Netty01-nio

2021-05-18 11:17:30 浏览数 (1)

一. NIO 基础

non-blocking io 非阻塞 IO

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

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channel

buffer

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

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多线程版

socket1

thread

socket2

thread

socket3

thread

⚠️ 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

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线程池版

socket1

thread

socket2

thread

socket3

socket4

⚠️ 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）

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selector 版

selector

thread

channel

channel

channel

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

代码语言：javascript复制
1234567890abcd

使用 FileChannel 来读取文件内容

代码语言：javascript复制
@Slf4j
public class ChannelDemo1 {
    public static void main(String[] args) {
        try (RandomAccessFile file = new RandomAccessFile("helloword/data.txt", "rw")) {
            FileChannel channel = file.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()) {
                    log.debug("{}", (char)buffer.get());
                }
                // 切换 buffer 写模式
                buffer.clear();
            } while (true);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

输出

代码语言：javascript复制
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 读到字节数：10
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 1
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 2
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 3
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 4
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 5
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 6
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 7
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 8
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 9
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 0
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 读到字节数：4
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - a
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - b
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - c
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - d
10:39:03 [DEBUG] [main] c.i.n.ChannelDemo1 - 读到字节数：-1

2.1 ByteBuffer 正确使用姿势

  1. 向 buffer 写入数据，例如调用 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

2.2 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
代码语言：javascript复制
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);
    }
}

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

代码语言：javascript复制
Bytebuffer buf = ByteBuffer.allocate(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
代码语言：javascript复制
int readBytes = channel.read(buf);

和

代码语言：javascript复制
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
代码语言：javascript复制
int writeBytes = channel.write(buf);

和

代码语言：javascript复制
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
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

字符串与 ByteBuffer 互转
代码语言：javascript复制
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

代码语言：javascript复制
          ------------------------------------------------- 
         |  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
⚠️ 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 3parts.txt

代码语言：javascript复制
onetwothree

使用如下方式读取，可以将数据填充至多个 buffer

代码语言：javascript复制
try (RandomAccessFile file = new RandomAccessFile("helloword/3parts.txt", "rw")) {
    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();
    debug(a);
    debug(b);
    debug(c);
} catch (IOException e) {
    e.printStackTrace();
}

结果

代码语言：javascript复制
          ------------------------------------------------- 
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
 -------- ------------------------------------------------- ---------------- 
|00000000| 6f 6e 65                                        |one             |
 -------- ------------------------------------------------- ---------------- 
          ------------------------------------------------- 
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
 -------- ------------------------------------------------- ---------------- 
|00000000| 74 77 6f                                        |two             |
 -------- ------------------------------------------------- ---------------- 
          ------------------------------------------------- 
         |  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

代码语言：javascript复制
try (RandomAccessFile file = new RandomAccessFile("helloword/3parts.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();
    debug(d);
    debug(e);
    channel.write(new ByteBuffer[]{d, e});
} catch (IOException e) {
    e.printStackTrace();
}

输出

代码语言：javascript复制
          ------------------------------------------------- 
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
 -------- ------------------------------------------------- ---------------- 
|00000000| 66 6f 75 72                                     |four            |
 -------- ------------------------------------------------- ---------------- 
          ------------------------------------------------- 
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
 -------- ------------------------------------------------- ---------------- 
|00000000| 66 69 76 65                                     |five            |
 -------- ------------------------------------------------- ----------------

文件内容

代码语言：javascript复制
onetwothreefourfive

2.6 练习

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,worldn
  • I’m zhangsann
  • How are you?n

变成了下面的两个 byteBuffer (黏包，半包)

  • Hello,worldnI’m zhangsannHo
  • 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

代码语言：javascript复制
public static void main(String[] args) {
    ByteBuffer source = ByteBuffer.allocate(32);
    //                     11            24
    source.put("Hello,worldnI'm zhangsannHo".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();
}

3. 文件编程

3.1 FileChannel

⚠️ FileChannel 工作模式

FileChannel 只能工作在阻塞模式下

获取

不能直接打开 FileChannel，必须通过 FileInputStream、FileOutputStream 或者 RandomAccessFile 来获取 FileChannel，它们都有 getChannel 方法

  • 通过 FileInputStream 获取的 channel 只能读
  • 通过 FileOutputStream 获取的 channel 只能写
  • 通过 RandomAccessFile 是否能读写根据构造 RandomAccessFile 时的读写模式决定
读取

会从 channel 读取数据填充 ByteBuffer，返回值表示读到了多少字节，-1 表示到达了文件的末尾

代码语言：javascript复制
int readBytes = channel.read(buffer);
写入

写入的正确姿势如下， SocketChannel

代码语言：javascript复制
ByteBuffer buffer = ...;
buffer.put(...); // 存入数据
buffer.flip();   // 切换读模式

while(buffer.hasRemaining()) {
    channel.write(buffer);
}

在 while 中调用 channel.write 是因为 write 方法并不能保证一次将 buffer 中的内容全部写入 channel

关闭

channel 必须关闭，不过调用了 FileInputStream、FileOutputStream 或者 RandomAccessFile 的 close 方法会间接地调用 channel 的 close 方法

位置

获取当前位置

代码语言：javascript复制
long pos = channel.position();

设置当前位置

代码语言：javascript复制
long newPos = ...;
channel.position(newPos);

设置当前位置时，如果设置为文件的末尾

  • 这时读取会返回 -1
  • 这时写入，会追加内容，但要注意如果 position 超过了文件末尾，再写入时在新内容和原末尾之间会有空洞（00）
大小

使用 size 方法获取文件的大小

强制写入

操作系统出于性能的考虑，会将数据缓存，不是立刻写入磁盘。可以调用 force(true) 方法将文件内容和元数据（文件的权限等信息）立刻写入磁盘

3.2 两个 Channel 传输数据

代码语言：javascript复制
String FROM = "helloword/data.txt";
String TO = "helloword/to.txt";
long start = System.nanoTime();
try (FileChannel from = new FileInputStream(FROM).getChannel();
     FileChannel to = new FileOutputStream(TO).getChannel();
    ) {
    from.transferTo(0, from.size(), to);
} catch (IOException e) {
    e.printStackTrace();
}
long end = System.nanoTime();
System.out.println("transferTo 用时："   (end - start) / 1000_000.0);

输出

代码语言：javascript复制
transferTo 用时：8.2011

超过 2g 大小的文件传输

代码语言：javascript复制
public class TestFileChannelTransferTo {
    public static void main(String[] args) {
        try (
                FileChannel from = new FileInputStream("data.txt").getChannel();
                FileChannel to = new FileOutputStream("to.txt").getChannel();
        ) {
            // 效率高，底层会利用操作系统的零拷贝进行优化
            long size = from.size();
            // left 变量代表还剩余多少字节
            for (long left = size; left > 0; ) {
                System.out.println("position:"   (size - left)   " left:"   left);
                left -= from.transferTo((size - left), left, to);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

实际传输一个超大文件

代码语言：javascript复制
position:0 left:7769948160
position:2147483647 left:5622464513
position:4294967294 left:3474980866
position:6442450941 left:1327497219

3.3 Path

jdk7 引入了 Path 和 Paths 类

  • Path 用来表示文件路径
  • Paths 是工具类，用来获取 Path 实例
代码语言：javascript复制
Path source = Paths.get("1.txt"); // 相对路径 使用 user.dir 环境变量来定位 1.txt

Path source = Paths.get("d:\1.txt"); // 绝对路径 代表了  d:1.txt

Path source = Paths.get("d:/1.txt"); // 绝对路径 同样代表了  d:1.txt

Path projects = Paths.get("d:\data", "projects"); // 代表了  d:dataprojects
  • . 代表了当前路径
  • .. 代表了上一级路径

例如目录结构如下

代码语言：javascript复制
d:
	|- data
		|- projects
			|- a
			|- b

代码

代码语言：javascript复制
Path path = Paths.get("d:\data\projects\a\..\b");
System.out.println(path);
System.out.println(path.normalize()); // 正常化路径

会输出

代码语言：javascript复制
d:dataprojectsa..b
d:dataprojectsb

3.4 Files

检查文件是否存在

代码语言：javascript复制
Path path = Paths.get("helloword/data.txt");
System.out.println(Files.exists(path));

创建一级目录

代码语言：javascript复制
Path path = Paths.get("helloword/d1");
Files.createDirectory(path);
  • 如果目录已存在，会抛异常 FileAlreadyExistsException
  • 不能一次创建多级目录，否则会抛异常 NoSuchFileException

创建多级目录用

代码语言：javascript复制
Path path = Paths.get("helloword/d1/d2");
Files.createDirectories(path);

拷贝文件

代码语言：javascript复制
Path source = Paths.get("helloword/data.txt");
Path target = Paths.get("helloword/target.txt");

Files.copy(source, target);
  • 如果文件已存在，会抛异常 FileAlreadyExistsException

如果希望用 source 覆盖掉 target，需要用 StandardCopyOption 来控制

代码语言：javascript复制
Files.copy(source, target, StandardCopyOption.REPLACE_EXISTING);

移动文件

代码语言：javascript复制
Path source = Paths.get("helloword/data.txt");
Path target = Paths.get("helloword/data.txt");

Files.move(source, target, StandardCopyOption.ATOMIC_MOVE);
  • StandardCopyOption.ATOMIC_MOVE 保证文件移动的原子性

删除文件

代码语言：javascript复制
Path target = Paths.get("helloword/target.txt");

Files.delete(target);
  • 如果文件不存在，会抛异常 NoSuchFileException

删除目录

代码语言：javascript复制
Path target = Paths.get("helloword/d1");

Files.delete(target);
  • 如果目录还有内容，会抛异常 DirectoryNotEmptyException

遍历目录文件

代码语言：javascript复制
public static void main(String[] args) throws IOException {
    Path path = Paths.get("C:\Program Files\Java\jdk1.8.0_91");
    AtomicInteger dirCount = new AtomicInteger();
    AtomicInteger fileCount = new AtomicInteger();
    Files.walkFileTree(path, new SimpleFileVisitor<Path>(){
        @Override
        public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) 
            throws IOException {
            System.out.println(dir);
            dirCount.incrementAndGet();
            return super.preVisitDirectory(dir, attrs);
        }

        @Override
        public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) 
            throws IOException {
            System.out.println(file);
            fileCount.incrementAndGet();
            return super.visitFile(file, attrs);
        }
    });
    System.out.println(dirCount); // 133
    System.out.println(fileCount); // 1479
}

统计 jar 的数目

代码语言：javascript复制
Path path = Paths.get("C:\Program Files\Java\jdk1.8.0_91");
AtomicInteger fileCount = new AtomicInteger();
Files.walkFileTree(path, new SimpleFileVisitor<Path>(){
    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) 
        throws IOException {
        if (file.toFile().getName().endsWith(".jar")) {
            fileCount.incrementAndGet();
        }
        return super.visitFile(file, attrs);
    }
});
System.out.println(fileCount); // 724

删除多级目录

代码语言：javascript复制
Path path = Paths.get("d:\a");
Files.walkFileTree(path, new SimpleFileVisitor<Path>(){
    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) 
        throws IOException {
        Files.delete(file);
        return super.visitFile(file, attrs);
    }

    @Override
    public FileVisitResult postVisitDirectory(Path dir, IOException exc) 
        throws IOException {
        Files.delete(dir);
        return super.postVisitDirectory(dir, exc);
    }
});
⚠️ 删除很危险

删除是危险操作，确保要递归删除的文件夹没有重要内容

拷贝多级目录

代码语言：javascript复制
long start = System.currentTimeMillis();
String source = "D:\Snipaste-1.16.2-x64";
String target = "D:\Snipaste-1.16.2-x64aaa";

Files.walk(Paths.get(source)).forEach(path -> {
    try {
        String targetName = path.toString().replace(source, target);
        // 是目录
        if (Files.isDirectory(path)) {
            Files.createDirectory(Paths.get(targetName));
        }
        // 是普通文件
        else if (Files.isRegularFile(path)) {
            Files.copy(path, Paths.get(targetName));
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
});
long end = System.currentTimeMillis();
System.out.println(end - start);

4. 网络编程

4.1 非阻塞 vs 阻塞

阻塞
  • 阻塞模式下，相关方法都会导致线程暂停
    • ServerSocketChannel.accept 会在没有连接建立时让线程暂停
    • SocketChannel.read 会在没有数据可读时让线程暂停
    • 阻塞的表现其实就是线程暂停了，暂停期间不会占用 cpu，但线程相当于闲置
  • 单线程下，阻塞方法之间相互影响，几乎不能正常工作，需要多线程支持
  • 但多线程下，有新的问题，体现在以下方面
    • 32 位 jvm 一个线程 320k，64 位 jvm 一个线程 1024k，如果连接数过多，必然导致 OOM，并且线程太多，反而会因为频繁上下文切换导致性能降低
    • 可以采用线程池技术来减少线程数和线程上下文切换，但治标不治本，如果有很多连接建立，但长时间 inactive，会阻塞线程池中所有线程，因此不适合长连接，只适合短连接

服务器端

代码语言：javascript复制
// 使用 nio 来理解阻塞模式, 单线程
// 0. ByteBuffer
ByteBuffer buffer = ByteBuffer.allocate(16);
// 1. 创建了服务器
ServerSocketChannel ssc = ServerSocketChannel.open();

// 2. 绑定监听端口
ssc.bind(new InetSocketAddress(8080));

// 3. 连接集合
List<SocketChannel> channels = new ArrayList<>();
while (true) {
    // 4. accept 建立与客户端连接， SocketChannel 用来与客户端之间通信
    log.debug("connecting...");
    SocketChannel sc = ssc.accept(); // 阻塞方法，线程停止运行
    log.debug("connected... {}", sc);
    channels.add(sc);
    for (SocketChannel channel : channels) {
        // 5. 接收客户端发送的数据
        log.debug("before read... {}", channel);
        channel.read(buffer); // 阻塞方法，线程停止运行
        buffer.flip();
        debugRead(buffer);
        buffer.clear();
        log.debug("after read...{}", channel);
    }
}

客户端

代码语言：javascript复制
SocketChannel sc = SocketChannel.open();
sc.connect(new InetSocketAddress("localhost", 8080));
System.out.println("waiting...");
非阻塞
  • 非阻塞模式下，相关方法都会不会让线程暂停
    • 在 ServerSocketChannel.accept 在没有连接建立时，会返回 null，继续运行
    • SocketChannel.read 在没有数据可读时，会返回 0，但线程不必阻塞，可以去执行其它 SocketChannel 的 read 或是去执行 ServerSocketChannel.accept
    • 写数据时，线程只是等待数据写入 Channel 即可，无需等 Channel 通过网络把数据发送出去
  • 但非阻塞模式下，即使没有连接建立，和可读数据，线程仍然在不断运行，白白浪费了 cpu
  • 数据复制过程中，线程实际还是阻塞的（AIO 改进的地方）

服务器端，客户端代码不变

代码语言：javascript复制
// 使用 nio 来理解非阻塞模式, 单线程
// 0. ByteBuffer
ByteBuffer buffer = ByteBuffer.allocate(16);
// 1. 创建了服务器
ServerSocketChannel ssc = ServerSocketChannel.open();
ssc.configureBlocking(false); // 非阻塞模式
// 2. 绑定监听端口
ssc.bind(new InetSocketAddress(8080));
// 3. 连接集合
List<SocketChannel> channels = new ArrayList<>();
while (true) {
    // 4. accept 建立与客户端连接， SocketChannel 用来与客户端之间通信
    SocketChannel sc = ssc.accept(); // 非阻塞，线程还会继续运行，如果没有连接建立，但sc是null
    if (sc != null) {
        log.debug("connected... {}", sc);
        sc.configureBlocking(false); // 非阻塞模式
        channels.add(sc);
    }
    for (SocketChannel channel : channels) {
        // 5. 接收客户端发送的数据
        int read = channel.read(buffer);// 非阻塞，线程仍然会继续运行，如果没有读到数据，read 返回 0
        if (read > 0) {
            buffer.flip();
            debugRead(buffer);
            buffer.clear();
            log.debug("after read...{}", channel);
        }
    }
}
多路复用

单线程可以配合 Selector 完成对多个 Channel 可读写事件的监控，这称之为多路复用

  • 多路复用仅针对网络 IO、普通文件 IO 没法利用多路复用
  • 如果不用 Selector 的非阻塞模式，线程大部分时间都在做无用功，而 Selector 能够保证
    • 有可连接事件时才去连接
    • 有可读事件才去读取
    • 有可写事件才去写入
      • 限于网络传输能力，Channel 未必时时可写，一旦 Channel 可写，会触发 Selector 的可写事件

4.2 Selector

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selector 版

selector

thread

channel

channel

channel

好处

  • 一个线程配合 selector 就可以监控多个 channel 的事件，事件发生线程才去处理。避免非阻塞模式下所做无用功
  • 让这个线程能够被充分利用
  • 节约了线程的数量
  • 减少了线程上下文切换
创建
代码语言：javascript复制
Selector selector = Selector.open();
绑定 Channel 事件

也称之为注册事件，绑定的事件 selector 才会关心

代码语言：javascript复制
channel.configureBlocking(false);
SelectionKey key = channel.register(selector, 绑定事件);
  • channel 必须工作在非阻塞模式
  • FileChannel 没有非阻塞模式，因此不能配合 selector 一起使用
  • 绑定的事件类型可以有
    • connect - 客户端连接成功时触发
    • accept - 服务器端成功接受连接时触发
    • read - 数据可读入时触发，有因为接收能力弱，数据暂不能读入的情况
    • write - 数据可写出时触发，有因为发送能力弱，数据暂不能写出的情况
监听 Channel 事件

可以通过下面三种方法来监听是否有事件发生，方法的返回值代表有多少 channel 发生了事件

方法1，阻塞直到绑定事件发生

代码语言：javascript复制
int count = selector.select();

方法2，阻塞直到绑定事件发生，或是超时（时间单位为 ms）

代码语言：javascript复制
int count = selector.select(long timeout);

方法3，不会阻塞，也就是不管有没有事件，立刻返回，自己根据返回值检查是否有事件

代码语言：javascript复制
int count = selector.selectNow();
? select 何时不阻塞

  • 事件发生时
    • 客户端发起连接请求，会触发 accept 事件
    • 客户端发送数据过来，客户端正常、异常关闭时，都会触发 read 事件，另外如果发送的数据大于 buffer 缓冲区，会触发多次读取事件
    • channel 可写，会触发 write 事件
    • 在 linux 下 nio bug 发生时
  • 调用 selector.wakeup()
  • 调用 selector.close()
  • selector 所在线程 interrupt

4.3 处理 accept 事件

客户端代码为

代码语言：javascript复制
public class Client {
    public static void main(String[] args) {
        try (Socket socket = new Socket("localhost", 8080)) {
            System.out.println(socket);
            socket.getOutputStream().write("world".getBytes());
            System.in.read();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

服务器端代码为

代码语言：javascript复制
@Slf4j
public class ChannelDemo6 {
    public static void main(String[] args) {
        try (ServerSocketChannel channel = ServerSocketChannel.open()) {
            channel.bind(new InetSocketAddress(8080));
            System.out.println(channel);
            Selector selector = Selector.open();
            channel.configureBlocking(false);
            channel.register(selector, SelectionKey.OP_ACCEPT);

            while (true) {
                int count = selector.select();
//                int count = selector.selectNow();
                log.debug("select count: {}", count);
//                if(count <= 0) {
//                    continue;
//                }

                // 获取所有事件
                Set<SelectionKey> keys = selector.selectedKeys();

                // 遍历所有事件，逐一处理
                Iterator<SelectionKey> iter = keys.iterator();
                while (iter.hasNext()) {
                    SelectionKey key = iter.next();
                    // 判断事件类型
                    if (key.isAcceptable()) {
                        ServerSocketChannel c = (ServerSocketChannel) key.channel();
                        // 必须处理
                        SocketChannel sc = c.accept();
                        log.debug("{}", sc);
                    }
                    // 处理完毕，必须将事件移除
                    iter.remove();
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}
? 事件发生后能否不处理

事件发生后，要么处理，要么取消（cancel），不能什么都不做，否则下次该事件仍会触发，这是因为 nio 底层使用的是水平触发

4.4 处理 read 事件

代码语言：javascript复制
@Slf4j
public class ChannelDemo6 {
    public static void main(String[] args) {
        try (ServerSocketChannel channel = ServerSocketChannel.open()) {
            channel.bind(new InetSocketAddress(8080));
            System.out.println(channel);
            Selector selector = Selector.open();
            channel.configureBlocking(false);
            channel.register(selector, SelectionKey.OP_ACCEPT);

            while (true) {
                int count = selector.select();
//                int count = selector.selectNow();
                log.debug("select count: {}", count);
//                if(count <= 0) {
//                    continue;
//                }

                // 获取所有事件
                Set<SelectionKey> keys = selector.selectedKeys();

                // 遍历所有事件，逐一处理
                Iterator<SelectionKey> iter = keys.iterator();
                while (iter.hasNext()) {
                    SelectionKey key = iter.next();
                    // 判断事件类型
                    if (key.isAcceptable()) {
                        ServerSocketChannel c = (ServerSocketChannel) key.channel();
                        // 必须处理
                        SocketChannel sc = c.accept();
                        sc.configureBlocking(false);
                        sc.register(selector, SelectionKey.OP_READ);
                        log.debug("连接已建立: {}", sc);
                    } else if (key.isReadable()) {
                        SocketChannel sc = (SocketChannel) key.channel();
                        ByteBuffer buffer = ByteBuffer.allocate(128);
                        int read = sc.read(buffer);
                        if(read == -1) {
                            key.cancel();
                            sc.close();
                        } else {
                            buffer.flip();
                            debug(buffer);
                        }
                    }
                    // 处理完毕，必须将事件移除
                    iter.remove();
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

开启两个客户端，修改一下发送文字，输出

代码语言：javascript复制
sun.nio.ch.ServerSocketChannelImpl[/0:0:0:0:0:0:0:0:8080]
21:16:39 [DEBUG] [main] c.i.n.ChannelDemo6 - select count: 1
21:16:39 [DEBUG] [main] c.i.n.ChannelDemo6 - 连接已建立: java.nio.channels.SocketChannel[connected local=/127.0.0.1:8080 remote=/127.0.0.1:60367]
21:16:39 [DEBUG] [main] c.i.n.ChannelDemo6 - select count: 1
          ------------------------------------------------- 
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
 -------- ------------------------------------------------- ---------------- 
|00000000| 68 65 6c 6c 6f                                  |hello           |
 -------- ------------------------------------------------- ---------------- 
21:16:59 [DEBUG] [main] c.i.n.ChannelDemo6 - select count: 1
21:16:59 [DEBUG] [main] c.i.n.ChannelDemo6 - 连接已建立: java.nio.channels.SocketChannel[connected local=/127.0.0.1:8080 remote=/127.0.0.1:60378]
21:16:59 [DEBUG] [main] c.i.n.ChannelDemo6 - select count: 1
          ------------------------------------------------- 
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
 -------- ------------------------------------------------- ---------------- 
|00000000| 77 6f 72 6c 64                                  |world           |
 -------- ------------------------------------------------- ----------------
? 为何要 iter.remove()

因为 select 在事件发生后，就会将相关的 key 放入 selectedKeys 集合，但不会在处理完后从 selectedKeys 集合中移除，需要我们自己编码删除。例如

  • 第一次触发了 ssckey 上的 accept 事件，没有移除 ssckey
  • 第二次触发了 sckey 上的 read 事件，但这时 selectedKeys 中还有上次的 ssckey ，在处理时因为没有真正的 serverSocket 连上了，就会导致空指针异常
? cancel 的作用

cancel 会取消注册在 selector 上的 channel，并从 keys 集合中删除 key 后续不会再监听事件

⚠️ 不处理边界的问题

以前有同学写过这样的代码，思考注释中两个问题，以 bio 为例，其实 nio 道理是一样的

代码语言：javascript复制
public class Server {
    public static void main(String[] args) throws IOException {
        ServerSocket ss=new ServerSocket(9000);
        while (true) {
            Socket s = ss.accept();
            InputStream in = s.getInputStream();
            // 这里这么写，有没有问题
            byte[] arr = new byte[4];
            while(true) {
                int read = in.read(arr);
                // 这里这么写，有没有问题
                if(read == -1) {
                    break;
                }
                System.out.println(new String(arr, 0, read));
            }
        }
    }
}

客户端

代码语言：javascript复制
public class Client {
    public static void main(String[] args) throws IOException {
        Socket max = new Socket("localhost", 9000);
        OutputStream out = max.getOutputStream();
        out.write("hello".getBytes());
        out.write("world".getBytes());
        out.write("你好".getBytes());
        max.close();
    }
}

输出

代码语言：javascript复制
hell
owor
ld�
�好

为什么？

处理消息的边界
  • 一种思路是固定消息长度，数据包大小一样，服务器按预定长度读取，缺点是浪费带宽
  • 另一种思路是按分隔符拆分，缺点是效率低
  • TLV 格式，即 Type 类型、Length 长度、Value 数据，类型和长度已知的情况下，就可以方便获取消息大小，分配合适的 buffer，缺点是 buffer 需要提前分配，如果内容过大，则影响 server 吞吐量
    • Http 1.1 是 TLV 格式
    • Http 2.0 是 LTV 格式

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.statediagram-cluster.statediagram-cluster-alt .inner{fill:#e0e0e0}#mermaid-svg-zvyxjk8qjDbgExa2 .statediagram-cluster .inner{rx:0;ry:0}#mermaid-svg-zvyxjk8qjDbgExa2 .statediagram-state rect.basic{rx:5px;ry:5px}#mermaid-svg-zvyxjk8qjDbgExa2 .statediagram-state rect.divider{stroke-dasharray:10,10;fill:#efefef}#mermaid-svg-zvyxjk8qjDbgExa2 .note-edge{stroke-dasharray:5}#mermaid-svg-zvyxjk8qjDbgExa2 .statediagram-note rect{fill:#fff5ad;stroke:#aa3;stroke-width:1px;rx:0;ry:0}:root{--mermaid-font-family: '"trebuchet ms", verdana, arial';--mermaid-font-family: "Comic Sans MS", "Comic Sans", cursive}#mermaid-svg-zvyxjk8qjDbgExa2 .error-icon{fill:#522}#mermaid-svg-zvyxjk8qjDbgExa2 .error-text{fill:#522;stroke:#522}#mermaid-svg-zvyxjk8qjDbgExa2 .edge-thickness-normal{stroke-width:2px}#mermaid-svg-zvyxjk8qjDbgExa2 .edge-thickness-thick{stroke-width:3.5px}#mermaid-svg-zvyxjk8qjDbgExa2 .edge-pattern-solid{stroke-dasharray:0}#mermaid-svg-zvyxjk8qjDbgExa2 .edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-zvyxjk8qjDbgExa2 .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-zvyxjk8qjDbgExa2 .marker{fill:#333}#mermaid-svg-zvyxjk8qjDbgExa2 .marker.cross{stroke:#333} :root { --mermaid-font-family: "trebuchet ms", verdana, arial;} #mermaid-svg-zvyxjk8qjDbgExa2 { color: rgba(0, 0, 0, 0.75); font: ; } 客户端1 服务器 ByteBuffer1 ByteBuffer2 发送 01234567890abcdef3333r 第一次 read 存入 01234567890abcdef 扩容 拷贝 01234567890abcdef 第二次 read 存入 3333r 01234567890abcdef3333r 客户端1 服务器 ByteBuffer1 ByteBuffer2

服务器端

代码语言：javascript复制
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 ByteBuffer
            ByteBuffer target = ByteBuffer.allocate(length);
            // 从 source 读，向 target 写
            for (int j = 0; j < length; j  ) {
                target.put(source.get());
            }
            debugAll(target);
        }
    }
    source.compact(); // 0123456789abcdef  position 16 limit 16
}

public static void main(String[] args) throws IOException {
    // 1. 创建 selector, 管理多个 channel
    Selector selector = Selector.open();
    ServerSocketChannel ssc = ServerSocketChannel.open();
    ssc.configureBlocking(false);
    // 2. 建立 selector 和 channel 的联系（注册）
    // SelectionKey 就是将来事件发生后，通过它可以知道事件和哪个channel的事件
    SelectionKey sscKey = ssc.register(selector, 0, null);
    // key 只关注 accept 事件
    sscKey.interestOps(SelectionKey.OP_ACCEPT);
    log.debug("sscKey:{}", sscKey);
    ssc.bind(new InetSocketAddress(8080));
    while (true) {
        // 3. select 方法, 没有事件发生，线程阻塞，有事件，线程才会恢复运行
        // select 在事件未处理时，它不会阻塞, 事件发生后要么处理，要么取消，不能置之不理
        selector.select();
        // 4. 处理事件, selectedKeys 内部包含了所有发生的事件
        Iterator<SelectionKey> iter = selector.selectedKeys().iterator(); // accept, read
        while (iter.hasNext()) {
            SelectionKey key = iter.next();
            // 处理key 时，要从 selectedKeys 集合中删除，否则下次处理就会有问题
            iter.remove();
            log.debug("key: {}", key);
            // 5. 区分事件类型
            if (key.isAcceptable()) { // 如果是 accept
                ServerSocketChannel channel = (ServerSocketChannel) key.channel();
                SocketChannel sc = channel.accept();
                sc.configureBlocking(false);
                ByteBuffer buffer = ByteBuffer.allocate(16); // attachment
                // 将一个 byteBuffer 作为附件关联到 selectionKey 上
                SelectionKey scKey = sc.register(selector, 0, buffer);
                scKey.interestOps(SelectionKey.OP_READ);
                log.debug("{}", sc);
                log.debug("scKey:{}", scKey);
            } else if (key.isReadable()) { // 如果是 read
                try {
                    SocketChannel channel = (SocketChannel) key.channel(); // 拿到触发事件的channel
                    // 获取 selectionKey 上关联的附件
                    ByteBuffer buffer = (ByteBuffer) key.attachment();
                    int read = channel.read(buffer); // 如果是正常断开，read 的方法的返回值是 -1
                    if(read == -1) {
                        key.cancel();
                    } else {
                        split(buffer);
                        // 需要扩容
                        if (buffer.position() == buffer.limit()) {
                            ByteBuffer newBuffer = ByteBuffer.allocate(buffer.capacity() * 2);
                            buffer.flip();
                            newBuffer.put(buffer); // 0123456789abcdef3333n
                            key.attach(newBuffer);
                        }
                    }

                } catch (IOException e) {
                    e.printStackTrace();
                    key.cancel();  // 因为客户端断开了,因此需要将 key 取消（从 selector 的 keys 集合中真正删除 key）
                }
            }
        }
    }
}

客户端

代码语言：javascript复制
SocketChannel sc = SocketChannel.open();
sc.connect(new InetSocketAddress("localhost", 8080));
SocketAddress address = sc.getLocalAddress();
// sc.write(Charset.defaultCharset().encode("hellonworldn"));
sc.write(Charset.defaultCharset().encode("0123n456789abcdef"));
sc.write(Charset.defaultCharset().encode("0123456789abcdef3333n"));
System.in.read();
ByteBuffer 大小分配
  • 每个 channel 都需要记录可能被切分的消息，因为 ByteBuffer 不能被多个 channel 共同使用，因此需要为每个 channel 维护一个独立的 ByteBuffer
  • ByteBuffer 不能太大，比如一个 ByteBuffer 1Mb 的话，要支持百万连接就要 1Tb 内存，因此需要设计大小可变的 ByteBuffer
    • 一种思路是首先分配一个较小的 buffer，例如 4k，如果发现数据不够，再分配 8k 的 buffer，将 4k buffer 内容拷贝至 8k buffer，优点是消息连续容易处理，缺点是数据拷贝耗费性能，参考实现 http://tutorials.jenkov.com/java-performance/resizable-array.html
    • 另一种思路是用多个数组组成 buffer，一个数组不够，把多出来的内容写入新的数组，与前面的区别是消息存储不连续解析复杂，优点是避免了拷贝引起的性能损耗

4.5 处理 write 事件

一次无法写完例子
  • 非阻塞模式下，无法保证把 buffer 中所有数据都写入 channel，因此需要追踪 write 方法的返回值（代表实际写入字节数）
  • 用 selector 监听所有 channel 的可写事件，每个 channel 都需要一个 key 来跟踪 buffer，但这样又会导致占用内存过多，就有两阶段策略
    • 当消息处理器第一次写入消息时，才将 channel 注册到 selector 上
    • selector 检查 channel 上的可写事件，如果所有的数据写完了，就取消 channel 的注册
    • 如果不取消，会每次可写均会触发 write 事件
代码语言：javascript复制
public class WriteServer {

    public static void main(String[] args) throws IOException {
        ServerSocketChannel ssc = ServerSocketChannel.open();
        ssc.configureBlocking(false);
        ssc.bind(new InetSocketAddress(8080));

        Selector selector = Selector.open();
        ssc.register(selector, SelectionKey.OP_ACCEPT);

        while(true) {
            selector.select();

            Iterator<SelectionKey> iter = selector.selectedKeys().iterator();
            while (iter.hasNext()) {
                SelectionKey key = iter.next();
                iter.remove();
                if (key.isAcceptable()) {
                    SocketChannel sc = ssc.accept();
                    sc.configureBlocking(false);
                    SelectionKey sckey = sc.register(selector, SelectionKey.OP_READ);
                    // 1. 向客户端发送内容
                    StringBuilder sb = new StringBuilder();
                    for (int i = 0; i < 3000000; i  ) {
                        sb.append("a");
                    }
                    ByteBuffer buffer = Charset.defaultCharset().encode(sb.toString());
                    int write = sc.write(buffer);
                    // 3. write 表示实际写了多少字节
                    System.out.println("实际写入字节:"   write);
                    // 4. 如果有剩余未读字节，才需要关注写事件
                    if (buffer.hasRemaining()) {
                        // read 1  write 4
                        // 在原有关注事件的基础上，多关注 写事件
                        sckey.interestOps(sckey.interestOps()   SelectionKey.OP_WRITE);
                        // 把 buffer 作为附件加入 sckey
                        sckey.attach(buffer);
                    }
                } else if (key.isWritable()) {
                    ByteBuffer buffer = (ByteBuffer) key.attachment();
                    SocketChannel sc = (SocketChannel) key.channel();
                    int write = sc.write(buffer);
                    System.out.println("实际写入字节:"   write);
                    if (!buffer.hasRemaining()) { // 写完了
                        key.interestOps(key.interestOps() - SelectionKey.OP_WRITE);
                        key.attach(null);
                    }
                }
            }
        }
    }
}

客户端

代码语言：javascript复制
public class WriteClient {
    public static void main(String[] args) throws IOException {
        Selector selector = Selector.open();
        SocketChannel sc = SocketChannel.open();
        sc.configureBlocking(false);
        sc.register(selector, SelectionKey.OP_CONNECT | SelectionKey.OP_READ);
        sc.connect(new InetSocketAddress("localhost", 8080));
        int count = 0;
        while (true) {
            selector.select();
            Iterator<SelectionKey> iter = selector.selectedKeys().iterator();
            while (iter.hasNext()) {
                SelectionKey key = iter.next();
                iter.remove();
                if (key.isConnectable()) {
                    System.out.println(sc.finishConnect());
                } else if (key.isReadable()) {
                    ByteBuffer buffer = ByteBuffer.allocate(1024 * 1024);
                    count  = sc.read(buffer);
                    buffer.clear();
                    System.out.println(count);
                }
            }
        }
    }
}
? write 为何要取消

只要向 channel 发送数据时，socket 缓冲可写，这个事件会频繁触发，因此应当只在 socket 缓冲区写不下时再关注可写事件，数据写完之后再取消关注

4.6 更进一步

? 利用多线程优化

现在都是多核 cpu，设计时要充分考虑别让 cpu 的力量被白白浪费

前面的代码只有一个选择器，没有充分利用多核 cpu，如何改进呢？

分两组选择器

  • 单线程配一个选择器，专门处理 accept 事件
  • 创建 cpu 核心数的线程，每个线程配一个选择器，轮流处理 read 事件
代码语言：javascript复制
public class ChannelDemo7 {
    public static void main(String[] args) throws IOException {
        new BossEventLoop().register();
    }


    @Slf4j
    static class BossEventLoop implements Runnable {
        private Selector boss;
        private WorkerEventLoop[] workers;
        private volatile boolean start = false;
        AtomicInteger index = new AtomicInteger();

        public void register() throws IOException {
            if (!start) {
                ServerSocketChannel ssc = ServerSocketChannel.open();
                ssc.bind(new InetSocketAddress(8080));
                ssc.configureBlocking(false);
                boss = Selector.open();
                SelectionKey ssckey = ssc.register(boss, 0, null);
                ssckey.interestOps(SelectionKey.OP_ACCEPT);
                workers = initEventLoops();
                new Thread(this, "boss").start();
                log.debug("boss start...");
                start = true;
            }
        }

        public WorkerEventLoop[] initEventLoops() {
//        EventLoop[] eventLoops = new EventLoop[Runtime.getRuntime().availableProcessors()];
            WorkerEventLoop[] workerEventLoops = new WorkerEventLoop[2];
            for (int i = 0; i < workerEventLoops.length; i  ) {
                workerEventLoops[i] = new WorkerEventLoop(i);
            }
            return workerEventLoops;
        }

        @Override
        public void run() {
            while (true) {
                try {
                    boss.select();
                    Iterator<SelectionKey> iter = boss.selectedKeys().iterator();
                    while (iter.hasNext()) {
                        SelectionKey key = iter.next();
                        iter.remove();
                        if (key.isAcceptable()) {
                            ServerSocketChannel c = (ServerSocketChannel) key.channel();
                            SocketChannel sc = c.accept();
                            sc.configureBlocking(false);
                            log.debug("{} connected", sc.getRemoteAddress());
                            workers[index.getAndIncrement() % workers.length].register(sc);
                        }
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    @Slf4j
    static class WorkerEventLoop implements Runnable {
        private Selector worker;
        private volatile boolean start = false;
        private int index;

        private final ConcurrentLinkedQueue<Runnable> tasks = new ConcurrentLinkedQueue<>();

        public WorkerEventLoop(int index) {
            this.index = index;
        }

        public void register(SocketChannel sc) throws IOException {
            if (!start) {
                worker = Selector.open();
                new Thread(this, "worker-"   index).start();
                start = true;
            }
            tasks.add(() -> {
                try {
                    SelectionKey sckey = sc.register(worker, 0, null);
                    sckey.interestOps(SelectionKey.OP_READ);
                    worker.selectNow();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            });
            worker.wakeup();
        }

        @Override
        public void run() {
            while (true) {
                try {
                    worker.select();
                    Runnable task = tasks.poll();
                    if (task != null) {
                        task.run();
                    }
                    Set<SelectionKey> keys = worker.selectedKeys();
                    Iterator<SelectionKey> iter = keys.iterator();
                    while (iter.hasNext()) {
                        SelectionKey key = iter.next();
                        if (key.isReadable()) {
                            SocketChannel sc = (SocketChannel) key.channel();
                            ByteBuffer buffer = ByteBuffer.allocate(128);
                            try {
                                int read = sc.read(buffer);
                                if (read == -1) {
                                    key.cancel();
                                    sc.close();
                                } else {
                                    buffer.flip();
                                    log.debug("{} message:", sc.getRemoteAddress());
                                    debugAll(buffer);
                                }
                            } catch (IOException e) {
                                e.printStackTrace();
                                key.cancel();
                                sc.close();
                            }
                        }
                        iter.remove();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}
? 如何拿到 cpu 个数

  • Runtime.getRuntime().availableProcessors() 如果工作在 docker 容器下，因为容器不是物理隔离的，会拿到物理 cpu 个数，而不是容器申请时的个数
  • 这个问题直到 jdk 10 才修复，使用 jvm 参数 UseContainerSupport 配置， 默认开启

4.7 UDP

  • UDP 是无连接的，client 发送数据不会管 server 是否开启
  • server 这边的 receive 方法会将接收到的数据存入 byte buffer，但如果数据报文超过 buffer 大小，多出来的数据会被默默抛弃

首先启动服务器端

代码语言：javascript复制
public class UdpServer {
    public static void main(String[] args) {
        try (DatagramChannel channel = DatagramChannel.open()) {
            channel.socket().bind(new InetSocketAddress(9999));
            System.out.println("waiting...");
            ByteBuffer buffer = ByteBuffer.allocate(32);
            channel.receive(buffer);
            buffer.flip();
            debug(buffer);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

输出

代码语言：javascript复制
waiting...

运行客户端

代码语言：javascript复制
public class UdpClient {
    public static void main(String[] args) {
        try (DatagramChannel channel = DatagramChannel.open()) {
            ByteBuffer buffer = StandardCharsets.UTF_8.encode("hello");
            InetSocketAddress address = new InetSocketAddress("localhost", 9999);
            channel.send(buffer, address);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

接下来服务器端输出

代码语言：javascript复制
          ------------------------------------------------- 
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
 -------- ------------------------------------------------- ---------------- 
|00000000| 68 65 6c 6c 6f                                  |hello           |
 -------- ------------------------------------------------- ----------------

5. NIO vs BIO

5.1 stream vs channel

  • stream 不会自动缓冲数据，channel 会利用系统提供的发送缓冲区、接收缓冲区（更为底层）
  • stream 仅支持阻塞 API，channel 同时支持阻塞、非阻塞 API，网络 channel 可配合 selector 实现多路复用
  • 二者均为全双工，即读写可以同时进行

5.2 IO 模型

同步阻塞、同步非阻塞、同步多路复用、异步阻塞（没有此情况）、异步非阻塞

  • 同步：线程自己去获取结果（一个线程）
  • 异步：线程自己不去获取结果，而是由其它线程送结果（至少两个线程）

当调用一次 channel.read 或 stream.read 后，会切换至操作系统内核态来完成真正数据读取，而读取又分为两个阶段，分别为：

  • 等待数据阶段
  • 复制数据阶段
  • 阻塞 IO
  • 非阻塞 IO
  • 多路复用
  • 信号驱动
  • 异步 IO
  • 阻塞 IO vs 多路复用
? 参考

UNIX 网络编程 - 卷 I

5.3 零拷贝

传统 IO 问题

传统的 IO 将一个文件通过 socket 写出

代码语言：javascript复制
File f = new File("helloword/data.txt");
RandomAccessFile file = new RandomAccessFile(file, "r");

byte[] buf = new byte[(int)f.length()];
file.read(buf);

Socket socket = ...;
socket.getOutputStream().write(buf);

内部工作流程是这样的：

  1. java 本身并不具备 IO 读写能力，因此 read 方法调用后，要从 java 程序的用户态切换至内核态，去调用操作系统（Kernel）的读能力，将数据读入内核缓冲区。这期间用户线程阻塞，操作系统使用 DMA（Direct Memory Access）来实现文件读，其间也不会使用 cpu DMA 也可以理解为硬件单元，用来解放 cpu 完成文件 IO
  2. 从内核态切换回用户态，将数据从内核缓冲区读入用户缓冲区（即 byte[] buf），这期间 cpu 会参与拷贝，无法利用 DMA
  3. 调用 write 方法，这时将数据从用户缓冲区（byte[] buf）写入 socket 缓冲区，cpu 会参与拷贝
  4. 接下来要向网卡写数据，这项能力 java 又不具备，因此又得从用户态切换至内核态，调用操作系统的写能力，使用 DMA 将 socket 缓冲区的数据写入网卡，不会使用 cpu

可以看到中间环节较多，java 的 IO 实际不是物理设备级别的读写，而是缓存的复制，底层的真正读写是操作系统来完成的

  • 用户态与内核态的切换发生了 3 次，这个操作比较重量级
  • 数据拷贝了共 4 次
NIO 优化

通过 DirectByteBuf

  • ByteBuffer.allocate(10) HeapByteBuffer 使用的还是 java 内存
  • ByteBuffer.allocateDirect(10) DirectByteBuffer 使用的是操作系统内存

大部分步骤与优化前相同，不再赘述。唯有一点：java 可以使用 DirectByteBuf 将堆外内存映射到 jvm 内存中来直接访问使用

  • 这块内存不受 jvm 垃圾回收的影响，因此内存地址固定，有助于 IO 读写
  • java 中的 DirectByteBuf 对象仅维护了此内存的虚引用，内存回收分成两步
    • DirectByteBuf 对象被垃圾回收，将虚引用加入引用队列
    • 通过专门线程访问引用队列，根据虚引用释放堆外内存
  • 减少了一次数据拷贝，用户态与内核态的切换次数没有减少

进一步优化（底层采用了 linux 2.1 后提供的 sendFile 方法），java 中对应着两个 channel 调用 transferTo/transferFrom 方法拷贝数据

  1. java 调用 transferTo 方法后，要从 java 程序的用户态切换至内核态，使用 DMA将数据读入内核缓冲区，不会使用 cpu
  2. 数据从内核缓冲区传输到 socket 缓冲区，cpu 会参与拷贝
  3. 最后使用 DMA 将 socket 缓冲区的数据写入网卡，不会使用 cpu

可以看到

  • 只发生了一次用户态与内核态的切换
  • 数据拷贝了 3 次

进一步优化（linux 2.4）

  1. java 调用 transferTo 方法后，要从 java 程序的用户态切换至内核态，使用 DMA将数据读入内核缓冲区，不会使用 cpu
  2. 只会将一些 offset 和 length 信息拷入 socket 缓冲区，几乎无消耗
  3. 使用 DMA 将 内核缓冲区的数据写入网卡，不会使用 cpu

整个过程仅只发生了一次用户态与内核态的切换，数据拷贝了 2 次。所谓的【零拷贝】，并不是真正无拷贝，而是在不会拷贝重复数据到 jvm 内存中，零拷贝的优点有

  • 更少的用户态与内核态的切换
  • 不利用 cpu 计算，减少 cpu 缓存伪共享
  • 零拷贝适合小文件传输

5.3 AIO

AIO 用来解决数据复制阶段的阻塞问题

  • 同步意味着，在进行读写操作时，线程需要等待结果，还是相当于闲置
  • 异步意味着，在进行读写操作时，线程不必等待结果，而是将来由操作系统来通过回调方式由另外的线程来获得结果

异步模型需要底层操作系统（Kernel）提供支持

  • Windows 系统通过 IOCP 实现了真正的异步 IO
  • Linux 系统异步 IO 在 2.6 版本引入，但其底层实现还是用多路复用模拟了异步 IO，性能没有优势
文件 AIO

先来看看 AsynchronousFileChannel

代码语言：javascript复制
@Slf4j
public class AioDemo1 {
    public static void main(String[] args) throws IOException {
        try{
            AsynchronousFileChannel s = 
                AsynchronousFileChannel.open(
                	Paths.get("1.txt"), StandardOpenOption.READ);
            ByteBuffer buffer = ByteBuffer.allocate(2);
            log.debug("begin...");
            s.read(buffer, 0, null, new CompletionHandler<Integer, ByteBuffer>() {
                @Override
                public void completed(Integer result, ByteBuffer attachment) {
                    log.debug("read completed...{}", result);
                    buffer.flip();
                    debug(buffer);
                }

                @Override
                public void failed(Throwable exc, ByteBuffer attachment) {
                    log.debug("read failed...");
                }
            });

        } catch (IOException e) {
            e.printStackTrace();
        }
        log.debug("do other things...");
        System.in.read();
    }
}

输出

代码语言：javascript复制
13:44:56 [DEBUG] [main] c.i.aio.AioDemo1 - begin...
13:44:56 [DEBUG] [main] c.i.aio.AioDemo1 - do other things...
13:44:56 [DEBUG] [Thread-5] c.i.aio.AioDemo1 - read completed...2
          ------------------------------------------------- 
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
 -------- ------------------------------------------------- ---------------- 
|00000000| 61 0d                                           |a.              |
 -------- ------------------------------------------------- ----------------

可以看到

  • 响应文件读取成功的是另一个线程 Thread-5
  • 主线程并没有 IO 操作阻塞
? 守护线程

默认文件 AIO 使用的线程都是守护线程，所以最后要执行 System.in.read() 以避免守护线程意外结束

网络 AIO
代码语言：javascript复制
public class AioServer {
    public static void main(String[] args) throws IOException {
        AsynchronousServerSocketChannel ssc = AsynchronousServerSocketChannel.open();
        ssc.bind(new InetSocketAddress(8080));
        ssc.accept(null, new AcceptHandler(ssc));
        System.in.read();
    }

    private static void closeChannel(AsynchronousSocketChannel sc) {
        try {
            System.out.printf("[%s] %s closen", Thread.currentThread().getName(), sc.getRemoteAddress());
            sc.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static class ReadHandler implements CompletionHandler<Integer, ByteBuffer> {
        private final AsynchronousSocketChannel sc;

        public ReadHandler(AsynchronousSocketChannel sc) {
            this.sc = sc;
        }

        @Override
        public void completed(Integer result, ByteBuffer attachment) {
            try {
                if (result == -1) {
                    closeChannel(sc);
                    return;
                }
                System.out.printf("[%s] %s readn", Thread.currentThread().getName(), sc.getRemoteAddress());
                attachment.flip();
                System.out.println(Charset.defaultCharset().decode(attachment));
                attachment.clear();
                // 处理完第一个 read 时，需要再次调用 read 方法来处理下一个 read 事件
                sc.read(attachment, attachment, this);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void failed(Throwable exc, ByteBuffer attachment) {
            closeChannel(sc);
            exc.printStackTrace();
        }
    }

    private static class WriteHandler implements CompletionHandler<Integer, ByteBuffer> {
        private final AsynchronousSocketChannel sc;

        private WriteHandler(AsynchronousSocketChannel sc) {
            this.sc = sc;
        }

        @Override
        public void completed(Integer result, ByteBuffer attachment) {
            // 如果作为附件的 buffer 还有内容，需要再次 write 写出剩余内容
            if (attachment.hasRemaining()) {
                sc.write(attachment);
            }
        }

        @Override
        public void failed(Throwable exc, ByteBuffer attachment) {
            exc.printStackTrace();
            closeChannel(sc);
        }
    }

    private static class AcceptHandler implements CompletionHandler<AsynchronousSocketChannel, Object> {
        private final AsynchronousServerSocketChannel ssc;

        public AcceptHandler(AsynchronousServerSocketChannel ssc) {
            this.ssc = ssc;
        }

        @Override
        public void completed(AsynchronousSocketChannel sc, Object attachment) {
            try {
                System.out.printf("[%s] %s connectedn", Thread.currentThread().getName(), sc.getRemoteAddress());
            } catch (IOException e) {
                e.printStackTrace();
            }
            ByteBuffer buffer = ByteBuffer.allocate(16);
            // 读事件由 ReadHandler 处理
            sc.read(buffer, buffer, new ReadHandler(sc));
            // 写事件由 WriteHandler 处理
            sc.write(Charset.defaultCharset().encode("server hello!"), ByteBuffer.allocate(16), new WriteHandler(sc));
            // 处理完第一个 accpet 时，需要再次调用 accept 方法来处理下一个 accept 事件
            ssc.accept(null, this);
        }

        @Override
        public void failed(Throwable exc, Object attachment) {
            exc.printStackTrace();
        }
    }
}

achment.clear(); // 处理完第一个 read 时，需要再次调用 read 方法来处理下一个 read 事件 sc.read(attachment, attachment, this); } catch (IOException e) { e.printStackTrace(); } }

代码语言：javascript复制
    @Override
    public void failed(Throwable exc, ByteBuffer attachment) {
        closeChannel(sc);
        exc.printStackTrace();
    }
}

private static class WriteHandler implements CompletionHandler<Integer, ByteBuffer> {
    private final AsynchronousSocketChannel sc;

    private WriteHandler(AsynchronousSocketChannel sc) {
        this.sc = sc;
    }

    @Override
    public void completed(Integer result, ByteBuffer attachment) {
        // 如果作为附件的 buffer 还有内容，需要再次 write 写出剩余内容
        if (attachment.hasRemaining()) {
            sc.write(attachment);
        }
    }

    @Override
    public void failed(Throwable exc, ByteBuffer attachment) {
        exc.printStackTrace();
        closeChannel(sc);
    }
}

private static class AcceptHandler implements CompletionHandler<AsynchronousSocketChannel, Object> {
    private final AsynchronousServerSocketChannel ssc;

    public AcceptHandler(AsynchronousServerSocketChannel ssc) {
        this.ssc = ssc;
    }

    @Override
    public void completed(AsynchronousSocketChannel sc, Object attachment) {
        try {
            System.out.printf("[%s] %s connectedn", Thread.currentThread().getName(), sc.getRemoteAddress());
        } catch (IOException e) {
            e.printStackTrace();
        }
        ByteBuffer buffer = ByteBuffer.allocate(16);
        // 读事件由 ReadHandler 处理
        sc.read(buffer, buffer, new ReadHandler(sc));
        // 写事件由 WriteHandler 处理
        sc.write(Charset.defaultCharset().encode("server hello!"), ByteBuffer.allocate(16), new WriteHandler(sc));
        // 处理完第一个 accpet 时，需要再次调用 accept 方法来处理下一个 accept 事件
        ssc.accept(null, this);
    }

    @Override
    public void failed(Throwable exc, Object attachment) {
        exc.printStackTrace();
    }
}

}

代码语言：javascript复制
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