XMLSocket在向ServerSocket发送消息的时候，对每个消息最后都会附加/0作为消息的结束
为了更便于日后的编程，我们对该特点进行特殊处理，专门封装出一个XMLSocketWriter类来完成
而是这种机制对外不可见，即便别人不知道这种特点，只要使用这个专门的Writer类都可以完成
以下是XMLSocketWriter的代码，是从JDK1.4.2.08中的源代码中修改而来的。
/*
 * @(#)BufferedWriter.java  1.24 03/01/23
 *
 * Copyright 2003 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
package com.inet.io;

/**
 * Write 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     1.24, 03/01/23
 * @author  Mark Reinhold
 * @since   JDK1.1
 */

import java.io.IOException;
import java.io.Writer;

/**
 *
 * <p>
 * Title: XMLSocketWriter
 * </p>
 * <p>
 * Description:
 * </p>
 * <p>
 * Copyright: Copyright (c) 2006
 * </p>
 * <p>
 * Company: INET
 * </p>
 *
 * 该类从JDK1.4.2_08的BufferedWriter拷贝而来并稍加修改
 *
 * @author chenyankui
 * @version 1.0
 */
public class XMLSocketWriter 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;

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

    /**
     * Create 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 XMLSocketWriter(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 = System.getProperty("line.separator");
    }

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

    /**
     * Flush 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;
            }
           
            // 写http-head结束标记
            out.write(MMFlash.HTTP_HEAD_END);
           
            out.write(cb, 0, nextChar);
           
            nextChar = 0;
        }
    }

    /**
     * Write 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;
    }

    /**
     * Write 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();
            }
        }
    }

    /**
     * Write a portion of a String.
     *
     * @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();
            }
        }
    }

    /**
     * Write 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);
    }

    /**
     * Flush the stream.
     *
     * @exception IOException
     *                If an I/O error occurs
     */
    public void flush() throws IOException
    {
        synchronized(lock)
        {
            if(nextChar == 0)
            {
                return;
            }
           
            // 写数据结束标记
            this.write(MMFlash.HTTP_DATA_END);
           
            flushBuffer();
           
            out.flush();
        }
    }

    /**
     * Close the stream.
     *
     * @exception IOException
     *                If an I/O error occurs
     */
    public void close() throws IOException
    {
        synchronized(lock)
        {
            if(out == null)
                return;
           
            flushBuffer();
           
            out.close();
            out = null;
            cb = null;
        }
    }
}