1、Buffer简介

Buffer本质是一个内存块，既可以写入数据，也可以读出数据。

java.nio中的Buffer类是一个非线程安全的类

Buffer类型说明：
**
Buffer是一个抽象类，对应于java的主要数据类型，nio中有8种缓冲区，分别如下

  • ByteBuffer : 使用最多的Buffer
  • CharBuffer
  • ShortBuffer
  • IntBuffer
  • DoubleBufffer
  • LongBuffer
  • FloatBuffer
  • MappedByteBuffer ：专门用于内存映射

Buffer属性说明
capacity 容量，即可以容纳的最大数据量；在缓冲区创建时设置并且不能改变
limit 上限，缓冲区中当前的数据量
position 读写指针，缓冲区中下一个要被读或写的元素的索引
mark 标记，调用 mark()方法来设置 mark=position，再调用 reset()可以让 position 恢复到 mark 标记的位置即 position=mark

ByteBuffer 有以下重要属性

  • capacity
  • position
  • limit

一开始
0021.png
写模式下，position 是写入位置，limit 等于容量，下图表示写入了 4 个字节后的状态
0018.png
flip 动作发生后，position 切换为读取位置，limit 切换为读取限制
0019.png
读取 4 个字节后，状态
0020.png

clear 动作发生后，状态
0021.png

compact 方法，是把未读完的部分向前压缩，然后切换至写模式

0022.png

Buffer使用步骤：

（1）使用创建子类实例对象的 allocate()方法，创建一个 Buffer 类的实例对象。
（2）调用 put 方法，将数据写入到缓冲区中。
（3）写入完成后，在开始读取数据前，调用 Buffer.flip()方法，将缓冲区转换为读模式。
（4）调用 get 方法，从缓冲区中读取数据。
（5）读取完成后，调用 Buffer.clear() 或 Buffer.compact()方法，将缓冲区转换为写入模式。

2、Buffer实战

2.1、Buffer调试工具类

  1. package com.jx.netty.niobase.util;
  3. import io.netty.util.internal.StringUtil;
  4. import java.nio.ByteBuffer;
  6. import static io.netty.util.internal.StringUtil.NEWLINE;
  7. import static io.netty.util.internal.MathUtil.isOutOfBounds;
  8. /**
  9.  * @Description:
  10.  * @Author: zhangjx
  11.  * @Date: 2021-05-07
  12.  **/
  13. public class ByteBufferUtil {
  14.     private static final char[] BYTE2CHAR = new char[256];
  15.     private static final char[] HEXDUMP_TABLE = new char[256 * 4];
  16.     private static final String[] HEXPADDING = new String[16];
  17.     private static final String[] HEXDUMP_ROWPREFIXES = new String[65536 >>> 4];
  18.     private static final String[] BYTE2HEX = new String[256];
  19.     private static final String[] BYTEPADDING = new String[16];
  21.     static {
  22.         final char[] DIGITS = "0123456789abcdef".toCharArray();
  23.         for (int i = 0; i < 256; i++) {
  24.             HEXDUMP_TABLE[i << 1] = DIGITS[i >>> 4 & 0x0F];
  25.             HEXDUMP_TABLE[(i << 1) + 1] = DIGITS[i & 0x0F];
  26.         }
  28.         int i;
  30.         // Generate the lookup table for hex dump paddings
  31.         for (i = 0; i < HEXPADDING.length; i++) {
  32.             int padding = HEXPADDING.length - i;
  33.             StringBuilder buf = new StringBuilder(padding * 3);
  34.             for (int j = 0; j < padding; j++) {
  35.                 buf.append("   ");
  36.             }
  37.             HEXPADDING[i] = buf.toString();
  38.         }
  40.         // Generate the lookup table for the start-offset header in each row (up to 64KiB).
  41.         for (i = 0; i < HEXDUMP_ROWPREFIXES.length; i++) {
  42.             StringBuilder buf = new StringBuilder(12);
  43.             buf.append(NEWLINE);
  44.             buf.append(Long.toHexString(i << 4 & 0xFFFFFFFFL | 0x100000000L));
  45.             buf.setCharAt(buf.length() - 9, '|');
  46.             buf.append('|');
  47.             HEXDUMP_ROWPREFIXES[i] = buf.toString();
  48.         }
  50.         // Generate the lookup table for byte-to-hex-dump conversion
  51.         for (i = 0; i < BYTE2HEX.length; i++) {
  52.             BYTE2HEX[i] = ' ' + StringUtil.byteToHexStringPadded(i);
  53.         }
  55.         // Generate the lookup table for byte dump paddings
  56.         for (i = 0; i < BYTEPADDING.length; i++) {
  57.             int padding = BYTEPADDING.length - i;
  58.             StringBuilder buf = new StringBuilder(padding);
  59.             for (int j = 0; j < padding; j++) {
  60.                 buf.append(' ');
  61.             }
  62.             BYTEPADDING[i] = buf.toString();
  63.         }
  65.         // Generate the lookup table for byte-to-char conversion
  66.         for (i = 0; i < BYTE2CHAR.length; i++) {
  67.             if (i <= 0x1f || i >= 0x7f) {
  68.                 BYTE2CHAR[i] = '.';
  69.             } else {
  70.                 BYTE2CHAR[i] = (char) i;
  71.             }
  72.         }
  73.     }
  75.     /**
  76.      * 打印所有内容
  77.      * @param buffer
  78.      */
  79.     public static void debugAll(ByteBuffer buffer) {
  80.         int oldlimit = buffer.limit();
  81.         buffer.limit(buffer.capacity());
  82.         StringBuilder origin = new StringBuilder(256);
  83.         appendPrettyHexDump(origin, buffer, 0, buffer.capacity());
  84.         System.out.println("+--------+-------------------- all ------------------------+----------------+");
  85.         System.out.printf("position: [%d], limit: [%d]\n", buffer.position(), oldlimit);
  86.         System.out.println(origin);
  87.         buffer.limit(oldlimit);
  88.     }
  90.     /**
  91.      * 打印可读取内容
  92.      * @param buffer
  93.      */
  94.     public static void debugRead(ByteBuffer buffer) {
  95.         StringBuilder builder = new StringBuilder(256);
  96.         appendPrettyHexDump(builder, buffer, buffer.position(), buffer.limit() - buffer.position());
  97.         System.out.println("+--------+-------------------- read -----------------------+----------------+");
  98.         System.out.printf("position: [%d], limit: [%d]\n", buffer.position(), buffer.limit());
  99.         System.out.println(builder);
  100.     }
  102.     private static void appendPrettyHexDump(StringBuilder dump, ByteBuffer buf, int offset, int length) {
  103.         if (isOutOfBounds(offset, length, buf.capacity())) {
  104.             throw new IndexOutOfBoundsException(
  105.                     "expected: " + "0 <= offset(" + offset + ") <= offset + length(" + length
  106.                             + ") <= " + "buf.capacity(" + buf.capacity() + ')');
  107.         }
  108.         if (length == 0) {
  109.             return;
  110.         }
  111.         dump.append(
  112.                 "         +-------------------------------------------------+" +
  113.                         NEWLINE + "         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |" +
  114.                         NEWLINE + "+--------+-------------------------------------------------+----------------+");
  116.         final int startIndex = offset;
  117.         final int fullRows = length >>> 4;
  118.         final int remainder = length & 0xF;
  120.         // Dump the rows which have 16 bytes.
  121.         for (int row = 0; row < fullRows; row++) {
  122.             int rowStartIndex = (row << 4) + startIndex;
  124.             // Per-row prefix.
  125.             appendHexDumpRowPrefix(dump, row, rowStartIndex);
  127.             // Hex dump
  128.             int rowEndIndex = rowStartIndex + 16;
  129.             for (int j = rowStartIndex; j < rowEndIndex; j++) {
  130.                 dump.append(BYTE2HEX[getUnsignedByte(buf, j)]);
  131.             }
  132.             dump.append(" |");
  134.             // ASCII dump
  135.             for (int j = rowStartIndex; j < rowEndIndex; j++) {
  136.                 dump.append(BYTE2CHAR[getUnsignedByte(buf, j)]);
  137.             }
  138.             dump.append('|');
  139.         }
  141.         // Dump the last row which has less than 16 bytes.
  142.         if (remainder != 0) {
  143.             int rowStartIndex = (fullRows << 4) + startIndex;
  144.             appendHexDumpRowPrefix(dump, fullRows, rowStartIndex);
  146.             // Hex dump
  147.             int rowEndIndex = rowStartIndex + remainder;
  148.             for (int j = rowStartIndex; j < rowEndIndex; j++) {
  149.                 dump.append(BYTE2HEX[getUnsignedByte(buf, j)]);
  150.             }
  151.             dump.append(HEXPADDING[remainder]);
  152.             dump.append(" |");
  154.             // Ascii dump
  155.             for (int j = rowStartIndex; j < rowEndIndex; j++) {
  156.                 dump.append(BYTE2CHAR[getUnsignedByte(buf, j)]);
  157.             }
  158.             dump.append(BYTEPADDING[remainder]);
  159.             dump.append('|');
  160.         }
  162.         dump.append(NEWLINE +
  163.                 "+--------+-------------------------------------------------+----------------+");
  164.     }
  166.     private static void appendHexDumpRowPrefix(StringBuilder dump, int row, int rowStartIndex) {
  167.         if (row < HEXDUMP_ROWPREFIXES.length) {
  168.             dump.append(HEXDUMP_ROWPREFIXES[row]);
  169.         } else {
  170.             dump.append(NEWLINE);
  171.             dump.append(Long.toHexString(rowStartIndex & 0xFFFFFFFFL | 0x100000000L));
  172.             dump.setCharAt(dump.length() - 9, '|');
  173.             dump.append('|');
  174.         }
  175.     }
  177.     public static short getUnsignedByte(ByteBuffer buffer, int index) {
  178.         return (short) (buffer.get(index) & 0xFF);
  179.     }
  180. }

2.2、Buffer使用示例

  1.  public static void main(String[] args) {
  2.         IntBuffer intBuffer = IntBuffer.allocate(10);
  4.         for (int i = 0; i < 5; i++) {
  5.             intBuffer.put(i);
  6.             System.out.println("写模式： position: " + intBuffer.position() + " limit: " + intBuffer.limit() + " capacity: " + intBuffer.capacity());
  8.         }
  10.         //将缓冲区切换为读模式
  11.         intBuffer.flip();
  13.         for (int i = 0; i < 5; i++) {
  14.             System.out.println("读取： " + intBuffer.get() + " 读模式： position: " + intBuffer.position() + " limit: " + intBuffer.limit() + " capacity: " + intBuffer.capacity());
  16.         }
  18.         //rewind 倒带已经读完的数据，再读一遍
  19.         intBuffer.rewind();
  20.         for (int i = 0; i < 5; i++) {
  21.             System.out.println("读取： " + intBuffer.get() + " 读模式： position: " + intBuffer.position() + " limit: " + intBuffer.limit() + " capacity: " + intBuffer.capacity());
  22.         }
  24.         //mark 和 reset
  25.         intBuffer.rewind();
  26.         for (int i = 0; i < 5; i++) {
  27.             if(i == 2){
  28.                 intBuffer.mark();
  29.             }
  30.             System.out.println("读取： " + intBuffer.get() + " 读模式： position: " + intBuffer.position() + " limit: " + intBuffer.limit() + " capacity: " + intBuffer.capacity());
  31.         }
  33.         intBuffer.reset();
  35.         System.out.println("读取： " + intBuffer.get() + " 读模式： position: " + intBuffer.position() + " limit: " + intBuffer.limit() + " capacity: " + intBuffer.capacity());
  37.         //clear 清空缓冲区
  38.         intBuffer.clear();
  39.         intBuffer.put(1);
  40.           System.out.println("读取： " + intBuffer.get() + " 读模式： position: " + intBuffer.position() + " limit: " + intBuffer.limit() + " capacity: " + intBuffer.capacity());
  41.     }

2.3、字符串和ByteBuffer互相转化

        //字符串转为ByteBuffer

        //1、转为字节数组，写入ByteBuffer，仍处于写模式
        ByteBuffer strBuffer1 = ByteBuffer.allocate(16);
        strBuffer1.put( "hello".getBytes());
        ByteBufferUtil.debugAll(strBuffer1);

        //2、Charset 自动切换为读模式
        ByteBuffer strBuffer2 = StandardCharsets.UTF_8.encode("hello");
        ByteBufferUtil.debugAll(strBuffer2);

        //3、wrap 自动切换为读模式
        ByteBuffer strBuffer3 = ByteBuffer.wrap("hello".getBytes());
        ByteBufferUtil.debugAll(strBuffer3);


        //ByteBuffer转为字符串


        //strBuffer1仍然处于写模式，无法转为字符串
        String s1 = StandardCharsets.UTF_8.decode(strBuffer1).toString();
        System.out.println("s1: " + s1);

        String s2 = StandardCharsets.UTF_8.decode(strBuffer2).toString();
        System.out.println("s2: " +s2);

2.4、ByteBuffer 黏包，半包问题

网络上有多条数据发送给服务端，数据之间使用 \n 进行分隔但由于某种原因这些数据在接收时，被进行了重新组合，例如原始数据有3条为

  • Hello,world\n
  • I’m zhangsan\n
  • How are you?\n

变成了下面的两个 byteBuffer (黏包，半包)

  • Hello,world\nI’m zhangsan\nHo
  • w are you?\n

现在要求你编写程序，将错乱的数据恢复成原始的按 \n 分隔的数据

public class ByteBufferExam {

    public static void main(String[] args) {
        ByteBuffer source = ByteBuffer.allocate(32);
        //                     11            24
        source.put("Hello,world\nI'm zhangsan\nHo".getBytes());
        split(source);

        source.put("w are you?\nhaha!\n".getBytes());
        split(source);
    }

    private static void split(ByteBuffer source) {
        source.flip();
        for (int i = 0; i < source.limit() ; i++) {
            if (source.get(i) == '\n'){
                int len = i + 1 - source.position();
                ByteBuffer buffer = ByteBuffer.allocate(len);


                for (int j = 0; j < len; j++) {
                    byte b = source.get();
                    buffer.put(b);
                }
                ByteBufferUtil.debugAll(buffer);
            }
        }
        source.compact();
    }
}