前言

關于Java的NIO與BIO在上一篇博客中已經簡單總結，這里開始梳理NIO的三個組件——buffer，channel與selector

Buffer

從名字來看其實就是一個用于緩存數據的類，Buffer類是一個抽象類，具體的實現在NIO中有8種緩沖區類，分別如下：ByteBuffer，CharBuffer，DoubleBuffer，FloatBuffer，IntBuffer，LongBuffer，ShortBuffer，MappedByteBuffer 前7種其實就對應的Java中的基本類型，第八種數據類型是專門用于內存映射的一種ByteBuffer類型。

三個關鍵的屬性

Buffer內部維護了三個關鍵的屬性，一個是capacity，一個是position，一個是limit，同時Buffer存在兩種不同的模式，可讀模式和可寫模式。

capacity

這個表示Buffer的容量大小，寫入對象的數量最多不能超過capacity，該屬性一旦被分配，就不會改變。

Buffer的創建是通過調用之類的allocate方法來分配。

position

這個表示當前的位置，但是position的值與Buffer的讀寫模式有關，在不同的模式下，position屬性的值是不同的。當Buffer進行讀寫模式的切換的時候position會進行調整。

幾個方法的調用都會觸發Buffer的讀寫模式的切換，后續會總結這個

limit

limit表示讀寫的最大上限。這個也與Buffer的讀寫模式有關，不同的模式limit的值不同。

寫模式下：limit表示寫入的數據最大上限，在Buffer剛進入到寫模式下，limit屬性值被設置為緩沖區的capacity容量。

讀模式下：limit表示最多能從Buffer中讀到多少數據。

下面以IntBuffer為例來說明

Buffer的容量分配

打印Buffer相關屬性的方法

/**
 * 打印出buffer各個屬性的位置
 * @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 =================");
}

通過allocate進行容量的分配

/**
 * buffer分配空間，buffer創建之后，默認進入寫模式
 */
public static void allocateBuffer(){
    //Buffer.allocate，分配指定空間的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);
}

運行結果如下：

[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的寫入

Buffer只要分配了內存空間，就是可寫入模式，寫入數據需要調用Buffer中的put方法，如下實例

/**
 * 往buffer中寫入數據
 * @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);
}

寫入指定數據之后，相關變量的變化如下

[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變成了6，表示寫入了6個元素，limit表示可寫入20個元素

Buffer的讀取

向緩沖區中寫入數據之后，并不能直接從Buffer中讀取數據，需要進行一個模式的切換，需要調用Buffer的flip方法完成模式的切換。

/**
 * 從buffer中讀取，從寫入模式切換到讀取模式，需要調用buffer的flip方法
 * @param buffer
 */
public static void readBuffer(IntBuffer buffer){
	//由寫入模式切換成讀取模式
    buffer.flip();
    int index = 0;
    printBufferPropertiesInfo(buffer);
    while(index<buffer.limit()){
       	//利用get()方法從Buffer中讀取數據
        log.info("read info {}",buffer.get());
        index++;
    }
    log.info("after read print buffer info");
    printBufferPropertiesInfo(buffer);
}

運行讀取數據之后的日志

# 可以看到由寫入模式切換成讀取模式之后。position置為0，limit置成了可讀取的個數
[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 =================

# 開始讀取數據
[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

# 讀取完成之后，position指向當前讀取到的元素位置，limit依舊為可讀取的個數。
[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 =================

倒帶，標記讀取

倒帶讀取

已經讀完的數據，如果需要再讀一遍，需要調用rewind方法，position的位置會被重新置為0，然后可以重新讀取緩存中的數據

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

}

運行日志如下：

[main] INFO com.learn.netty.component.buffer.BufferDemo - after rewind
# 倒帶之后的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 =================

# 倒帶之后讀取的數據
[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

標記讀取

可以通過mark方法，暫存需要讀取的位置。然后reset方法之后，position可以置回mark方法記錄的位置，之后從mark標記的位置開始讀取

代碼如下所示：

/**
 * 測試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();//這里標記的一個位置
        }
        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());
    }
}

運行結果

[main] INFO com.learn.netty.component.buffer.BufferDemo - normal read
# 正常讀取的position等相關屬性
[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
## 這里是標志位
[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 =================

##正常讀取完成之后的 position等相關屬性
[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標記為
[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 =================
##從第3個元素開始讀取
[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方法會清空Buffer中的數據，同時Buffer會切換入寫入模式

小結

關于Buffer的模式切換，這里做一個簡單的總結

Channel

在NIO中一個連接就是一個channel，如果熟悉Linux的話，其實一個channel就是對應一個文件描述符。在Java NIO中對應不同的網絡傳輸協議，對應不同的channel。幾種常用的channel實現類如下：FileChannel，SocketChannel，ServerSocketChannel，DatagramChannel。NIO中的讀取數據，都會通過channel與Buffer進行交互。

FileChannel是文件通道，SocketChannel是套接字通道，ServerSocketChannel是服務套接字通道，DatagramChannel是數據報通道。下面我們重點通過實例說明FileChannel，SocketChannel和ServerSocketChannel。

FileChannel

獲取FileChannel

通常來講獲取FileChannel有兩種方式，一種是根據文件輸入輸出流進行獲取，另一種是根據文件隨機訪問類進行獲取。需要說明的是FileChannel沒有非阻塞模式，都是阻塞模式。

1、根據文件輸入輸出流獲取文件channel

需要注意的是根據輸入文件流獲取的channel對文件只有讀取的權限，沒有寫入的權限。

//1.構建文件輸入流
FileInputStream fileInputStream = new FileInputStream(new File(fileName));
//2.獲取文件channel
FileChannel fileChannel = fileInputStream.getChannel();

//1.構建文件輸出流
FileOutputStream fileOutputStream = new FileOutputStream(new File(fileName));
//2.獲取文件channel
FileChannel fileChannel = fileInputStream.getChannel();

2、通過RandomAccessFile獲取FileChannel

如果需要channel對文件進行可讀寫，則需要指定rw訪問權限。

//這里的targetFileName是文件的絕對路徑
RandomAccessFile randomAccessFile = new RandomAccessFile(targetFileName,"rw");
//通過RandomAccessFile獲取FileChannel
FileChannel fileChannel = randomAccessFile.getChannel();

通過FileChannel讀取文件內容

調用指定的read方法，將文件中的內容讀取到buffer中即可

//，因此我們每次在讀取文件內容的時候，需要先分配一個Buffer緩沖區。
fileChannel.read(buffer)

通過FileChannel寫入文件內容

在將channel中的內容寫入到磁盤的時候，需要將buffer變成讀模式。

//將buffer中的內容讀取出來，寫入到FileChannel指定的文件中，因此在讀取之前，需要將buffer變成可讀模式
buffer.flip();
fileChannel.write(buffer)

實例

可以通過以下實例來熟悉FileChannel。下述代碼的讀取和寫入都是用的同一個buffer

/**
 * 先讀取文件內容，然后將簡單的一句話附加到文件末尾
 */
public static void simpleAppendInfo2File(){
    String targetFileName = "F:\\git_learn\\index_newName_bak.html";
    FileInputStream fileInputStream=null;
    FileChannel fileChannel = null;
    try{
		//1、創建FileChannel
        RandomAccessFile randomAccessFile = new RandomAccessFile(targetFileName,"rw");
        fileChannel = randomAccessFile.getChannel();

        //2.實例化Buffer緩沖區
        ByteBuffer fileBuffer = ByteBuffer.allocate(1024);//文件緩沖區

		//3、文件內容的讀取
        while(fileChannel.read(fileBuffer)!=-1){
            fileBuffer.flip();//fileBuffer變成可讀模式
            //讀取buffer中的內容
            while(fileBuffer.position()<fileBuffer.limit()){
                System.out.print((char)fileBuffer.get());
            }
			//清空緩沖區
            fileBuffer.clear();
        }

		//4、在文件的末尾附加內容，依舊可以用同一個buffer操作
        fileBuffer.put("\nthis is append coment".getBytes("UTF-8"));
        fileBuffer.flip();//這里需要切換成可讀模式
        fileChannel.write(fileBuffer);
    }catch (Exception e){
        e.printStackTrace();
    }finally {
        try {
            fileChannel.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

ServerSocketChannel和SocketChannel

NIO中涉及到網絡的通道有兩個，一個是ServerSocketChannel，一個是SocketChannel。前者負責連接的監聽，后者負責連接的傳輸。ServerSocketChannel位于服務端，而SocketChannel則位于兩端。需要說明的是這兩者有阻塞和非阻塞兩種模式，但是接下來的代碼實例我們只介紹阻塞模式下二者的操作。這里還是采用一個簡單的網絡通信實例

客戶端代碼：

/**
 * autor:liman
 * createtime:2020/8/26
 * comment:SocketChannel的實例
 */
@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));

            //這里實例化了兩個buffer，一個用于從channel中讀取數據，一個用于往channel中寫入數據。
            ByteBuffer writeBuffer = ByteBuffer.allocate(1024);
            ByteBuffer readBuffer = ByteBuffer.allocate(1024);

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

            writeBuffer.flip();//變成讀模式
            socketChannel.write(writeBuffer);//將客戶端的數據發送到服務端的SocketChannel

            readBuffer.clear();//clear操作之后，buffer會變成寫模式
            String serverInfo = readInfoFromBuffer(socketChannel,readBuffer);
            log.info("從服務端接受到的消息:{}",serverInfo);
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            try {
                socketChannel.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 從buffer讀取相關內容
     * @param socketChannel
     * @param buffer
     * @return
     * @throws IOException
     */
    public static String readInfoFromBuffer(SocketChannel socketChannel,ByteBuffer buffer) throws IOException {
        StringBuilder stringBuilder = new StringBuilder();
        socketChannel.read(buffer);//這里的buffer是可寫模式，可以直接read
        buffer.flip();//將buffer變成可讀模式
        while(buffer.hasRemaining()){
            while(buffer.position()<buffer.limit()) {
                stringBuilder.append((char) buffer.get());
            }
        }
        return stringBuilder.toString();
    }
}

服務端代碼：

/**
 * autor:liman
 * createtime:2020/8/26
 * comment:ServerSocketChannel的實例
 */
@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);

            //讀取數據
            String messageFromClient = readInfoFromClient(socketChannel, readBuffer);
            log.info("從客戶端讀取到的信息為:{}",messageFromClient);
            System.in.read();
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            try {
                socketChannel.close();
                serverSocketChannel.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    /**
    	從buffer中讀取數據
    */
    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();
    }

}

二者其實差異不大，只需要注意buffer的操作即可，同時需要注意SocketChannel與ServerSocketChannel的構建即可。

Selector

Selector是完成IO多路復用的關鍵組件，通過上面的Channel實例我們可以看出，其實一個channel可以代表一條連接通道，但是通過一個Selector就可以同時監控多個通道的網絡IO操作。

通道和選擇器的關系通過注冊完成，注冊到選擇器的通道，必須處于非阻塞模式，這也意味著FileChannel無法注冊到選擇器。同時只有繼承了SelectableChannel的通道才能注冊到選擇器上 關于Selector的實例需要詳細參考其他資料。

選擇器實例的獲取

通過Selector的靜態open方法來獲取Selector實例。

Selector selector = Selector.open()

通道注冊到Selector

如果將通道注冊到Selector，需要提前準備好對應的channel，相關代碼如下所示

selector = Selector.open();

serverSocketChannel = ServerSocketChannel.open();
//注冊到選擇器的通道必須是非阻塞模式
serverSocketChannel.configureBlocking(false);
serverSocketChannel.socket().bind(new InetSocketAddress(port), 1024);
//將serverSocketChannel注冊到selector上
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

遍歷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:處理相關事件
            // ......
        } catch (Exception e) {
            log.error("通道出現異常，異常信息為:{}",e);
            if(key!=null){
                key.cancel();
                if(key.channel()!=null){
                    key.channel().close();
                }
            }
        }
        keysIterator.remove();
    }
} catch (Exception e) {
    e.printStackTrace();
}

Selector更多的東西需要多多領悟，有時候通過文字似乎不好表述。還是直接上實例（該實例參照《Netty 權威指南》）。

實例

服務端代碼

/**
 * 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) {
        //構造方法中將ServerSocketChannel注冊到Selector
        try {
            selector = Selector.open();
            serverSocketChannel = ServerSocketChannel.open();
            //注冊到選擇器的通道必須是非阻塞模式
            serverSocketChannel.configureBlocking(false);
            serverSocketChannel.socket().bind(new InetSocketAddress(port), 1024);
            //將serverSocketChannel注冊到selector上
            serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
            log.info("The time server is start in port : {}", port);
        } catch (IOException e) {
            log.error("服務啟動出行異常，異常信息為:{}", e);
            System.exit(1);
        }
    }

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

    /**
    	開辟線程處理Selector上注冊的事件
    */
    @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("通道出現異常，異常信息為:{}",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();
            }
        }
    }

    /**
     * 處理Selector上的注冊事件
     * @param key
     * @throws IOException
     */
    private void handleInput(SelectionKey key) throws IOException {
        String currentTime = null;
        if (key.isValid()) {
			//處理服務端ServerSocketChannel的Accept事件。
            if(key.isAcceptable()){
                ServerSocketChannel serverSocketChannel = (ServerSocketChannel) key.channel();
                //將與ServerSocketChannel建立連接的SocketChannel注冊到selector，并注冊READ事件
                SocketChannel socketChannel = serverSocketChannel.accept();
                socketChannel.configureBlocking(false);
                socketChannel.register(selector,SelectionKey.OP_READ);
            }

            //處理ServerSocketChannel的可讀事件，讀取SocketChannel中的數據
            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();
            }
        }
    }

    /*
    	將SocketChannel中的內容寫入到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);
        }
    }
}

啟動服務端的代碼

/**
 * autor:liman
 * createtime:2020/8/12
 * comment:時間回顯服務端啟動
 */
@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();
    }
}

客戶端代碼

客戶端的線程代碼

/**
 * 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 {
            //構造方法中，將SocketChannel注冊到selector
            selector = Selector.open();
            socketChannel = SocketChannel.open();
            socketChannel.configureBlocking(false);
        } catch (Exception e) {
            log.error("客戶端初始化異常，異常信息為:{}",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("流關閉異常，異常信息為：{}",e);
            }
        }
    }

    private void handleInput(SelectionKey key) throws IOException {
        if (key.isValid()) {
            SocketChannel socketChannel = (SocketChannel) key.channel();
            if (key.isConnectable()) {//如果可連接，則繼續注冊可讀事件。
                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()) {//可讀事件
                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();
            }
        }
    }

    /**
    	將客戶端的連接事件注冊到Selector上
    	這里只注冊了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);
        }
    }

  	/**
  		將指定的發送命令寫入到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.");
        }
    }
}

注冊事件

關于注冊事件，主要有以下四種

可讀事件	OP_READ
可寫事件	OP_WRITE
連接事件	OP_CONNECT
接收事件	OP_ACCEPT

但每一個通道，其實并不需要關注所有的事件，比如ServerSocketChannel，僅僅支持接收事件。SocketChannel支持接收事件，可讀與可寫事件，并不支持接收事件。

總結

本篇博客簡單總結了一下Java NIO中的三個核心組件，大部分參考了《Netty、Redis、Zookeeper高并發實戰》與《Netty權威指南》兩本書。