NIO笔记02-ByteBuffer

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前言

创建data.txt文件，内容为

0123456789123asd

使用 FileChannel 来读取文件内容
代码中的position指针后面会详细讲解

@Slf4j
public class Test01ByteBuffer {public static void main(String[] args) {// FileChannel// 1. 输入输出流， 2. RandomAccessFiletry (FileChannel channel = new FileInputStream("data.txt").getChannel()) {// 准备缓冲区ByteBuffer buffer = ByteBuffer.allocate(10);//缓冲区设为10字节while(true) {// 从 channel 读取数据，向 buffer 写入int len = channel.read(buffer);log.debug("读取到的字节数 {}", len);if(len == -1) { // 没有内容了break;}// 打印 buffer 的内容buffer.flip(); // 切换至读模式(将position = 0)while(buffer.hasRemaining()) { // 是否还有剩余未读数据byte b = buffer.get();log.debug("实际字节 {}", (char) b);}buffer.clear(); // 切换为写模式}} catch (IOException e) {e.printStackTrace();}}
}

输出：

22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 读取到的字节数 10
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 0
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 1
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 2
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 3
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 4
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 5
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 6
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 7
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 8
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 9
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 读取到的字节数 6
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 1
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 2
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 3
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 a
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 s
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 实际字节 d
22:22:59 [DEBUG] [main] c.i.n.c.Test01ByteBuffer - 读取到的字节数 -1

1. ByteBuffer 正确使用姿势

1. 向 buffer 写入数据，例如前面代码中调用 channel.read(buffer)
2. 调用 flip() 切换至读模式
3. 从 buffer 读取数据，例如调用 buffer.get()
4. 调用 clear() 或 compact() 切换至写模式
5. 重复 1~4 步骤

2. ByteBuffer 结构

ByteBuffer 有以下重要属性

• capacity
• position
• limit

一开始创建ByteBuffer时为：position = 0

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
position = 4

调用flip()后，position 切换为读取位置，limit 切换为读取限制
position = 0
limit = 4

读取 4 个字节后，状态：
position = 4
limit = 4

调用clear()方法后，状态：position = 0

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 ★

使用该类查看ByteBuffer中的可读取内容

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;/*** 调试工具类*/
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 paddingsfor (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 conversionfor (i = 0; i < BYTE2HEX.length; i++) {BYTE2HEX[i] = ' ' + StringUtil.byteToHexStringPadded(i);}// Generate the lookup table for byte dump paddingsfor (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 conversionfor (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);}public static void main(String[] args) {ByteBuffer buffer = ByteBuffer.allocate(10);buffer.put(new byte[]{97, 98, 99, 100});debugAll(buffer);}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 dumpint rowEndIndex = rowStartIndex + 16;for (int j = rowStartIndex; j < rowEndIndex; j++) {dump.append(BYTE2HEX[getUnsignedByte(buf, j)]);}dump.append(" |");// ASCII dumpfor (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 dumpint rowEndIndex = rowStartIndex + remainder;for (int j = rowStartIndex; j < rowEndIndex; j++) {dump.append(BYTE2HEX[getUnsignedByte(buf, j)]);}dump.append(HEXPADDING[remainder]);dump.append(" |");// Ascii dumpfor (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);}
}

3. ByteBuffer 常见方法

分配空间allocate

1.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);//底层是返回了new HeapByteBuffer(capacity, capacity)对象
//或
ByteBuffer byteBuffer = ByteBuffer.allocateDirect(16)//底层使用了DirectByteBuffer  

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

DirectByteBuffer缺点：

1. 因为是直接内存(系统内存)，所以分配的速度比较慢，因为要调用操作系统的函数
2. 使用不当会导致内存泄漏，DirectByteBuffer使用后必须要合理的释放。

netty对DirectByteBuffer进行了封装，读写和分配效率进行了优化。
netty采用了对象池进行分配，尽可能减少了DirectByteBuffer的分配频率，进而提高分配效率。同时对象池会对DirectByteBuffer进行回收，减少内存泄漏问题。

向 buffer 写入数据

有两种办法

• 调用 channel 的 read 方法

int readBytes = channel.read(buf);

• 调用 buffer 自己的 put 方法

buf.put((byte)127);

从 buffer 读取数据

同样有两种办法

• 调用 channel 的 write 方法

int writeBytes = channel.write(buf);

• 调用 buffer 自己的 get 方法

byte b = buf.get();

注意
get 方法会让 position 读指针向后走，如果想重复读取数据
1.可以调用 rewind 方法将 position 重新置为 0
2.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

mark 和 reset

mark 是在读取时，做一个标记，即使 position 改变后，只要调用 reset 就能回到 mark 的位置
注意： rewind 和 flip 都会清除 mark 位置

代码实现

import java.nio.ByteBuffer;
import static cn.qf.nio.ByteBufferUtil.debugAll;
public class ByteBufferRead {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]);//读取4字节的数据
//        debugAll(buffer);
//        buffer.rewind();//底层源码是将position赋值为0
//        System.out.println((char)buffer.get());// mark() & reset()// mark() 做一个标记，记录 position 位置， reset() 是将 position 重置到 mark 的位置//mark()和reset()结合使用实现标记跳转功能//position从0开始
//        System.out.println((char) buffer.get());//a
//        System.out.println((char) buffer.get());//b
//        buffer.mark(); // 加标记，索引2 的位置。但position值不变
//        System.out.println((char) buffer.get());//c
//        System.out.println((char) buffer.get());//d
//        buffer.reset(); // 将 position 重置到索引 2
//        System.out.println((char) buffer.get());//c
//        System.out.println((char) buffer.get());//d// get(i) 不会改变读索引的位置//position从0开始System.out.println((char) buffer.get(3));debugAll(buffer);//position依然等于0}
}

字符串与 ByteBuffer 互转

import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;
import static cn.qf.nio.c2.ByteBufferUtil.debugAll;public class ByteBufferString {public static void main(String[] args) {// 1. 字符串转为 ByteBufferByteBuffer buffer1 = ByteBuffer.allocate(16);buffer1.put("hello".getBytes());debugAll(buffer1);// 2. CharsetByteBuffer buffer2 = StandardCharsets.UTF_8.encode("hello");debugAll(buffer2);//会自动切换到读模式 position = 0// 3. wrapByteBuffer buffer3 = ByteBuffer.wrap("hello".getBytes());debugAll(buffer3);//会自动切换到读模式 position = 0// 4. 转为字符串String str1 = StandardCharsets.UTF_8.decode(buffer2).toString();System.out.println(str1);//hellobuffer1.flip();String str2 = StandardCharsets.UTF_8.decode(buffer1).toString();System.out.println(str2);//helloString str3 = new String(buffer3.array());System.out.println(str3);//hello}
}

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...........|
+--------+-------------------------------------------------+----------------+
+--------+-------------------- 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
hello
hello

注意：Buffer 是非线程安全的

4. Scattering Reads(分散读取ByteBuffer )

分散读取，创建一个文本文件 words.txt

onetwothree

使用如下方式读取，可以将数据填充至多个 buffer

import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import static cn.qf.nio.c2.ByteBufferUtil.debugAll;
public class Test06ScatteringReads {public static void main(String[] args) {try (FileChannel channel = new RandomAccessFile("words.txt", "r").getChannel()) {ByteBuffer b1 = ByteBuffer.allocate(3);ByteBuffer b2 = ByteBuffer.allocate(3);ByteBuffer b3 = ByteBuffer.allocate(5);channel.read(new ByteBuffer[]{b1, b2, b3});b1.flip();b2.flip();b3.flip();debugAll(b1);debugAll(b2);debugAll(b3);} catch (IOException e) {}}
}

输出

+--------+-------------------- 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           |
+--------+-------------------------------------------------+----------------+

5. Gathering Writes(批量ByteBuffer写入)

使用如下方式写入，可以将多个 buffer 的数据填充至 channel

import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.StandardCharsets;
public class Test07GatheringWrites {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("你好呀");try (FileChannel channel = new RandomAccessFile("words2.txt", "rw").getChannel()) {channel.write(new ByteBuffer[]{b1, b2, b3});} catch (IOException e) {}}
}

运行代码输出到words2.txt文件中。

6. 练习(黏包和半包问题) ★

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

import java.nio.ByteBuffer;
import static cn.itcast.nio.c2.ByteBufferUtil.debugAll;
public class Test08ByteBufferExam {public static void main(String[] args) {/*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\nI'm zhangsan\nHow are you?\n变成了下面的两个 byteBuffer (黏包，半包)Hello,world\nI'm zhangsan\nHow 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*/ByteBuffer source = ByteBuffer.allocate(32);source.put("Hello,world\nI'm zhangsan\nHo".getBytes());split(source);source.put("w are you?\n".getBytes());split(source);}private static void split(ByteBuffer source) {source.flip();for (int i = 0; i < source.limit(); i++) {// 找到一条完整消息if (source.get(i) == '\n') {int length = i + 1 - source.position();// 把这条完整消息存入新的 ByteBufferByteBuffer target = ByteBuffer.allocate(length);// 从 source 读，向 target 写for (int j = 0; j < length; j++) {target.put(source.get());}System.out.println("---------------↓输出的数据内容↓-----------");ByteBufferUtil.debugAll(result);System.out.println("---------------↓传递的数据内容↓-----------");ByteBufferUtil.debugAll(buffer);System.out.println("----------------------------------------------------");}}source.compact();}
}

输出

---------------↓输出的数据内容↓-----------+--------+-------------------- all ------------------------+----------------+
position: [12], limit: [12]+-------------------------------------------------+|  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 48 65 6c 6c 6f 2c 77 6f 72 6c 64 0a             |Hello,world.    |
+--------+-------------------------------------------------+----------------+
---------------↓传递的数据内容↓-----------
+--------+-------------------- all ------------------------+----------------+
position: [12], limit: [27]+-------------------------------------------------+|  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 48 65 6c 6c 6f 2c 77 6f 72 6c 64 0a 49 27 6d 20 |Hello,world.I'm |
|00000010| 7a 68 61 6e 67 73 61 6e 0a 48 6f 00 00 00 00 00 |zhangsan.Ho.....|
+--------+-------------------------------------------------+----------------+
----------------------------------------------------
---------------↓输出的数据内容↓-----------
+--------+-------------------- all ------------------------+----------------+
position: [13], limit: [13]+-------------------------------------------------+|  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 49 27 6d 20 7a 68 61 6e 67 73 61 6e 0a          |I'm zhangsan.   |
+--------+-------------------------------------------------+----------------+
---------------↓传递的数据内容↓-----------
+--------+-------------------- all ------------------------+----------------+
position: [25], limit: [27]+-------------------------------------------------+|  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 48 65 6c 6c 6f 2c 77 6f 72 6c 64 0a 49 27 6d 20 |Hello,world.I'm |
|00000010| 7a 68 61 6e 67 73 61 6e 0a 48 6f 00 00 00 00 00 |zhangsan.Ho.....|
+--------+-------------------------------------------------+----------------+
----------------------------------------------------
---------------↓输出的数据内容↓-----------
+--------+-------------------- all ------------------------+----------------+
position: [13], limit: [13]+-------------------------------------------------+|  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 48 6f 77 20 61 72 65 20 79 6f 75 3f 0a          |How are you?.   |
+--------+-------------------------------------------------+----------------+
---------------↓传递的数据内容↓-----------
+--------+-------------------- all ------------------------+----------------+
position: [13], limit: [13]+-------------------------------------------------+|  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 48 6f 77 20 61 72 65 20 79 6f 75 3f 0a 27 6d 20 |How are you?.'m |
|00000010| 7a 68 61 6e 67 73 61 6e 0a 48 6f 00 00 00 00 00 |zhangsan.Ho.....|
+--------+-------------------------------------------------+----------------+
----------------------------------------------------

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