Java-IO之BufferedReader（字符缓冲输入流）

BufferedReader是缓冲字符输入流，继承于Reader，BufferedReader的作用是为其他字符输入流添加一些缓冲功能。

BufferedReader主要的函数列表：

BufferedReader(Reader in)
BufferedReader(Reader in, int size)

void     close()
void     mark(int markLimit)
boolean  markSupported()
int      read()
int      read(char[] buffer, int offset, int length)
String   readLine()
boolean ready()
void     reset()
long     skip(long charCount)

示例代码：

public class BufferedReaderTest {

    private static final int LEN = 5;

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

    /**
     * BufferedReader的API测试函数
    */
    private static void testBufferedReader() {

    // 创建BufferedReader字符流，内容是ArrayLetters数组
        try {
            File file = new File("out.txt");
            BufferedReader in =new BufferedReader(new FileReader(file));

            // 从字符流中读取5个字符。“abcde”
            for (int i=0; i<LEN; i++) {
                // 若能继续读取下一个字符，则读取下一个字符
                if (in.ready()) {
                    // 读取“字符流的下一个字符”
                    int tmp = in.read();
                    System.out.printf("%d : %c\n", i, tmp);
                }
            }

            // 若“该字符流”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个字符。
            in.skip(22);

            // 读取5个字符
            char[] buf = new char[LEN];
            in.read(buf, 0, LEN);
            System.out.printf("buf=%s\n", String.valueOf(buf));
            // 读取该行剩余的数据
            System.out.printf("readLine=%s\n", in.readLine());

            // 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“重置后的字符流”中读取5个字符到buf中。即读取“fghij”
            in.read(buf, 0, LEN);
            System.out.printf("buf=%s\n", String.valueOf(buf));

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

运行结果：

0 : a

1 : b

2 : c

3 : d

4 : e

buf=

基于JDK8的BufferedReader源码分析：

public class BufferedReader extends Reader {

        private Reader in;

        private char cb[];//字符缓冲数组
        private int nChars, nextChar;

        private static final int INVALIDATED = -2;
        private static final int UNMARKED = -1;
        private int markedChar = UNMARKED;
        private int readAheadLimit = 0; /* Valid only when markedChar > 0 */

        /** If the next character is a line feed, skip it */
        private boolean skipLF = false;\\如果是换行，跳过

        /** The skipLF flag when the mark was set */
        private boolean markedSkipLF = false;

        private static int defaultCharBufferSize = 8192;//默认字符数组缓冲大小
        private static int defaultExpectedLineLength = 80;

        //构造函数，字符输入流，参数是Reader和输入缓冲大小
        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;
        }
        //默认大小，参数Reader
        public BufferedReader(Reader in) {
                this(in, defaultCharBufferSize);
        }

        /** Checks to make sure that the stream has not been closed */
        //判断输入流没有被关闭
        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.
         */
        //
        private void fill() throws IOException {
                int dst;
                if (markedChar <= UNMARKED) {
                    /* No mark */
                    dst = 0;
                } 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 {
                        n = in.read(cb, dst, cb.length - dst);
                } while (n == 0);
                if (n > 0) {
                    nChars = dst + n;
                    nextChar = dst;
                }
        }
        //读单个字符
        public int read() throws IOException {
            synchronized (lock) {
                        ensureOpen();
                        for (;;) {
                            if (nextChar >= nChars) {
                                        fill();
                                        if (nextChar >= nChars)
                                            return -1;
                            }
                            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.
         */
        //读到数组cbuf中
        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. */
                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;
                        }
                }
                int n = Math.min(len, nChars - nextChar);
                System.arraycopy(cb, nextChar, cbuf, off, n);
                nextChar += n;
                return n;
        }
        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;
                        while ((n < len) && in.ready()) {
                                int n1 = read1(cbuf, off + n, len - n);
                                if (n1 <= 0) break;
                                n += n1;
                        }
                        return n;
            }
        }
        //读一行
        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;
                                }
                                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:
                    for (i = nextChar; i < nChars; i++) {
                                        c = cb[i];
                                        if ((c == '\n') || (c == '\r')) {
                                                eol = true;
                                                break charLoop;
                                        }
                                }

                                startChar = nextChar;
                                nextChar = i;

                                if (eol) {
                                        String str;
                                        if (s == null) {
                                                str = new String(cb, startChar, i - startChar);
                                        } else {
                                                s.append(cb, startChar, i - startChar);
                                                str = s.toString();
                                        }
                                        nextChar++;
                                        if (c == '\r') {
                                            skipLF = true;
                                        }
                                        return str;
                                }

                                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
         */
        //读一行，直到'\n'，'\r',或者回车
        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
         */
        //跳过多长的字符
        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;
                                if (r <= d) {
                                        nextChar += r;
                                        r = 0;
                                        break;
                                }
                                else {
                                        r -= d;
                                        nextChar = nChars;
                                    }
                        }
                        return n - r;
                    }
        }
        //是否可读
        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.
         */
        //是否支持标记
        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
         */
        //
        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
         */
        //重置到最近一次标记
        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;
                }
        }
        //关闭资源
        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
         */
        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);
    }
}