Netty拾遗（二）——Java NIO的三个组件（Buffer，Channel，Selector）

前言

关于Java的NIO与BIO在上一篇博客中已经简单总结，这里开始梳理NIO的三个组件——buffer，channel与selector

Buffer

从名字来看其实就是一个用于缓存数据的类，Buffer类是一个抽象类，具体的实现在NIO中有8种缓冲区类，分别如下：ByteBuffer，CharBuffer，DoubleBuffer，FloatBuffer，IntBuffer，LongBuffer，ShortBuffer，MappedByteBuffer 前7种其实就对应的Java中的基本类型，第八种数据类型是专门用于内存映射的一种ByteBuffer类型。

三个关键的属性

Buffer内部维护了三个关键的属性，一个是capacity，一个是position，一个是limit，同时Buffer存在两种不同的模式，可读模式和可写模式。

capacity

这个表示Buffer的容量大小，写入对象的数量最多不能超过capacity，该属性一旦被分配，就不会改变。

Buffer的创建是通过调用之类的allocate方法来分配。

position

这个表示当前的位置，但是position的值与Buffer的读写模式有关，在不同的模式下，position属性的值是不同的。当Buffer进行读写模式的切换的时候position会进行调整。

几个方法的调用都会触发Buffer的读写模式的切换，后续会总结这个

limit

limit表示读写的最大上限。这个也与Buffer的读写模式有关，不同的模式limit的值不同。

写模式下：limit表示写入的数据最大上限，在Buffer刚进入到写模式下，limit属性值被设置为缓冲区的capacity容量。

读模式下：limit表示最多能从Buffer中读到多少数据。

下面以IntBuffer为例来说明

Buffer的容量分配

打印Buffer相关属性的方法

/**
 * 打印出buffer各个属性的位置
 * @param buffer
 */
public static void printBufferPropertiesInfo(Buffer buffer){
     
    int position = buffer.position();
    int limit = buffer.limit();
    int capacity = buffer.capacity();
    log.info("================= print buffer info start =================");
    log.info("position:{}",position);
    log.info("limit:{}",limit);
    log.info("capacity:{}",capacity);
    log.info("================= print buffer info end =================");
}

通过allocate进行容量的分配

/**
 * buffer分配空间，buffer创建之后，默认进入写模式
 */
public static void allocateBuffer(){
     
    //Buffer.allocate，分配指定空间的buffer
    intBuffer = IntBuffer.allocate(20);
    log.info("buffer allocate finished");
    log.info("buffer mode : {}",intBuffer.isReadOnly());
    intBuffer.flip();
    log.info("buffer mode : {}",intBuffer.isReadOnly());
    printBufferPropertiesInfo(intBuffer);
}

运行结果如下：

[main] INFO com.learn.netty.component.buffer.BufferDemo - buffer allocate finished
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info start =================
[main] INFO com.learn.netty.component.buffer.BufferDemo - position:0
[main] INFO com.learn.netty.component.buffer.BufferDemo - limit:20
[main] INFO com.learn.netty.component.buffer.BufferDemo - capacity:20
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info end =================

Buffer的写入

Buffer只要分配了内存空间，就是可写入模式，写入数据需要调用Buffer中的put方法，如下实例

/**
 * 往buffer中写入数据
 * @param buffer
 * @param count
 */
public static void writeBuffer(IntBuffer buffer,int count){
     
    int i = 0;
    while(i<count){
     
        buffer.put(i+1);
        i++;
    }
    printBufferPropertiesInfo(buffer);
}

写入指定数据之后，相关变量的变化如下

[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info start =================
[main] INFO com.learn.netty.component.buffer.BufferDemo - position:6
[main] INFO com.learn.netty.component.buffer.BufferDemo - limit:20
[main] INFO com.learn.netty.component.buffer.BufferDemo - capacity:20
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info end =================

可以看到position变成了6，表示写入了6个元素，limit表示可写入20个元素

Buffer的读取

向缓冲区中写入数据之后，并不能直接从Buffer中读取数据，需要进行一个模式的切换，需要调用Buffer的flip方法完成模式的切换。

/**
 * 从buffer中读取，从写入模式切换到读取模式，需要调用buffer的flip方法
 * @param buffer
 */
public static void readBuffer(IntBuffer buffer){
     
	//由写入模式切换成读取模式
    buffer.flip();
    int index = 0;
    printBufferPropertiesInfo(buffer);
    while(index<buffer.limit()){
     
       	//利用get()方法从Buffer中读取数据
        log.info("read info {}",buffer.get());
        index++;
    }
    log.info("after read print buffer info");
    printBufferPropertiesInfo(buffer);
}

运行读取数据之后的日志

# 可以看到由写入模式切换成读取模式之后。position置为0，limit置成了可读取的个数
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info start =================
[main] INFO com.learn.netty.component.buffer.BufferDemo - position:0
[main] INFO com.learn.netty.component.buffer.BufferDemo - limit:6
[main] INFO com.learn.netty.component.buffer.BufferDemo - capacity:20
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info end =================

# 开始读取数据
[main] INFO com.learn.netty.component.buffer.BufferDemo - read info 1
[main] INFO com.learn.netty.component.buffer.BufferDemo - read info 2
[main] INFO com.learn.netty.component.buffer.BufferDemo - read info 3
[main] INFO com.learn.netty.component.buffer.BufferDemo - read info 4
[main] INFO com.learn.netty.component.buffer.BufferDemo - read info 5
[main] INFO com.learn.netty.component.buffer.BufferDemo - read info 6
[main] INFO com.learn.netty.component.buffer.BufferDemo - after read print buffer info

# 读取完成之后，position指向当前读取到的元素位置，limit依旧为可读取的个数。
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info start =================
[main] INFO com.learn.netty.component.buffer.BufferDemo - position:6
[main] INFO com.learn.netty.component.buffer.BufferDemo - limit:6
[main] INFO com.learn.netty.component.buffer.BufferDemo - capacity:20
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info end =================

倒带，标记读取

倒带读取

已经读完的数据，如果需要再读一遍，需要调用rewind方法，position的位置会被重新置为0，然后可以重新读取缓存中的数据

/**
 * 倒带读取
 * @param buffer
 */
public static void readWind(IntBuffer buffer){
     
    log.info("after rewind");
    buffer.rewind();//倒带的操作
    printBufferPropertiesInfo(buffer);
    log.info("rewind read buffer info");
    int index = 0;
    while(index<buffer.limit()){
     
        log.info("buffer item : {}",buffer.get(index++));
    }

}

运行日志如下：

[main] INFO com.learn.netty.component.buffer.BufferDemo - after rewind
# 倒带之后的position和limit的值
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info start =================
[main] INFO com.learn.netty.component.buffer.BufferDemo - position:0
[main] INFO com.learn.netty.component.buffer.BufferDemo - limit:6
[main] INFO com.learn.netty.component.buffer.BufferDemo - capacity:20
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info end =================

# 倒带之后读取的数据
[main] INFO com.learn.netty.component.buffer.BufferDemo - rewind read buffer info
[main] INFO com.learn.netty.component.buffer.BufferDemo - buffer item : 1
[main] INFO com.learn.netty.component.buffer.BufferDemo - buffer item : 2
[main] INFO com.learn.netty.component.buffer.BufferDemo - buffer item : 3
[main] INFO com.learn.netty.component.buffer.BufferDemo - buffer item : 4
[main] INFO com.learn.netty.component.buffer.BufferDemo - buffer item : 5
[main] INFO com.learn.netty.component.buffer.BufferDemo - buffer item : 6

标记读取

可以通过mark方法，暂存需要读取的位置。然后reset方法之后，position可以置回mark方法记录的位置，之后从mark标记的位置开始读取

代码如下所示：

/**
 * 测试mark和reset的操作
 * @param buffer
 */
public static void markAndResetRead(IntBuffer buffer){
     
    buffer.flip();
    int i = 0;
    log.info("normal read");
    printBufferPropertiesInfo(buffer);
    while(i<buffer.limit()){
     
        if(i==2){
     
            log.info("mark index");
            buffer.mark();//这里标记的一个位置
        }
        log.info("mark read item:{}",buffer.get());
        i++;
    }
    printBufferPropertiesInfo(buffer);
    log.info("reset read");
    buffer.reset();//回退到上一次mark的位置
    printBufferPropertiesInfo(buffer);
    while(buffer.position()<buffer.limit()){
     
        log.info("after reset read item:{}",buffer.get());
    }
}

运行结果

[main] INFO com.learn.netty.component.buffer.BufferDemo - normal read
# 正常读取的position等相关属性
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info start =================
[main] INFO com.learn.netty.component.buffer.BufferDemo - position:0
[main] INFO com.learn.netty.component.buffer.BufferDemo - limit:6
[main] INFO com.learn.netty.component.buffer.BufferDemo - capacity:20
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info end =================
[main] INFO com.learn.netty.component.buffer.BufferDemo - mark read item:1
[main] INFO com.learn.netty.component.buffer.BufferDemo - mark read item:2
## 这里是标志位
[main] INFO com.learn.netty.component.buffer.BufferDemo - mark index
[main] INFO com.learn.netty.component.buffer.BufferDemo - mark read item:3
[main] INFO com.learn.netty.component.buffer.BufferDemo - mark read item:4
[main] INFO com.learn.netty.component.buffer.BufferDemo - mark read item:5
[main] INFO com.learn.netty.component.buffer.BufferDemo - mark read item:6
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info start =================

##正常读取完成之后的 position等相关属性
[main] INFO com.learn.netty.component.buffer.BufferDemo - position:6
[main] INFO com.learn.netty.component.buffer.BufferDemo - limit:6
[main] INFO com.learn.netty.component.buffer.BufferDemo - capacity:20
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info end =================

##reset标记为
[main] INFO com.learn.netty.component.buffer.BufferDemo - reset read
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info start =================
[main] INFO com.learn.netty.component.buffer.BufferDemo - position:2
[main] INFO com.learn.netty.component.buffer.BufferDemo - limit:6
[main] INFO com.learn.netty.component.buffer.BufferDemo - capacity:20
[main] INFO com.learn.netty.component.buffer.BufferDemo - ================= print buffer info end =================
##从第3个元素开始读取
[main] INFO com.learn.netty.component.buffer.BufferDemo - after reset read item:3
[main] INFO com.learn.netty.component.buffer.BufferDemo - after reset read item:4
[main] INFO com.learn.netty.component.buffer.BufferDemo - after reset read item:5
[main] INFO com.learn.netty.component.buffer.BufferDemo - after reset read item:6

清空

clear方法会清空Buffer中的数据，同时Buffer会切换入写入模式

小结

关于Buffer的模式切换，这里做一个简单的总结

Channel

在NIO中一个连接就是一个channel，如果熟悉Linux的话，其实一个channel就是对应一个文件描述符。在Java NIO中对应不同的网络传输协议，对应不同的channel。几种常用的channel实现类如下：FileChannel，SocketChannel，ServerSocketChannel，DatagramChannel。NIO中的读取数据，都会通过channel与Buffer进行交互。

FileChannel是文件通道，SocketChannel是套接字通道，ServerSocketChannel是服务套接字通道，DatagramChannel是数据报通道。下面我们重点通过实例说明FileChannel，SocketChannel和ServerSocketChannel。

FileChannel

获取FileChannel

通常来讲获取FileChannel有两种方式，一种是根据文件输入输出流进行获取，另一种是根据文件随机访问类进行获取。需要说明的是FileChannel没有非阻塞模式，都是阻塞模式。

1、根据文件输入输出流获取文件channel

需要注意的是根据输入文件流获取的channel对文件只有读取的权限，没有写入的权限。

//1.构建文件输入流
FileInputStream fileInputStream = new FileInputStream(new File(fileName));
//2.获取文件channel
FileChannel fileChannel = fileInputStream.getChannel();

//1.构建文件输出流
FileOutputStream fileOutputStream = new FileOutputStream(new File(fileName));
//2.获取文件channel
FileChannel fileChannel = fileInputStream.getChannel();

2、通过RandomAccessFile获取FileChannel

如果需要channel对文件进行可读写，则需要指定rw访问权限。

//这里的targetFileName是文件的绝对路径
RandomAccessFile randomAccessFile = new RandomAccessFile(targetFileName,"rw");
//通过RandomAccessFile获取FileChannel
FileChannel fileChannel = randomAccessFile.getChannel();

通过FileChannel读取文件内容

调用指定的read方法，将文件中的内容读取到buffer中即可

//，因此我们每次在读取文件内容的时候，需要先分配一个Buffer缓冲区。
fileChannel.read(buffer)

通过FileChannel写入文件内容

在将channel中的内容写入到磁盘的时候，需要将buffer变成读模式。

//将buffer中的内容读取出来，写入到FileChannel指定的文件中，因此在读取之前，需要将buffer变成可读模式
buffer.flip();
fileChannel.write(buffer)

实例

可以通过以下实例来熟悉FileChannel。下述代码的读取和写入都是用的同一个buffer

/**
 * 先读取文件内容，然后将简单的一句话附加到文件末尾
 */
public static void simpleAppendInfo2File(){
     
    String targetFileName = "F:\\git_learn\\index_newName_bak.html";
    FileInputStream fileInputStream=null;
    FileChannel fileChannel = null;
    try{
     
		//1、创建FileChannel
        RandomAccessFile randomAccessFile = new RandomAccessFile(targetFileName,"rw");
        fileChannel = randomAccessFile.getChannel();

        //2.实例化Buffer缓冲区
        ByteBuffer fileBuffer = ByteBuffer.allocate(1024);//文件缓冲区

		//3、文件内容的读取
        while(fileChannel.read(fileBuffer)!=-1){
     
            fileBuffer.flip();//fileBuffer变成可读模式
            //读取buffer中的内容
            while(fileBuffer.position()<fileBuffer.limit()){
     
                System.out.print((char)fileBuffer.get());
            }
			//清空缓冲区
            fileBuffer.clear();
        }

		//4、在文件的末尾附加内容，依旧可以用同一个buffer操作
        fileBuffer.put("\nthis is append coment".getBytes("UTF-8"));
        fileBuffer.flip();//这里需要切换成可读模式
        fileChannel.write(fileBuffer);
    }catch (Exception e){
     
        e.printStackTrace();
    }finally {
     
        try {
     
            fileChannel.close();
        } catch (IOException e) {
     
            e.printStackTrace();
        }
    }
}

ServerSocketChannel和SocketChannel

NIO中涉及到网络的通道有两个，一个是ServerSocketChannel，一个是SocketChannel。前者负责连接的监听，后者负责连接的传输。ServerSocketChannel位于服务端，而SocketChannel则位于两端。需要说明的是这两者有阻塞和非阻塞两种模式，但是接下来的代码实例我们只介绍阻塞模式下二者的操作。这里还是采用一个简单的网络通信实例

客户端代码：

/**
 * autor:liman
 * createtime:2020/8/26
 * comment:SocketChannel的实例
 */
@Slf4j
public class SocketChannelSelf {
     

    public static void main(String[] args) {
     
        SocketChannel socketChannel = null;
        try {
     
            socketChannel = SocketChannel.open();
            socketChannel.connect(new InetSocketAddress("127.0.0.1",8899));

            //这里实例化了两个buffer，一个用于从channel中读取数据，一个用于往channel中写入数据。
            ByteBuffer writeBuffer = ByteBuffer.allocate(1024);
            ByteBuffer readBuffer = ByteBuffer.allocate(1024);

            String clientMessage = "i am client ";
            writeBuffer.put(clientMessage.getBytes("UTF-8"));

            writeBuffer.flip();//变成读模式
            socketChannel.write(writeBuffer);//将客户端的数据发送到服务端的SocketChannel

            readBuffer.clear();//clear操作之后，buffer会变成写模式
            String serverInfo = readInfoFromBuffer(socketChannel,readBuffer);
            log.info("从服务端接受到的消息:{}",serverInfo);
        }catch (Exception e){
     
            e.printStackTrace();
        }finally {
     
            try {
     
                socketChannel.close();
            } catch (IOException e) {
     
                e.printStackTrace();
            }
        }
    }

    /**
     * 从buffer读取相关内容
     * @param socketChannel
     * @param buffer
     * @return
     * @throws IOException
     */
    public static String readInfoFromBuffer(SocketChannel socketChannel,ByteBuffer buffer) throws IOException {
     
        StringBuilder stringBuilder = new StringBuilder();
        socketChannel.read(buffer);//这里的buffer是可写模式，可以直接read
        buffer.flip();//将buffer变成可读模式
        while(buffer.hasRemaining()){
     
            while(buffer.position()<buffer.limit()) {
     
                stringBuilder.append((char) buffer.get());
            }
        }
        return stringBuilder.toString();
    }
}

服务端代码：

/**
 * autor:liman
 * createtime:2020/8/26
 * comment:ServerSocketChannel的实例
 */
@Slf4j
public class ServerSocketChannelSelf {
     

    public static void main(String[] args) {
     
        ServerSocketChannel  serverSocketChannel = null;
        SocketChannel socketChannel = null;
        try {
     
            serverSocketChannel = ServerSocketChannel.open();
            serverSocketChannel.socket().bind(new InetSocketAddress("127.0.0.1",8899));

            socketChannel = serverSocketChannel.accept();
            ByteBuffer writeBuffer = ByteBuffer.allocate(1024);
            ByteBuffer readBuffer = ByteBuffer.allocate(1024);
            writeBuffer.put("hello this is Server,I'm listening in port 8899".getBytes("UTF-8"));
            writeBuffer.flip();
            socketChannel.write(writeBuffer);

            //读取数据
            String messageFromClient = readInfoFromClient(socketChannel, readBuffer);
            log.info("从客户端读取到的信息为:{}",messageFromClient);
            System.in.read();
        } catch (IOException e) {
     
            e.printStackTrace();
        }finally {
     
            try {
     
                socketChannel.close();
                serverSocketChannel.close();
            } catch (IOException e) {
     
                e.printStackTrace();
            }
        }
    }

    /**
    	从buffer中读取数据
    */
    public static String readInfoFromClient(SocketChannel socketChannel,ByteBuffer buffer) throws IOException {
     

        StringBuilder stringBuilder = new StringBuilder();
        socketChannel.read(buffer);
        buffer.flip();
        while(buffer.hasRemaining()){
     
            while(buffer.position()<buffer.limit()) {
     
                stringBuilder.append((char) buffer.get());
            }
        }
        return stringBuilder.toString();
    }

}

二者其实差异不大，只需要注意buffer的操作即可，同时需要注意SocketChannel与ServerSocketChannel的构建即可。

Selector

Selector是完成IO多路复用的关键组件，通过上面的Channel实例我们可以看出，其实一个channel可以代表一条连接通道，但是通过一个Selector就可以同时监控多个通道的网络IO操作。

通道和选择器的关系通过注册完成，注册到选择器的通道，必须处于非阻塞模式，这也意味着FileChannel无法注册到选择器。同时只有继承了SelectableChannel的通道才能注册到选择器上 关于Selector的实例需要详细参考其他资料。

选择器实例的获取

通过Selector的静态open方法来获取Selector实例。

Selector selector = Selector.open()

通道注册到Selector

如果将通道注册到Selector，需要提前准备好对应的channel，相关代码如下所示

selector = Selector.open();

serverSocketChannel = ServerSocketChannel.open();
//注册到选择器的通道必须是非阻塞模式
serverSocketChannel.configureBlocking(false);
serverSocketChannel.socket().bind(new InetSocketAddress(port), 1024);
//将serverSocketChannel注册到selector上
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

遍历Selector中的事件

try {
     
    selector.select(1000);
    Set<SelectionKey> selectionKeys = selector.selectedKeys();
    Iterator<SelectionKey> keysIterator = selectionKeys.iterator();
    SelectionKey key = null;
    while (keysIterator.hasNext()) {
     
        key = keysIterator.next();
        try {
     
			//TODO:处理相关事件
            // ......
        } catch (Exception e) {
     
            log.error("通道出现异常，异常信息为:{}",e);
            if(key!=null){
     
                key.cancel();
                if(key.channel()!=null){
     
                    key.channel().close();
                }
            }
        }
        keysIterator.remove();
    }
} catch (Exception e) {
     
    e.printStackTrace();
}

Selector更多的东西需要多多领悟，有时候通过文字似乎不好表述。还是直接上实例（该实例参照《Netty 权威指南》）。

实例

服务端代码

/**
 * autor:liman
 * createtime:2020/8/12
 * comment:
 */
@Slf4j
public class MultiplexerTimeServer implements Runnable {
     

    private Selector selector;
    private ServerSocketChannel serverSocketChannel;
    private volatile boolean stop;

    public MultiplexerTimeServer(int port) {
     
        //构造方法中将ServerSocketChannel注册到Selector
        try {
     
            selector = Selector.open();
            serverSocketChannel = ServerSocketChannel.open();
            //注册到选择器的通道必须是非阻塞模式
            serverSocketChannel.configureBlocking(false);
            serverSocketChannel.socket().bind(new InetSocketAddress(port), 1024);
            //将serverSocketChannel注册到selector上
            serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
            log.info("The time server is start in port : {}", port);
        } catch (IOException e) {
     
            log.error("服务启动出行异常，异常信息为:{}", e);
            System.exit(1);
        }
    }

    public void stop() {
     
        this.stop = true;
    }

    /**
    	开辟线程处理Selector上注册的事件
    */
    @Override
    public void run() {
     
        while (!stop) {
     
            try {
     
                selector.select(1000);
                Set<SelectionKey> selectionKeys = selector.selectedKeys();
                Iterator<SelectionKey> keysIterator = selectionKeys.iterator();
                SelectionKey key = null;
                while (keysIterator.hasNext()) {
     
                    key = keysIterator.next();
                    try {
     
                        handleInput(key);
                    } catch (Exception e) {
     
                        log.error("通道出现异常，异常信息为:{}",e);
                        if(key!=null){
     
                            key.cancel();
                            if(key.channel()!=null){
     
                                key.channel().close();
                            }
                        }
                    }
                    keysIterator.remove();
                }
            } catch (Exception e) {
     

            }
        }

        if (selector != null) {
     
            try {
     
                selector.close();
            } catch (IOException e) {
     
                e.printStackTrace();
            }
        }
    }

    /**
     * 处理Selector上的注册事件
     * @param key
     * @throws IOException
     */
    private void handleInput(SelectionKey key) throws IOException {
     
        String currentTime = null;
        if (key.isValid()) {
     
			//处理服务端ServerSocketChannel的Accept事件。
            if(key.isAcceptable()){
     
                ServerSocketChannel serverSocketChannel = (ServerSocketChannel) key.channel();
                //将与ServerSocketChannel建立连接的SocketChannel注册到selector，并注册READ事件
                SocketChannel socketChannel = serverSocketChannel.accept();
                socketChannel.configureBlocking(false);
                socketChannel.register(selector,SelectionKey.OP_READ);
            }

            //处理ServerSocketChannel的可读事件，读取SocketChannel中的数据
            if (key.isReadable()) {
     
                SocketChannel socketChannel = (SocketChannel) key.channel();
                ByteBuffer readBuffer = ByteBuffer.allocate(1024);
                int readBytes = socketChannel.read(readBuffer);
                if (readBytes > 0) {
     
                    readBuffer.flip();
                    byte[] bytes = new byte[readBuffer.remaining()];
                    readBuffer.get(bytes);
                    String body = new String(bytes, "UTF-8");
                    log.info("the time server receive order:{}", body);
                    if ("QUERY TIME".equalsIgnoreCase(body)) {
     
                        currentTime = LocalDateTime.now().toString();
                    } else {
     
                        currentTime = "BAD ORDER";
                    }
                    doWrite(socketChannel, currentTime);
                } else if (readBytes < 0) {
     
                    key.channel();

                }
                socketChannel.close();
            }
        }
    }

    /*
    	将SocketChannel中的内容写入到buffer中
    */
    private void doWrite(SocketChannel socketChannel, String response) throws IOException {
     
        if (response != null && response.trim().length() > 0) {
     
            byte[] bytes = response.getBytes();
            ByteBuffer writeBuffer = ByteBuffer.allocate(bytes.length);
            writeBuffer.put(bytes);
            writeBuffer.flip();
            socketChannel.write(writeBuffer);
        }
    }
}

启动服务端的代码

/**
 * autor:liman
 * createtime:2020/8/12
 * comment:时间回显服务端启动
 */
@Slf4j
public class TimeServer {
     
    public static void main(String[] args) {
     
        int port = 8999;
        MultiplexerTimeServer timeServer = new MultiplexerTimeServer(port);
        new Thread(timeServer,"NIO-Time-HandlerServer-001").start();
    }
}

客户端代码

客户端的线程代码

/**
 * autor:liman
 * createtime:2020/8/12
 * comment:
 */
@Slf4j
public class TimeClientHandler implements Runnable {
     

    private String host;
    private int port;
    private Selector selector;
    private SocketChannel socketChannel;
    private volatile boolean stop;

    public TimeClientHandler(String host, int port) {
     
        this.host = host;
        this.port = port;
        try {
     
            //构造方法中，将SocketChannel注册到selector
            selector = Selector.open();
            socketChannel = SocketChannel.open();
            socketChannel.configureBlocking(false);
        } catch (Exception e) {
     
            log.error("客户端初始化异常，异常信息为:{}",e);
        }
    }

    @Override
    public void run() {
     
        try {
     
            doConnect();
        } catch (IOException e) {
     
            e.printStackTrace();
        }

        while(!stop){
     
            try {
     
                selector.select(1000);
                Set<SelectionKey> selectionKeys = selector.selectedKeys();
                Iterator<SelectionKey> selectorKeyIterator = selectionKeys.iterator();
                SelectionKey key = null;
                while(selectorKeyIterator.hasNext()){
     
                    key = selectorKeyIterator.next();
                    try{
     
                        handleInput(key);
                    }catch (Exception e){
     
                        if(key!=null){
     
                            key.cancel();
                            if(key.channel()!=null){
     
                                key.channel().close();
                            }
                        }
                    }
                    selectorKeyIterator.remove();
                }

            } catch (IOException e) {
     
                e.printStackTrace();
            }
        }

        if(selector!=null){
     
            try{
     
                selector.close();
            }catch (Exception e){
     
                log.error("流关闭异常，异常信息为：{}",e);
            }
        }
    }

    private void handleInput(SelectionKey key) throws IOException {
     
        if (key.isValid()) {
     
            SocketChannel socketChannel = (SocketChannel) key.channel();
            if (key.isConnectable()) {
     //如果可连接，则继续注册可读事件。
                if (socketChannel.finishConnect()) {
     
                    socketChannel.register(selector, SelectionKey.OP_READ);
                    log.info("client connect to server ,client time is {}", LocalDateTime.now().toString());
                    doWrite(socketChannel);
                } else {
     
                    System.exit(1);
                }
            }

            if (key.isReadable()) {
     //可读事件
                ByteBuffer readBuffer = ByteBuffer.allocate(1024);
                int readBytes = socketChannel.read(readBuffer);
                if (readBytes > 0) {
     
                    readBuffer.flip();
                    byte[] bytes = new byte[readBuffer.remaining()];
                    readBuffer.get(bytes);
                    String body = new String(bytes,"UTF-8");
                    log.info("receive server message,Now is : {}",body);
                    this.stop = true;
                }else if(readBytes < 0){
     
                    key.cancel();
                }
                socketChannel.close();
            }
        }
    }

    /**
    	将客户端的连接事件注册到Selector上
    	这里只注册了CONNECT和READ事件。
    */
    private void doConnect() throws IOException {
     
        if (socketChannel.connect(new InetSocketAddress(host, port))) {
     
            socketChannel.register(selector, SelectionKey.OP_READ);
            doWrite(socketChannel);
        } else {
     
            socketChannel.register(selector, SelectionKey.OP_CONNECT);
        }
    }

  	/**
  		将指定的发送命令写入到SocketChannel
  	*/
    private void doWrite(SocketChannel socketChannel) throws IOException {
     
        byte[] req = "QUERY TIME".getBytes();
        ByteBuffer writeBuffer = ByteBuffer.allocate(req.length);
        writeBuffer.put(req);
        writeBuffer.flip();
        socketChannel.write(writeBuffer);
        if (!writeBuffer.hasRemaining()) {
     
            log.info("send order 2 server succeed.");
        }
    }
}

注册事件

关于注册事件，主要有以下四种

可读事件	OP_READ
可写事件	OP_WRITE
连接事件	OP_CONNECT
接收事件	OP_ACCEPT

但每一个通道，其实并不需要关注所有的事件，比如ServerSocketChannel，仅仅支持接收事件。SocketChannel支持接收事件，可读与可写事件，并不支持接收事件。

总结

本篇博客简单总结了一下Java NIO中的三个核心组件，大部分参考了《Netty、Redis、Zookeeper高并发实战》与《Netty权威指南》两本书。