedChar = UNMARKED;        /*        從markedChar開始可以最多可以讀取的字元 如果超過該長度限制 那麼markdChar會失效 調用reset()方法會拋異常      */      private int readAheadLimit = 0; /* Valid only when markedChar > 0 */        /** If the next character is a line feed, skip it */      private boolean skipLF = false;        /** The skipLF flag when the mark was set */      private boolean markedSkipLF = false;        // 默認快取數組cb[]大小      private static int defaultCharBufferSize = 8192;        // 用在readLine()方法中 默認一行佔80個字元      private static int defaultExpectedLineLength = 80;        /**       * Creates a buffering character-input stream that uses an input buffer of       * the specified size.       *       * @param  in   A Reader       * @param  sz   Input-buffer size       *       * @exception  IllegalArgumentException  If {@code sz <= 0}       */      public BufferedReader(Reader in, int sz) {          super(in);          if (sz <= 0)              throw new IllegalArgumentException("Buffer size <= 0");          this.in = in;          cb = new char[sz];          nextChar = nChars = 0;      }        /**       * Creates a buffering character-input stream that uses a default-sized       * input buffer.       *       * @param  in   A Reader       */      public BufferedReader(Reader in) {          this(in, defaultCharBufferSize);      }        /** Checks to make sure that the stream has not been closed */      private void ensureOpen() throws IOException {          if (in == null)              throw new IOException("Stream closed");      }        /**       * Fills the input buffer, taking the mark into account if it is valid.       * 該方法的目的就是 填充cb[]快取數組       * 注意當markedChar有效時[markedChar, nextChar)必須保留，不能覆蓋       */      private void fill() throws IOException {          int dst; // 數據填充的起始位置          if (markedChar <= UNMARKED) { // 說明沒有標記 可以從cb[0]開始填充數組              /* No mark */              dst = 0;          } else {              /* Marked */              int delta = nextChar - markedChar;              if (delta >= readAheadLimit) {                  /* Gone past read-ahead limit: Invalidate mark */                  markedChar = INVALIDATED; // 標記失效 從cb[0]開始填充數組                  readAheadLimit = 0;                  dst = 0;              } else {                  // 說明mark標記有效 且該cb[]數組還有剩餘空間                  if (readAheadLimit <= cb.length) {                      /* Shuffle in the current buffer */                      // 將[markedChar, nextChar)保留 其餘放棄                      System.arraycopy(cb, markedChar, cb, 0, delta);                      markedChar = 0; // markdChar被移至cb[0]                      dst = delta;                  } else { // 說明 readAheadLimit超過了cb[]長度 需要擴容cb[]數組                      /* 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 {              n = in.read(cb, dst, cb.length - dst); // 從cb[dst] 開始填充數組          } while (n == 0);          if (n > 0) {              nChars = dst + n;              nextChar = dst;          }      }        /**       * Reads a single character.       *       * @return The character read, as an integer in the range       *         0 to 65535 (<tt>0x00-0xffff</tt>), or -1 if the       *         end of the stream has been reached       * @exception  IOException  If an I/O error occurs       */      public int read() throws IOException {          synchronized (lock) {              ensureOpen();              for (;;) {                  if (nextChar >= nChars) {                      fill();                      if (nextChar >= nChars)                          // 說明沒有數據了                          return -1;                  }                  /*                      skipLF默認是false 全局搜索skipLF = ture 會發現只有readLine()會使用它                      只要不混用read()和readLine()就不會跳過換行                  */                  if (skipLF) {                      skipLF = false;                      if (cb[nextChar] == 'n') {                          nextChar++;                          continue;                      }                  }                  return cb[nextChar++];              }          }      }        /**       * Reads characters into a portion of an array, reading from the underlying       * stream if necessary.       */      private int read1(char[] cbuf, int off, int len) throws IOException {          if (nextChar >= nChars) {              /* If the requested length is at least as large as the buffer, and                 if there is no mark/reset activity, and if line feeds are not                 being skipped, do not bother to copy the characters into the                 local buffer.  In this way buffered streams will cascade                 harmlessly.                 如果len長度大於快取數組長度 並且沒有標記 並且 skipLF為false                 那麼就直接從底層流中讀取數據*/              if (len >= cb.length && markedChar <= UNMARKED && !skipLF) {                  return in.read(cbuf, off, len);              }              fill();          }          if (nextChar >= nChars) return -1;          /*              skipLF默認是false 全局搜索skipLF = ture 會發現只有readLine()會使用它              只要不混用read1()和readLine()就不會跳過換行          */          if (skipLF) {              skipLF = false;              if (cb[nextChar] == 'n') {                  nextChar++;                  if (nextChar >= nChars)                      fill();                  if (nextChar >= nChars)                      return -1;              }          }          // 讀取的個數 不能超過快取數組cb[]有效字元個數          int n = Math.min(len, nChars - nextChar);          System.arraycopy(cb, nextChar, cbuf, off, n);          nextChar += n;          return n;      }        /**       * Reads characters into a portion of an array.       *       * <p> This method implements the general contract of the corresponding       * <code>{@link Reader#read(char[], int, int) read}</code> method of the       * <code>{@link Reader}</code> class.  As an additional convenience, it       * attempts to read as many characters as possible by repeatedly invoking       * the <code>read</code> method of the underlying stream.  This iterated       * <code>read</code> continues until one of the following conditions becomes       * true: <ul>       *       *   <li> The specified number of characters have been read,       *       *   <li> The <code>read</code> method of the underlying stream returns       *   <code>-1</code>, indicating end-of-file, or       *       *   <li> The <code>ready</code> method of the underlying stream       *   returns <code>false</code>, indicating that further input requests       *   would block.       *       * </ul> If the first <code>read</code> on the underlying stream returns       * <code>-1</code> to indicate end-of-file then this method returns       * <code>-1</code>.  Otherwise this method returns the number of characters       * actually read.       *       * <p> Subclasses of this class are encouraged, but not required, to       * attempt to read as many characters as possible in the same fashion.       *       * <p> Ordinarily this method takes characters from this stream's character       * buffer, filling it from the underlying stream as necessary.  If,       * however, the buffer is empty, the mark is not valid, and the requested       * length is at least as large as the buffer, then this method will read       * characters directly from the underlying stream into the given array.       * Thus redundant <code>BufferedReader</code>s will not copy data       * unnecessarily.       *       * @param      cbuf  Destination buffer       * @param      off   Offset at which to start storing characters       * @param      len   Maximum number of characters to read       *       * @return     The number of characters read, or -1 if the end of the       *             stream has been reached       *       * @exception  IOException  If an I/O error occurs       */      /*          此方法判斷數組越界情況 並調用read1(char[] cbuf, int off, int len)      */      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;              }                int n = read1(cbuf, off, len);              if (n <= 0) return n;              // 當讀取的n小於期望的len時並底層流沒有被阻塞 會嘗試繼續讀取數據              while ((n < len) && in.ready()) {                  int n1 = read1(cbuf, off + n, len - n);                  if (n1 <= 0) break;                  n += n1;              }              return n;          }      }        /**       * Reads a line of text.  A line is considered to be terminated by any one       * of a line feed ('n'), a carriage return ('r'), or a carriage return       * followed immediately by a linefeed.       *       * @param      ignoreLF  If true, the next 'n' will be skipped       *       * @return     A String containing the contents of the line, not including       *             any line-termination characters, or null if the end of the       *             stream has been reached       *       * @see        java.io.LineNumberReader#readLine()       *       * @exception  IOException  If an I/O error occurs       */      String readLine(boolean ignoreLF) throws IOException {          StringBuffer s = null;          int startChar;            synchronized (lock) {              ensureOpen();              boolean omitLF = ignoreLF || skipLF;            bufferLoop:              for (;;) {                    if (nextChar >= nChars)                      fill();                  if (nextChar >= nChars) { /* EOF */                      if (s != null && s.length() > 0)                          return s.toString();                      else                          return null;                  }                  // eol 表示是否遇到 換行符                  boolean eol = false;                  char c = 0;                  int i;                    /* Skip a leftover 'n', if necessary */                  if (omitLF && (cb[nextChar] == 'n'))                      nextChar++;                  skipLF = false;                  omitLF = false;                charLoop: // 這裡有兩種情況跳出循環 1 快取數組cb[]有效字元被讀完 2 遇到 n or r                  for (i = nextChar; i < nChars; i++) {                      c = cb[i];                      if ((c == 'n') || (c == 'r')) {                          eol = true;                          break charLoop;                      }                  }                    startChar = nextChar;                  nextChar = i;                  // 遇到換行符了 需要返回該行的內容了                  if (eol) {                      String str;                      if (s == null) {                          str = new String(cb, startChar, i - startChar);                      } else { // 執行else 必須先執行 341行的邏輯                          s.append(cb, startChar, i - startChar);                          str = s.toString();                      }                      nextChar++;                      if (c == 'r') {                          skipLF = true;                      }                      return str;                  }                  // 說明沒有遇到換行符 但是快取數組cb[]已經被讀完 需要再次fill()                  if (s == null)                      s = new StringBuffer(defaultExpectedLineLength);                  s.append(cb, startChar, i - startChar);              }          }      }        /**       * Reads a line of text.  A line is considered to be terminated by any one       * of a line feed ('n'), a carriage return ('r'), or a carriage return       * followed immediately by a linefeed.       *       * @return     A String containing the contents of the line, not including       *             any line-termination characters, or null if the end of the       *             stream has been reached       *       * @exception  IOException  If an I/O error occurs       *       * @see java.nio.file.Files#readAllLines       */      public String readLine() throws IOException {          return readLine(false);      }        /**       * Skips characters.       *       * @param  n  The number of characters to skip       *       * @return    The number of characters actually skipped       *       * @exception  IllegalArgumentException  If <code>n</code> is negative.       * @exception  IOException  If an I/O error occurs       */      public long skip(long n) throws IOException {          if (n < 0L) {              throw new IllegalArgumentException("skip value is negative");          }          synchronized (lock) {              ensureOpen();              long r = n;              while (r > 0) {                  if (nextChar >= nChars)                      fill();                  if (nextChar >= nChars) /* EOF */                      break;                  if (skipLF) {                      skipLF = false;                      if (cb[nextChar] == 'n') {                          nextChar++;                      }                  }                  long d = nChars - nextChar;                  // 說明cb[]有效字元個數 大於skip(long n) 跳過的字元個數                  if (r <= d) {                      nextChar += r;                      r = 0;                      break;                  }                  // cb[]有效字元個數不夠 需要再次fill()                  else {                      r -= d;                      nextChar = nChars;                  }              }              return n - r;          }      }        /**       * Tells whether this stream is ready to be read.  A buffered character       * stream is ready if the buffer is not empty, or if the underlying       * character stream is ready.       *       * @exception  IOException  If an I/O error occurs       */      public boolean ready() throws IOException {          synchronized (lock) {              ensureOpen();                /*               * If newline needs to be skipped and the next char to be read               * is a newline character, then just skip it right away.               */              if (skipLF) {                  /* Note that in.ready() will return true if and only if the next                   * read on the stream will not block.                   */                  if (nextChar >= nChars && in.ready()) {                      fill();                  }                  if (nextChar < nChars) {                      if (cb[nextChar] == 'n')                          nextChar++;                      skipLF = false;                  }              }              return (nextChar < nChars) || in.ready();          }      }        /**       * Tells whether this stream supports the mark() operation, which it does.       */      public boolean markSupported() {          return true;      }        /**       * Marks the present position in the stream.  Subsequent calls to reset()       * will attempt to reposition the stream to this point.       *       * @param readAheadLimit   Limit on the number of characters that may be       *                         read while still preserving the mark. An attempt       *                         to reset the stream after reading characters       *                         up to this limit or beyond may fail.       *                         A limit value larger than the size of the input       *                         buffer will cause a new buffer to be allocated       *                         whose size is no smaller than limit.       *                         Therefore large values should be used with care.       *       * @exception  IllegalArgumentException  If {@code readAheadLimit < 0}       * @exception  IOException  If an I/O error occurs       */      /*          IO流中的mark(int readAheadLimit)一定要注意 該readAheadLimit表示          mark方法調用過後，最多能讀取的字元個數          比如執行mark(10)，如果你讀取了11字元，那麼mark標記就無效了 但是不會報錯          但是當你執行reset()方法時 會拋出異常      */      public void mark(int readAheadLimit) throws IOException {          if (readAheadLimit < 0) {              throw new IllegalArgumentException("Read-ahead limit < 0");          }          synchronized (lock) {              ensureOpen();              this.readAheadLimit = readAheadLimit;              markedChar = nextChar;              markedSkipLF = skipLF;          }      }        /**       * Resets the stream to the most recent mark.       *       * @exception  IOException  If the stream has never been marked,       *                          or if the mark has been invalidated       */      public void reset() throws IOException {          synchronized (lock) {              ensureOpen();              if (markedChar < 0)                  throw new IOException((markedChar == INVALIDATED)                                        ? "Mark invalid"                                        : "Stream not marked");              nextChar = markedChar;              skipLF = markedSkipLF;          }      }        public void close() throws IOException {          synchronized (lock) {              if (in == null)                  return;              try {                  in.close();              } finally {                  in = null;                  cb = null;              }          }      }        /**       * Returns a {@code Stream}, the elements of which are lines read from       * this {@code BufferedReader}.  The {@link Stream} is lazily populated,       * i.e., read only occurs during the       * <a href="../util/stream/package-summary.html#StreamOps">terminal       * stream operation</a>.       *       * <p> The reader must not be operated on during the execution of the       * terminal stream operation. Otherwise, the result of the terminal stream       * operation is undefined.       *       * <p> After execution of the terminal stream operation there are no       * guarantees that the reader will be at a specific position from which to       * read the next character or line.       *       * <p> If an {@link IOException} is thrown when accessing the underlying       * {@code BufferedReader}, it is wrapped in an {@link       * UncheckedIOException} which will be thrown from the {@code Stream}       * method that caused the read to take place. This method will return a       * Stream if invoked on a BufferedReader that is closed. Any operation on       * that stream that requires reading from the BufferedReader after it is       * closed, will cause an UncheckedIOException to be thrown.       *       * @return a {@code Stream<String>} providing the lines of text       *         described by this {@code BufferedReader}       *       * @since 1.8       */      /*          流操作      */      public Stream<String> lines() {          Iterator<String> iter = new Iterator<String>() {              String nextLine = null;                @Override              public boolean hasNext() {                  if (nextLine != null) {                      return true;                  } else {                      try {                          nextLine = readLine();                          return (nextLine != null);                      } catch (IOException e) {                          throw new UncheckedIOException(e);                      }                  }              }                @Override              public String next() {                  if (nextLine != null || hasNext()) {                      String line = nextLine;                      nextLine = null;                      return line;                  } else {                      throw new NoSuchElementException();                  }              }          };          return StreamSupport.stream(Spliterators.spliteratorUnknownSize(                  iter, Spliterator.ORDERED | Spliterator.NONNULL), false);      }  }  

3 意外發現

我在debugBufferedReader程式碼的時候，發現JVM在啟動的時候，會調用BufferedReader的相關方法。

整個Java項目，僅僅在BufferedReader 第116行打上斷點，然後debug運行Demo程式

直接debug Demo程式，我們會發現，程式會自動進入BufferedReader這個斷點下，說明JVM啟動的時候，初始化了這個類，並用到了該類的某些方法。