Java NIO三大组件与ByteBuffer深入理解及使用

目录

• 1、三大组件
• 1.1 Channel & Buffer
• 1.2 Selector
• 2、ByteBuffer
• 2.1 ByteBuffer 正确使用姿势
• 2.2 ByteBuffer 结构
• 2.3 ByteBuffer 常见方法
• 2.4 Scattering Reads
• 2.5 Gathering Writes
• 2.6 黏包半包现象

1、三大组件

1.1 Channel & Buffer

channel 有一点类似于 stream，它就是读写数据的双向通道，可以从 channel 将数据读入 buffer，也可以将 buffer 的数据写入 channel，而之前的 stream 要么是输入，要么是输出，channel 比 stream 更为底层

常见的 Channel 有

• FileChannel
• DatagramChannel
• SocketChannel
• ServerSocketChannel

buffer 则用来缓冲读写数据，常见的 buffer 有

• ByteBuffer
• MappedByteBuffer
• DirectByteBuffer
• HeapByteBuffer
• ShortBuffer
• IntBuffer
• LongBuffer
• FloatBuffer
• DoubleBuffer
• CharBuffer

1.2 Selector

selector 单从字面意思不好理解，需要结合服务器的设计演化来理解它的用途

多线程版设计

多线程版缺点

• 内存占用高
• 线程上下文切换成本高
• 只适合连接数少的场景

线程池版设计

线程池版缺点

• 阻塞模式下，线程仅能处理一个 socket 连接
• 仅适合短连接场景

selector 版设计

selector 的作用就是配合一个线程来管理多个 channel，获取这些 channel 上发生的事件，这些 channel 工作在非阻塞模式下，不会让线程吊死在一个 channel 上。适合连接数特别多，但流量低的场景（low traffic）

调用 selector 的 select() 会阻塞直到 channel 发生了读写就绪事件，这些事件发生，select 方法就会返回这些事件交给 thread 来处理

2、ByteBuffer

有一普通文本文件 data.txt，内容为

1234567890abcd

使用 FileChannel 来读取文件内容

package org.example.demo1;
import lombok.extern.slf4j.Slf4j;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
@Slf4j
public class ChannelDemo1 {
    public static void main(String[] args) {
        try (FileChannel channel = new FileInputStream("data.txt").getChannel()) {
            ByteBuffer buffer = ByteBuffer.allocate(10);
            do {
                // 向 buffer 写入
                int len = channel.read(buffer);
                log.debug("读到字节数：{}", len);
                if (len == -1) {
                    break;
                }
                // 切换 buffer 读模式
                buffer.flip();
                while(buffer.hasRemaining()) {
                    byte b = buffer.get();
                    log.debug("实际字节{}", (char)b);
                }
                // 切换 buffer 写模式
                buffer.clear();
            } while (true);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

输出

15:03:39.467 [main] DEBUG org.example.demo1.ChannelDemo1 - 读到字节数：10
15:03:39.475 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节1
15:03:39.475 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节2
15:03:39.476 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节3
15:03:39.476 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节4
15:03:39.476 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节5
15:03:39.476 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节6
15:03:39.476 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节7
15:03:39.476 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节8
15:03:39.476 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节9
15:03:39.476 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节0
15:03:39.476 [main] DEBUG org.example.demo1.ChannelDemo1 - 读到字节数：4
15:03:39.477 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节a
15:03:39.477 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节b
15:03:39.477 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节c
15:03:39.477 [main] DEBUG org.example.demo1.ChannelDemo1 - 实际字节d
15:03:39.477 [main] DEBUG org.example.demo1.ChannelDemo1 - 读到字节数：-1

2.1 ByteBuffer 正确使用姿势

• 向 buffer 写入数据，例如调用 channel.read(buffer)
• 调用 flip() 切换至读模式
• 从 buffer 读取数据，例如调用 buffer.get()
• 调用 clear() 或 compact() 切换至写模式
• 重复 1~4 步骤

2.2 ByteBuffer 结构

ByteBuffer 有以下重要属性

• capacity
• position
• limit

一开始

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

flip 动作发生后，position 切换为读取位置，limit 切换为读取限制

读取 4 个字节后，状态

clear 动作发生后，状态

compact 方法，是把未读完的部分向前压缩，然后切换至写模式

调试工具类

package org.example.utils;
import io.netty.util.internal.StringUtil;
import java.nio.ByteBuffer;
import static io.netty.util.internal.MathUtil.isOutOfBounds;
import static io.netty.util.internal.StringUtil.NEWLINE;
public class ByteBufferUtil {
    private static final char[] BYTE2CHAR = new char[256];
    private static final char[] HEXDUMP_TABLE = new char[256 * 4];
    private static final String[] HEXPADDING = new String[16];
    private static final String[] HEXDUMP_ROWPREFIXES = new String[65536 >>> 4];
    private static final String[] BYTE2HEX = new String[256];
    private static final String[] BYTEPADDING = new String[16];
    static {
        final char[] DIGITS = "0123456789abcdef".toCharArray();
        for (int i = 0; i < 256; i++) {
            HEXDUMP_TABLE[i << 1] = DIGITS[i >>> 4 & 0x0F];
            HEXDUMP_TABLE[(i << 1) + 1] = DIGITS[i & 0x0F];
        }
        int i;
        // Generate the lookup table for hex dump paddings
        for (i = 0; i < HEXPADDING.length; i++) {
            int padding = HEXPADDING.length - i;
            StringBuilder buf = new StringBuilder(padding * 3);
            for (int j = 0; j < padding; j++) {
                buf.append("   ");
            }
            HEXPADDING[i] = buf.toString();
        }
        // Generate the lookup table for the start-offset header in each row (up to 64KiB).
        for (i = 0; i < HEXDUMP_ROWPREFIXES.length; i++) {
            StringBuilder buf = new StringBuilder(12);
            buf.append(NEWLINE);
            buf.append(Long.toHexString(i << 4 & 0xFFFFFFFFL | 0x100000000L));
            buf.setCharAt(buf.length() - 9, '|');
            buf.append('|');
            HEXDUMP_ROWPREFIXES[i] = buf.toString();
        }
        // Generate the lookup table for byte-to-hex-dump conversion
        for (i = 0; i < BYTE2HEX.length; i++) {
            BYTE2HEX[i] = ' ' + StringUtil.byteToHexStringPadded(i);
        }
        // Generate the lookup table for byte dump paddings
        for (i = 0; i < BYTEPADDING.length; i++) {
            int padding = BYTEPADDING.length - i;
            StringBuilder buf = new StringBuilder(padding);
            for (int j = 0; j < padding; j++) {
                buf.append(' ');
            }
            BYTEPADDING[i] = buf.toString();
        }
        // Generate the lookup table for byte-to-char conversion
        for (i = 0; i < BYTE2CHAR.length; i++) {
            if (i <= 0x1f || i >= 0x7f) {
                BYTE2CHAR[i] = '.';
            } else {
                BYTE2CHAR[i] = (char) i;
            }
        }
    }
    /**
     * 打印所有内容
     * @param buffer
     */
    public static void debugAll(ByteBuffer buffer) {
        int oldlimit = buffer.limit();
        buffer.limit(buffer.capacity());
        StringBuilder origin = new StringBuilder(256);
        appendPrettyHexDump(origin, buffer, 0, buffer.capacity());
        System.out.println("+--------+-------------------- all ------------------------+----------------+");
        System.out.printf("position: [%d], limit: [%d]\n", buffer.position(), oldlimit);
        System.out.println(origin);
        buffer.limit(oldlimit);
    }
    /**
     * 打印可读取内容
     * @param buffer
     */
    public static void debugRead(ByteBuffer buffer) {
        StringBuilder builder = new StringBuilder(256);
        appendPrettyHexDump(builder, buffer, buffer.position(), buffer.limit() - buffer.position());
        System.out.println("+--------+-------------------- read -----------------------+----------------+");
        System.out.printf("position: [%d], limit: [%d]\n", buffer.position(), buffer.limit());
        System.out.println(builder);
    }
    private static void appendPrettyHexDump(StringBuilder dump, ByteBuffer buf, int offset, int length) {
        if (isOutOfBounds(offset, length, buf.capacity())) {
            throw new IndexOutOfBoundsException(
                    "expected: " + "0 <= offset(" + offset + ") <= offset + length(" + length
                            + ") <= " + "buf.capacity(" + buf.capacity() + ')');
        }
        if (length == 0) {
            return;
        }
        dump.append(
                "         +-------------------------------------------------+" +
                        NEWLINE + "         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |" +
                        NEWLINE + "+--------+-------------------------------------------------+----------------+");
        final int startIndex = offset;
        final int fullRows = length >>> 4;
        final int remainder = length & 0xF;
        // Dump the rows which have 16 bytes.
        for (int row = 0; row < fullRows; row++) {
            int rowStartIndex = (row << 4) + startIndex;
            // Per-row prefix.
            appendHexDumpRowPrefix(dump, row, rowStartIndex);
            // Hex dump
            int rowEndIndex = rowStartIndex + 16;
            for (int j = rowStartIndex; j < rowEndIndex; j++) {
                dump.append(BYTE2HEX[getUnsignedByte(buf, j)]);
            }
            dump.append(" |");
            // ASCII dump
            for (int j = rowStartIndex; j < rowEndIndex; j++) {
                dump.append(BYTE2CHAR[getUnsignedByte(buf, j)]);
            }
            dump.append('|');
        }
        // Dump the last row which has less than 16 bytes.
        if (remainder != 0) {
            int rowStartIndex = (fullRows << 4) + startIndex;
            appendHexDumpRowPrefix(dump, fullRows, rowStartIndex);
            // Hex dump
            int rowEndIndex = rowStartIndex + remainder;
            for (int j = rowStartIndex; j < rowEndIndex; j++) {
                dump.append(BYTE2HEX[getUnsignedByte(buf, j)]);
            }
            dump.append(HEXPADDING[remainder]);
            dump.append(" |");
            // Ascii dump
            for (int j = rowStartIndex; j < rowEndIndex; j++) {
                dump.append(BYTE2CHAR[getUnsignedByte(buf, j)]);
            }
            dump.append(BYTEPADDING[remainder]);
            dump.append('|');
        }
        dump.append(NEWLINE +
                "+--------+-------------------------------------------------+----------------+");
    }
    private static void appendHexDumpRowPrefix(StringBuilder dump, int row, int rowStartIndex) {
        if (row < HEXDUMP_ROWPREFIXES.length) {
            dump.append(HEXDUMP_ROWPREFIXES[row]);
        } else {
            dump.append(NEWLINE);
            dump.append(Long.toHexString(rowStartIndex & 0xFFFFFFFFL | 0x100000000L));
            dump.setCharAt(dump.length() - 9, '|');
            dump.append('|');
        }
    }
    public static short getUnsignedByte(ByteBuffer buffer, int index) {
        return (short) (buffer.get(index) & 0xFF);
    }
}

测试如下：

package org.example.demo1;
import java.nio.ByteBuffer;
import static org.example.utils.ByteBufferUtil.debugAll;
public class TestByteBufferReadWrite {
    public static void main(String[] args){
        ByteBuffer byteBuffer = ByteBuffer.allocate(10);
        byteBuffer.put((byte) 0x61);// a
        debugAll(byteBuffer);
        byteBuffer.put(new byte[]{0x62,0x63,0x64});
        debugAll(byteBuffer);
        byteBuffer.get();
        debugAll(byteBuffer);
        //切换为读的状态
        byteBuffer.flip();
        byteBuffer.get();
        debugAll(byteBuffer);
        byteBuffer.compact();
        debugAll(byteBuffer);
    }
}

运行结果如下：

18:12:55.063 [main] DEBUG io.netty.util.internal.logging.InternalLoggerFactory - Using SLF4J as the default logging framework
+--------+-------------------- all ------------------------+----------------+
position: [1], limit: [10]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 61 00 00 00 00 00 00 00 00 00                   |a.........      |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [4], limit: [10]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 61 62 63 64 00 00 00 00 00 00                   |abcd......      |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [5], limit: [10]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 61 62 63 64 00 00 00 00 00 00                   |abcd......      |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [1], limit: [5]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 61 62 63 64 00 00 00 00 00 00                   |abcd......      |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [4], limit: [10]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 62 63 64 00 00 00 00 00 00 00                   |bcd.......      |
+--------+-------------------------------------------------+----------------+
 
Process finished with exit code 0

2.3 ByteBuffer 常见方法

分配空间

可以使用 allocate 方法为 ByteBuffer 分配空间，其它 buffer 类也有该方法

Bytebuffer buf = ByteBuffer.allocate(16);

例子：

package org.example.demo1;
import java.nio.ByteBuffer;
public class TestByteBufferAllocate {
    public static void main(String[] args){
        System.out.println(ByteBuffer.allocate(16).getClass());
        System.out.println(ByteBuffer.allocateDirect(16).getClass());
    }
}

运行结果如下：

注意：

class java.nio.HeapByteBuffer -java 堆内存，读写效率低，受到GC的影响 class java.nio.DirectByteBuffer -直接内存，读写效率高（少一次拷贝），不会受GC影响，分配的效率低

向 buffer 写入数据

有两种办法

• 调用 channel 的 read 方法
• 调用 buffer 自己的 put 方法

int readBytes = channel.read(buf);

和

buf.put((byte)127);

从 buffer 读取数据

同样有两种办法

• 调用channel的write方法
• 调用buffer自己的get方法

int writeBytes = channel.write(buf);

和

byte b = buf.get();

get 方法会让 position 读指针向后走，如果想重复读取数据

可以调用 rewind 方法将 position 重新置为 0

package org.example.demo1;
import java.nio.ByteBuffer;
import static org.example.utils.ByteBufferUtil.debugAll;
public class TestByteBufferRead {
    public static void main(String[] args){
        ByteBuffer buffer = ByteBuffer.allocate(10);
        buffer.put(new byte[]{'a','b','c','d'});
        buffer.flip();
        //rewind 从头开始读
        buffer.get(new  byte[4]);
        debugAll(buffer);
        System.out.println("===============================rewind================================");
        buffer.rewind();
        System.out.println((char)buffer.get());
    }
}

调用结果：

+--------+-------------------- all ------------------------+----------------+
position: [4], limit: [4]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 61 62 63 64 00 00 00 00 00 00                   |abcd......      |
+--------+-------------------------------------------------+----------------+
===============================rewind================================
a

或者调用 get(int i) 方法获取索引 i 的内容，它不会移动读指针

package org.example.demo1;
import java.nio.ByteBuffer;
import static org.example.utils.ByteBufferUtil.debugAll;
public class TestByteBufferRead {
    public static void main(String[] args){
        ByteBuffer buffer = ByteBuffer.allocate(10);
        buffer.put(new byte[]{'a','b','c','d'});
        buffer.flip();
        //get(i) 不会改变读索引的位置
        System.out.println((char) buffer.get(3));
        debugAll(buffer);
    }
}

调用结果：

mark 和 reset

mark 是在读取时，做一个标记，即使 position 改变，只要调用 reset 就能回到 mark 的位置

package org.example.demo1;
import java.nio.ByteBuffer;
import static org.example.utils.ByteBufferUtil.debugAll;
public class TestByteBufferRead {
    public static void main(String[] args){
        ByteBuffer buffer = ByteBuffer.allocate(10);
        buffer.put(new byte[]{'a','b','c','d'});
        buffer.flip();
        //mark & reset
        //mark 做一个标记，记录position位置，reset 是将position重置到mark的位置
        System.out.println((char) buffer.get());
        System.out.println((char) buffer.get());
        buffer.mark();//加标记，索引2的位置
        System.out.println((char) buffer.get());
        System.out.println((char) buffer.get());
        buffer.reset();//将position重置到索引2
        System.out.println((char) buffer.get());
        System.out.println((char) buffer.get());
    }
}

测试结果：

a
b
c
d
c
d

注意

rewind 和 flip 都会清除 mark 位置

字符串与ByteBuffer互转

package org.example.demo1;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import static org.example.utils.ByteBufferUtil.debugAll;
public class TestByteBufferString {
    public static void main(String[] args){
        ByteBuffer buffer = ByteBuffer.allocate(16);
        buffer.put("hello".getBytes());
        debugAll(buffer);
        buffer.flip();
        CharBuffer charBuffer = StandardCharsets.UTF_8.decode(buffer);
        String charBufferstr = charBuffer.toString();
        System.out.println(charBufferstr);
        //2.Charset
        ByteBuffer buffer2 = StandardCharsets.UTF_8.encode("hello");
        debugAll(buffer2);
        CharBuffer charBuffer1 = StandardCharsets.UTF_8.decode(buffer2);
        String buffer1 = charBuffer1.toString();
        System.out.println(buffer1);
        //3.wrap
        ByteBuffer buffer3 = ByteBuffer.wrap("hello".getBytes());
        debugAll(buffer3);
        CharBuffer charBuffer3 = StandardCharsets.UTF_8.decode(buffer3);
        String bufferstr3 = charBuffer3.toString();
        System.out.println(bufferstr3);
    }
}

输出：

+--------+-------------------- all ------------------------+----------------+
position: [5], limit: [16]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 68 65 6c 6c 6f 00 00 00 00 00 00 00 00 00 00 00 |hello...........|
+--------+-------------------------------------------------+----------------+
hello
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [5]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 68 65 6c 6c 6f                                  |hello           |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [5]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 68 65 6c 6c 6f                                  |hello           |
+--------+-------------------------------------------------+----------------+
hello

Buffer的线程安全

Buffer是非线程安全的

2.4 Scattering Reads

分散读取，有一个文本文件parts.txt

onetwothree

使用如下方式读取，可以将数据填充至多个 buffer

package org.example.demo1;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import static org.example.utils.ByteBufferUtil.debugAll;
public class TestByteBufferReads {
    public static void main(String[] args){
        try (RandomAccessFile file = new RandomAccessFile("parts.txt", "r")) {
            FileChannel channel = file.getChannel();
            ByteBuffer a = ByteBuffer.allocate(3);
            ByteBuffer b = ByteBuffer.allocate(3);
            ByteBuffer c = ByteBuffer.allocate(5);
            channel.read(new ByteBuffer[]{a, b, c});
            a.flip();
            b.flip();
            c.flip();
            debugAll(a);
            debugAll(b);
            debugAll(c);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

结果：

+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [3]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 6f 6e 65                                        |one             |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [3]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 74 77 6f                                        |two             |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [5]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 74 68 72 65 65                                  |three           |
+--------+-------------------------------------------------+----------------+

2.5 Gathering Writes

使用如下方式写入，可以将多个 buffer 的数据填充至 channel

package org.example.demo1;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.StandardCharsets;
public class TestGatheringWrites {
    public static void main(String[] args){
        ByteBuffer b1 = StandardCharsets.UTF_8.encode("hello");
        ByteBuffer b2 = StandardCharsets.UTF_8.encode("world");
        ByteBuffer b3 = StandardCharsets.UTF_8.encode("你好");
        try(FileChannel channel = new RandomAccessFile("words2.txt","rw").getChannel()){
            channel.write(new ByteBuffer[]{b1,b2,b3});
        }catch (IOException ex){
        }
    }
}

输出结果：

2.6 黏包半包现象

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 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();
    int oldLimit = source.limit();
    for (int i = 0; i < oldLimit; i++) {
        if (source.get(i) == '\n') {
            System.out.println(i);
            ByteBuffer target = ByteBuffer.allocate(i + 1 - source.position());
            // 0 ~ limit
            source.limit(i + 1);
            target.put(source); // 从source 读，向 target 写
            debugAll(target);
            source.limit(oldLimit);
        }
    }
    source.compact();
}

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