NIO基础

NIO：non- blocking io非阻塞

1.三大组件

1.1 Channel & Buffer

channel（通道的意思）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（文件传输通道）
• DatagramChannel（UDP传输通道）
• Socke Channel（TCP传输通道）
• ServersocketChannel（TCP传输通道）

buffer则用来缓冲读写数据，常见的 buffer有

• Bytebuffer

  • MappedbyteBuffer
  • Directbytebuffero
  • HeapbyteBuffer
• ShortBuffer
• IntBuffer
• Longbuffer
• FloatBuffer
• DoubleBuffer

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

线程池设计

• 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 I的 select0会阻塞直到 channel发生了读写就緒事件，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

1.ByteBuffer代码

package com.gaoxu.netty;

import lombok.extern.slf4j.Slf4j;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
@Slf4j
public class TestBuffer {
    public static void main(String[] args) throws FileNotFoundException {
        //FileChannel
        //1.输入输出流，2.RandomAccessFile
        try (FileChannel channel = new FileInputStream("data.txt").getChannel()) {
            //准备缓冲区
            ByteBuffer buffer = ByteBuffer.allocate(10);

            while (true) {
                //从channel读取文件，向buffer写入
                int len = channel.read(buffer);
                log.debug("读取的字节：{}",len);
                if (len<1){
                    break;
                }
                //打印buffer内容
                buffer.flip();//切换buffer的读模式

                while (buffer.hasRemaining()) {//buffer.hasRemaining()判断是否有剩余
                    byte b = buffer.get();//获取一个字节
                    System.out.print((char) b);
                    log.debug("读取的字节：{}",(char) b);
                }
                buffer.clear();//切换为写模式
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

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

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

3.ByteBuffer常用方法

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);

package com.gaoxu.netty;

import java.nio.ByteBuffer;

public class Testbytebufferallocate {
    public static void main(String[] args) {
       /*
            1.class java nio. Heapbytebuffer
                java堆内存，读写效率较低，受到GC的影响
            2.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
          */
        System.out.println(ByteBuffer.allocate(16));
        System.out.println(ByteBuffer.allocateDirect(16));
    }
}

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(i)方法不会改变指针位置
  byte c = buf.get(i);
  

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

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

注意

rewind 和 flip 都会清除 mark 位置

bytebuffer工具类

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);
    }
}

测试类

package com.gaoxu.netty;
import java.nio.ByteBuffer;
import static com.gaoxu.netty.util.ByteBufferUtil.debugAll;
public class Main {
    public static void main(String[] args) {
        ByteBuffer buffer = ByteBuffer.allocate(10);
        buffer.put((byte) 0x61);
        buffer.put(new byte[]{0x62,0x63,0x64});
        debugAll(buffer);
        buffer.flip();
        System.out.println(buffer.get());
        debugAll(buffer);
        buffer.compact();
        debugAll(buffer);
        buffer.put(new byte[]{0x65,0x6a});
        debugAll(buffer);

    }
}

运行结果

2021-11-04 01:41:15,049 DEBUG [io.netty.util.internal.logging.InternalLoggerFactory] - Using SLF4J as the default logging framework
+--------+-------------------- 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
+--------+-------------------- 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 6a 00 00 00 00 00                   |bcdej.....      |
+--------+-------------------------------------------------+----------------+

Process finished with exit code 0