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© 赵国刚 中级黑马   /  2013-8-12 20:20  /  1906 人查看  /  2 人回复  /   0 人收藏 转载请遵从CC协议 禁止商业使用本文

BufferReader  中有一个readLine() 这个可以直接读取一行数据 从而提高效率 这个清楚了

请教 BufferWriter 只比Writer方法多了一个newLine() 的方法 别貌似都跟Writer方法都一样 不晓得提高效率反映在哪?

如果说  BufferWriter 是先把数据写入缓冲区 flush() 后一次性写入硬盘的方法来来提高效率
那貌似 Writer 也是先把数据写入流 在flush() 或者close() 刷新后才写入硬盘的  这么说来貌似Writer 也有缓冲的能力。

再有BufferWriter  缓冲区有多大 ,如果一个很大分文件要写入 而不再中间进行flush()   缓冲区会不会满 ,如果满了是不是自动写入硬盘?

还有 BufferOutputStream 有是怎么提高OutputStream的效率的  ?

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1, BufferedWriter 提高效率就是因为把数据写入了缓冲区  下面是源码 你可以看看。
2,如果要是文件很大的话,应该需要拆分处理。http://www.oschina.net/code/snippet_54100_7938
3. BufferOutputStream 也是因为缓冲区才提高效率 具体也得看源码

/*
* @(#)BufferedWriter.java        1.29 05/11/30
*
* Copyright 2006 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/

package java.io;


/**
* Writes text to a character-output stream, buffering characters so as to
* provide for the efficient writing of single characters, arrays, and strings.
*
* <p> The buffer size may be specified, or the default size may be accepted.
* The default is large enough for most purposes.
*
* <p> A newLine() method is provided, which uses the platform's own notion of
* line separator as defined by the system property <tt>line.separator</tt>.
* Not all platforms use the newline character ('\n') to terminate lines.
* Calling this method to terminate each output line is therefore preferred to
* writing a newline character directly.
*
* <p> In general, a Writer sends its output immediately to the underlying
* character or byte stream.  Unless prompt output is required, it is advisable
* to wrap a BufferedWriter around any Writer whose write() operations may be
* costly, such as FileWriters and OutputStreamWriters.  For example,
*
* <pre>
* PrintWriter out
*   = new PrintWriter(new BufferedWriter(new FileWriter("foo.out")));
* </pre>
*
* will buffer the PrintWriter's output to the file.  Without buffering, each
* invocation of a print() method would cause characters to be converted into
* bytes that would then be written immediately to the file, which can be very
* inefficient.
*
* @see PrintWriter
* @see FileWriter
* @see OutputStreamWriter
*
* @version         1.29, 05/11/30
* @author        Mark Reinhold
* @since        JDK1.1
*/

public class BufferedWriter extends Writer {

    private Writer out;

    private char cb[];
    private int nChars, nextChar;

    private static int defaultCharBufferSize = 8192;

    /**
     * Line separator string.  This is the value of the line.separator
     * property at the moment that the stream was created.
     */
    private String lineSeparator;

    /**
     * Creates a buffered character-output stream that uses a default-sized
     * output buffer.
     *
     * @param  out  A Writer
     */
    public BufferedWriter(Writer out) {
        this(out, defaultCharBufferSize);
    }

    /**
     * Creates a new buffered character-output stream that uses an output
     * buffer of the given size.
     *
     * @param  out  A Writer
     * @param  sz   Output-buffer size, a positive integer
     *
     * @exception  IllegalArgumentException  If sz is <= 0
     */
    public BufferedWriter(Writer out, int sz) {
        super(out);
        if (sz <= 0)
            throw new IllegalArgumentException("Buffer size <= 0");
        this.out = out;
        cb = new char[sz];
        nChars = sz;
        nextChar = 0;

        lineSeparator =        (String) java.security.AccessController.doPrivileged(
               new sun.security.action.GetPropertyAction("line.separator"));
    }

    /** Checks to make sure that the stream has not been closed */
    private void ensureOpen() throws IOException {
        if (out == null)
            throw new IOException("Stream closed");
    }

    /**
     * Flushes the output buffer to the underlying character stream, without
     * flushing the stream itself.  This method is non-private only so that it
     * may be invoked by PrintStream.
     */
    void flushBuffer() throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (nextChar == 0)
                return;
            out.write(cb, 0, nextChar);
            nextChar = 0;
        }
    }

    /**
     * Writes a single character.
     *
     * @exception  IOException  If an I/O error occurs
     */
    public void write(int c) throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (nextChar >= nChars)
                flushBuffer();
            cb[nextChar++] = (char) c;
        }
    }

    /**
     * Our own little min method, to avoid loading java.lang.Math if we've run
     * out of file descriptors and we're trying to print a stack trace.
     */
    private int min(int a, int b) {
        if (a < b) return a;
        return b;
    }

    /**
     * Writes a portion of an array of characters.
     *
     * <p> Ordinarily this method stores characters from the given array into
     * this stream's buffer, flushing the buffer to the underlying stream as
     * needed.  If the requested length is at least as large as the buffer,
     * however, then this method will flush the buffer and write the characters
     * directly to the underlying stream.  Thus redundant
     * <code>BufferedWriter</code>s will not copy data unnecessarily.
     *
     * @param  cbuf  A character array
     * @param  off   Offset from which to start reading characters
     * @param  len   Number of characters to write
     *
     * @exception  IOException  If an I/O error occurs
     */
    public void write(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;
            }

            if (len >= nChars) {
                /* If the request length exceeds the size of the output buffer,
                   flush the buffer and then write the data directly.  In this
                   way buffered streams will cascade harmlessly. */
                flushBuffer();
                out.write(cbuf, off, len);
                return;
            }

            int b = off, t = off + len;
            while (b < t) {
                int d = min(nChars - nextChar, t - b);
                System.arraycopy(cbuf, b, cb, nextChar, d);
                b += d;
                nextChar += d;
                if (nextChar >= nChars)
                    flushBuffer();
            }
        }
    }

    /**
     * Writes a portion of a String.
     *
     * <p> If the value of the <tt>len</tt> parameter is negative then no
     * characters are written.  This is contrary to the specification of this
     * method in the {@linkplain java.io.Writer#write(java.lang.String,int,int)
     * superclass}, which requires that an {@link IndexOutOfBoundsException} be
     * thrown.
     *
     * @param  s     String to be written
     * @param  off   Offset from which to start reading characters
     * @param  len   Number of characters to be written
     *
     * @exception  IOException  If an I/O error occurs
     */
    public void write(String s, int off, int len) throws IOException {
        synchronized (lock) {
            ensureOpen();

            int b = off, t = off + len;
            while (b < t) {
                int d = min(nChars - nextChar, t - b);
                s.getChars(b, b + d, cb, nextChar);
                b += d;
                nextChar += d;
                if (nextChar >= nChars)
                    flushBuffer();
            }
        }
    }

    /**
     * Writes a line separator.  The line separator string is defined by the
     * system property <tt>line.separator</tt>, and is not necessarily a single
     * newline ('\n') character.
     *
     * @exception  IOException  If an I/O error occurs
     */
    public void newLine() throws IOException {
        write(lineSeparator);
    }

    /**
     * Flushes the stream.
     *
     * @exception  IOException  If an I/O error occurs
     */
    public void flush() throws IOException {
        synchronized (lock) {
            flushBuffer();
            out.flush();
        }
    }

    public void close() throws IOException {
        synchronized (lock) {
            if (out == null) {
                return;
            }
            try {
                flushBuffer();
            } finally {
                out.close();
                out = null;
                cb = null;
            }
        }
    }
}

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在BufferedReader类中封装一个数组,用来缓存数据,大小是8192(1024*4)字节的.
当用bufferedReader进行读数据时,先将数据读到缓冲区中,在一次写出.
bufferedWriter也是一样,封装了一个数组.对于大文件写出时,缓冲区写满后会自动进行写出,
只有最后一次,没写满,不会自动写出,需要flush或close().
加Buffer的原理都一样.

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