深入研究BufferedReader底层源码

目录

• 1 概述
• 2 BufferedReader源码分析
• 3 意外发现

1 概述

最近研究JDK IO流这一块源码，发现真的比较简单，而且还有很多意外发现，如果大家对JDK源码感兴趣，不妨从IO流这一块入手，说不定你会爱上JDK源码。今天我所分享的就是BufferedReader

2 BufferedReader源码分析

public class BufferedReader extends Reader {

	// 真正干活的流，这里使用到了装饰着模式
    private Reader in;

    //缓存字符数组
    private char cb[];

    /*
	  nchars cb[]数组目前存储的字符总数
	  nextChar cb[]数组下一个字符
    */
    private int nChars, nextChar;

    /*
	  INVALIDATED代表无效 当markedChar=INVALIDATED代表无效时 调用reset()方法会抛出异常
    */
    private static final int INVALIDATED = -2;

    // 该流中没有任何标记
    private static final int UNMARKED = -1;

    // mark标记
    private int markedChar = UNMARKED;

    /*
	  从markedChar开始可以最多可以读取的字符 如果超过该长度限制 那么markdChar会失效 调用reset()方法会抛异常
    */
    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;

    // 默认缓存数组cb[]大小
    private static int defaultCharBufferSize = 8192;

    // 用在readLine()方法中 默认一行占80个字符
    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.
     * 该方法的目的就是 填充cb[]缓存数组
     * 注意当markedChar有效时[markedChar, nextChar)必须保留，不能覆盖
     */
    private void fill() throws IOException {
        int dst; // 数据填充的起始位置
        if (markedChar <= UNMARKED) { // 说明没有标记 可以从cb[0]开始填充数组
            /* No mark */
            dst = 0;
        } else {
            /* Marked */
            int delta = nextChar - markedChar;
            if (delta >= readAheadLimit) {
                /* Gone past read-ahead limit: Invalidate mark */
                markedChar = INVALIDATED; // 标记失效 从cb[0]开始填充数组
                readAheadLimit = 0;
                dst = 0;
            } else {
            	// 说明mark标记有效 且该cb[]数组还有剩余空间
                if (readAheadLimit <= cb.length) {
                    /* Shuffle in the current buffer */
                    // 将[markedChar, nextChar)保留 其余放弃
                    System.arraycopy(cb, markedChar, cb, 0, delta);
                    markedChar = 0; // markdChar被移至cb[0]
                    dst = delta;
                } else { // 说明 readAheadLimit超过了cb[]长度 需要扩容cb[]数组
                    /* 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); // 从cb[dst] 开始填充数组
        } 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)
                    	// 说明没有数据了
                        return -1;
                }
                /*
					skipLF默认是false 全局搜索skipLF = ture 会发现只有readLine()会使用它
					只要不混用read()和readLine()就不会跳过换行
				*/
                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长度大于缓存数组长度 并且没有标记 并且 skipLF为false
               那么就直接从底层流中读取数据*/
            if (len >= cb.length && markedChar <= UNMARKED && !skipLF) {
                return in.read(cbuf, off, len);
            }
            fill();
        }
        if (nextChar >= nChars) return -1;
        /*
			skipLF默认是false 全局搜索skipLF = ture 会发现只有readLine()会使用它
			只要不混用read1()和readLine()就不会跳过换行
		*/
        if (skipLF) {
            skipLF = false;
            if (cb[nextChar] == '\n') {
                nextChar++;
                if (nextChar >= nChars)
                    fill();
                if (nextChar >= nChars)
                    return -1;
            }
        }
        // 读取的个数 不能超过缓存数组cb[]有效字符个数
        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
     */
    /*
		此方法判断数组越界情况 并调用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;
            // 当读取的n小于期望的len时并底层流没有被阻塞 会尝试继续读取数据
            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 换行符
                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: // 这里有两种情况跳出循环 1 缓存数组cb[]有效字符被读完 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;
                // 遇到换行符了 需要返回该行的内容了
                if (eol) {
                    String str;
                    if (s == null) {
                        str = new String(cb, startChar, i - startChar);
                    } else { // 执行else 必须先执行 341行的逻辑
                        s.append(cb, startChar, i - startChar);
                        str = s.toString();
                    }
                    nextChar++;
                    if (c == '\r') {
                        skipLF = true;
                    }
                    return str;
                }
                // 说明没有遇到换行符 但是缓存数组cb[]已经被读完 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;
                // 说明cb[]有效字符个数 大于skip(long n) 跳过的字符个数
                if (r <= d) {
                    nextChar += r;
                    r = 0;
                    break;
                }
                // cb[]有效字符个数不够 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 该readAheadLimit表示
		mark方法调用过后，最多能读取的字符个数
		比如执行mark(10)，如果你读取了11字符，那么mark标记就无效了 但是不会报错
		但是当你执行reset()方法时 会抛出异常
    */
    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 意外发现

我在debugBufferedReader代码的时候，发现JVM在启动的时候，会调用BufferedReader的相关方法。

整个Java项目，仅仅在BufferedReader 第116行打上断点，然后debug运行Demo程序

直接debug Demo程序，我们会发现，程序会自动进入BufferedReader这个断点下，说明JVM启动的时候，初始化了这个类，并用到了该类的某些方法。

作者：一杯热咖啡AAA

出处：https://www.cnblogs.com/AdaiCoffee/

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