04.NIO之bytebuffer常见方法演示

Leefs 2022-05-25 PM 1278℃ 0条

[TOC]

#### 1. 分配内存空间

可以使用allocate() 和 allocateDirect()方法为ByteBuffer分配空间，其他buffer类也有该方法

+ **allocate()**: 使用的是java的堆内存，堆内字节缓冲区，读写效率低，会受到GC的影响
+ **allocateDirect()**:使用的是直接内存，直接内存字节缓冲区，读写效率高（零拷贝），不会受GC影响,因为是系统直接内存，所以分配内存要调用操作系统函数，所以分配内存的速度较慢，如果使用不当（资源没得到合理释放），会造成内存泄漏。

```java
import java.nio.ByteBuffer;

/**
 * Created by lilinchao
 * Date 2022/5/25
 * Description 分配空间
 */
public class ByteBufferAllocateDemo {
    public static void main(String[] args) {
        System.out.println(ByteBuffer.allocate(16).getClass());
        System.out.println(ByteBuffer.allocateDirect(16).getClass());
    }
}
```

**输出结果**

```basic
class java.nio.HeapByteBuffer
class java.nio.DirectByteBuffer
```

#### 2. 向buffer写入数据

有两种办法

* 调用 channel 的 read 方法

```java
  int readBytes = channel.read(buf);
  ```

* 调用 buffer 自己的 put 方法

````java
  buf.put((byte)127);
  ````

#### 3. 从buffer读取数据

同样有两种办法

+ 调用 channel 的 write 方法

```java
  int writeBytes = channel.write(buf);
  ```

+ 调用 buffer 自己的 get 方法

```java
  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
* 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

**读写示例**

```java
import java.nio.ByteBuffer;

import static com.lilinchao.nio.bytebuffer_2.ByteBufferUtil.debugAll;

/**
 * Created by lilinchao
 * Date 2022/5/25
 * Description bytebuffer读写示例
 */
public class TestByteBufferReadWrite {
    public static void main(String[] args) {
        ByteBuffer buffer = ByteBuffer.allocate(10);
        buffer.put((byte) 0x61);    //a
        debugAll(buffer);
        buffer.put(new byte[]{0x62,0x63,0x64}); //b、c、d
        debugAll(buffer);
        //flip:切换对缓冲区的操作模式 写模式 --> 读模式
        buffer.flip();
        System.out.println(buffer.get());
        //get(i):不会改变索引的位置
        System.out.println(buffer.get(2));
        debugAll(buffer);
        //compact:会把未读完的数据向前压缩，然后切换到写模式
        buffer.compact();
        debugAll(buffer);
        buffer.put(new byte[]{0x65,0x6f});
        debugAll(buffer);
        // rewind 从头开始读
        buffer.flip();
        System.out.println((char)buffer.get());
        System.out.println((char)buffer.get());
        buffer.rewind();
        System.out.println((char)buffer.get());
    }
}
```

**运行结果**

```
+--------+-------------------- 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......      |
+--------+-------------------------------------------------+----------------+
97
99
+--------+-------------------- all ------------------------+----------------+
position: [1], 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......      |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [3], limit: [10]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 62 63 64 64 00 00 00 00 00 00                   |bcdd......      |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [5], limit: [10]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 62 63 64 65 6f 00 00 00 00 00                   |bcdeo.....      |
+--------+-------------------------------------------------+----------------+
b
c
b
```

#### 4. 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

```java
import java.nio.ByteBuffer;

/**
 * Created by lilinchao
 * Date 2022/5/25
 * Description mark 和 reset方法
 */
public class TestByteBufferMarkAndReset {
    public static void main(String[] args) {
        ByteBuffer byteBuffer = ByteBuffer.allocate(10);
        byteBuffer.put(new byte[]{'a', 'b', 'c', 'd'});
        byteBuffer.flip();

System.out.println((char) byteBuffer.get()); // 读取 a
        System.out.println((char) byteBuffer.get()); // 读取 b
        byteBuffer.mark(); // 加标记  索引为2 的位置
        System.out.println((char) byteBuffer.get()); // 读取 c
        System.out.println((char) byteBuffer.get()); // 读取 d
        byteBuffer.reset(); // 将position 重置到索引为2的位置
        System.out.println((char) byteBuffer.get()); // 读取 c
        System.out.println((char) byteBuffer.get()); // 读取 d
    }
}
```

**运行结果**

```
a
b
c
d
c
d
```

#### 5. 字符串与 ByteBuffer 互转

```java
ByteBuffer buffer1 = StandardCharsets.UTF_8.encode("你好");
ByteBuffer buffer2 = Charset.forName("utf-8").encode("你好");

debug(buffer1);
debug(buffer2);

CharBuffer buffer3 = StandardCharsets.UTF_8.decode(buffer1);
System.out.println(buffer3.getClass());
System.out.println(buffer3.toString());
```

**输出结果**

```
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| e4 bd a0 e5 a5 bd                               |......          |
+--------+-------------------------------------------------+----------------+
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| e4 bd a0 e5 a5 bd                               |......          |
+--------+-------------------------------------------------+----------------+
class java.nio.HeapCharBuffer
你好
```

**示例**

```java
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.StandardCharsets;

import static com.lilinchao.nio.bytebuffer_2.ByteBufferUtil.debugAll;

/**
 * Created by lilinchao
 * Date 2022/5/25
 * Description 字符串与 ByteBuffer 互转 Demo
 */
public class TestByteBufferString {
    public static void main(String[] args) {
        //1、字符串转为 ByteBuffer，还是写模式
        ByteBuffer buffer = ByteBuffer.allocate(16);
        buffer.put("hello".getBytes());
        debugAll(buffer);

// 2、Charset 字符集类，自动切换到读模式
        ByteBuffer helloBuffer = StandardCharsets.UTF_8.encode("hello");
        debugAll(helloBuffer);

// 3、wrap，自动切换到读模式
        ByteBuffer buffer3 = ByteBuffer.wrap("hello".getBytes());
        debugAll(buffer3);
        // 4、转换为字符串
        CharBuffer charBuffer = StandardCharsets.UTF_8.decode(helloBuffer);
        System.out.println(charBuffer.toString());

// 不起作用
        buffer.flip(); // 切换读模式，起作用
        CharBuffer charBuffer1 = StandardCharsets.UTF_8.decode(buffer);
        System.out.println(charBuffer1.toString());
    }
}
```

**运行结果**

```
+--------+-------------------- 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
```

#### 6. Scattering Reads（分散读）

分散读取集中写的方法不重要，重要的是思想，可以减少在ByteBuffer之间的拷贝，减少数据的复制次数，提高效率。

**示例**

+ 创建一个文本文件words.txt

```
onetwothree
```

+ 分散读取示例代码

```java
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

import static com.lilinchao.nio.bytebuffer_2.ByteBufferUtil.debugAll;

/**
 * Created by lilinchao
 * Date 2022/5/25
 * Description 分散读 Demo
 */
public class ScatteringReadsDemo {
    public static void main(String[] args) {

try(FileChannel channel = new RandomAccessFile("datas/words.txt", "r").getChannel()) {
            ByteBuffer b1 = ByteBuffer.allocate(3); //  one
            ByteBuffer b2 = ByteBuffer.allocate(3); //  two
            ByteBuffer b3 = ByteBuffer.allocate(5); //  three
            channel.read(new ByteBuffer[]{b1, b2, b3});
            b1.flip();
            b2.flip();
            b3.flip();
            debugAll(b1);
            debugAll(b2);
            debugAll(b3);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}
```

**运行结果**

```
+--------+-------------------- 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           |
+--------+-------------------------------------------------+----------------+
```

#### 7. Gathering Writes（集中写）

使用如下方式写入，可以将多个 buffer 的数据填充至 channel

```java
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

import static com.lilinchao.nio.bytebuffer_2.ByteBufferUtil.debugAll;

/**
 * Created by lilinchao
 * Date 2022/5/25
 * Description 集中写Demo
 */
public class GatheringWritesDemo {
    public static void main(String[] args) {
        try (RandomAccessFile file = new RandomAccessFile("datas/words.txt", "rw")) {
            FileChannel channel = file.getChannel();
            ByteBuffer d = ByteBuffer.allocate(4);
            ByteBuffer e = ByteBuffer.allocate(4);
            channel.position(11);

d.put(new byte[]{'f', 'o', 'u', 'r'});
            e.put(new byte[]{'f', 'i', 'v', 'e'});
            d.flip();
            e.flip();
            debugAll(d);
            debugAll(e);
            channel.write(new ByteBuffer[]{d, e});
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}
```

**运行结果**

```
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [4]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 66 6f 75 72                                     |four            |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [4]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 66 69 76 65                                     |five            |
+--------+-------------------------------------------------+----------------+
```

#### 最后：调试工具类

```java
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;

/**
 * @Author: lilinchao
 * @Date: 2022/5/25
 * @Description: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 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);
    }
}
```

标签: Netty, NIO

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04.NIO之bytebuffer常见方法演示

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