Java 網路編程 —— 實現非阻塞式的伺服器

## 創建阻塞的伺服器當 `ServerSocketChannel` 與 `SockelChannel` 採用預設的阻塞模式時，為了同時處理多個客戶的連接，必須使用多線程 ```java public class EchoServer { private int port = 8000; priv ...

創建阻塞的伺服器

當 ServerSocketChannel 與 SockelChannel 採用預設的阻塞模式時，為了同時處理多個客戶的連接，必須使用多線程

public class EchoServer {
    
	private int port = 8000;
    private ServerSocketChannel serverSocketChannel = null;
    private ExecutorService executorService; //線程池
    private static final int POOL_MULTIPLE = 4; //線程池中工作線程的數目
    
    public EchoServer() throws IOException {
        //創建一個線程池
        executorService = Executors.newFixedThreadPool(
            Runtime.getRuntime().availableProcessors() * POOL_MULTIPLE);
        //創建一個ServerSocketChannel對象
        serverSocketChannel = ServerSocketChannel.open();
        //使得在同一個主機上關閉了伺服器程式，緊接著再啟動該伺服器程式時，可以順利綁定相同的埠
        serverSocketChannel.socket().setReuseAddress(true);
        //把伺服器進程與一個本地埠綁定
        serverSocketChannel.socket().bind(new InetSocketAddress(port));
        System.out.println("伺服器啟動");
    }
    
    public void service() {
        while (true) {
            SocketChannel socketChannel = null;
            try {
                socketChannel = serverSocketChannel.accept();
                //處理客戶連接
                executorService.execute(new Handler(socketChannel));
            } catch(IOException e) {
                e.printStackTrace();
            }
        }
    }
    
    public static void main(String args[])throws IOException {
        new EchoServer().service();
    }
    
    //處理客戶連按
    class Handler implements Runnable {

        private SocketChannel socketChannel;
		
        public Handler(SocketChannel socketChannel) {
            this.socketChannel = socketChannel;
        }
        
        public void run() {
            handle(socketChannel);
        }
        
        public void handle(SocketChannel socketChannel) {
            try {
                //獲得與socketChannel關聯的Socket對象
                Socket socket = socketChannel.socket();
                System.out.println("接收到客戶連接，來自:" + socket.getInetAddress() + ":" + socket.getPort());
                
                BufferedReader br = getReader(socket);
                PrintWriter pw = getWriter(socket);
                
                String msg = null;
                while ((msg = br.readLine()) != null) {
                    System.out.println(msg);
                    pw.println(echo(msg));
                    if (msg.equals("bye")) {
                        break;
                    }
                }
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                try {
                    if(socketChannel != null) {
                        socketChannel.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    } 
    
    private PrintWriter getWriter(Socket socket) throws IOException {
        OutputStream socketOut = socket.getOutputStream();
        return new PrintWriter(socketOut,true);
    }
    
    private BufferedReader getReader(Socket socket) throws IOException {
        InputStream socketIn = socket.getInputStream();
        return new BufferedReader(new InputStreamReader(socketIn));
    }
    
    public String echo(String msg) {
        return "echo:" + msg;
    }
}

創建非阻塞的伺服器

在非阻塞模式下，EchoServer 只需要啟動一個主線程，就能同時處理三件事：

接收客戶的連接
接收客戶發送的數據
向客戶發迴響應數據

EchoServer 委托 Selector 來負責監控接收連接就緒事件、讀就緒事件和寫就緒事件如果有特定事件發生，就處理該事件

// 創建一個Selector對象
selector = Selector.open();
//創建一個ServerSocketChannel對象
serverSocketChannel = ServerSocketChannel.open();
//使得在同一個主機上關閉了伺服器程式，緊接著再啟動該伺服器程式時
//可以順利綁定到相同的埠
serverSocketChannel.socket().setReuseAddress(true);
//使ServerSocketChannel工作於非阻塞模式
serverSocketChannel.configureBlocking(false):
//把伺服器進程與一個本地埠綁定
serverSocketChannelsocket().bind(new InetSocketAddress(port));

EchoServer 類的 service() 方法負責處理本節開頭所說的三件事，體現其主要流程的代碼如下：

public void service() throws IOException {
    serverSocketChannel.reqister(selector, SelectionKey.OP_ACCEPT);
    //第1層while迴圈
    while(selector.select() > 0) {
        //獲得Selector的selected-keys集合
        Set readyKeys = selector.selectedKeys();
        Iterator it = readyKeys.iterator();
        //第2層while迴圈
        while (it.hasNext()) {
            SelectionKey key = null;
            //處理SelectionKey
            try {
                //取出一個SelectionKey
                key = (SelectionKey) it.next();
                //把 SelectionKey從Selector 的selected-key 集合中刪除
                it.remove();
                1f (key.isAcceptable()) { 處理接收連接就緒事件; }
                if (key.isReadable()) { 處理讀就緒水件; }
                if (key.isWritable()) { 處理寫就緒事件; }
            } catch(IOException e) {
                e.printStackTrace();
                try {
                    if(key != null) {
                        //使這個SelectionKey失效
                        key.cancel();
                        //關閉與這個SelectionKey關聯的SocketChannel
                        key.channel().close();
                    }
                } catch(Exception ex) { 
                    e.printStackTrace();
                }
            }
        }
    }
}

首先由 ServerSocketChannel 向 Selector 註冊接收連接就緒事件，如果 Selector 監控到該事件發生,就會把相應的 SelectionKey 對象加入 selected-keys 集合
第一層 while 迴圈，不斷詢問 Selector 已經發生的事件，select() 方法返回當前相關事件已經發生的 SelectionKey 的個數，如果當前沒有任何事件發生，該方法會阻塞下去，直到至少有一個事件發生。Selector 的 selectedKeys() 方法返回 selected-keys 集合，它存放了相關事件已經發生的 SelectionKey 對象
第二層 while 迴圈，從 selected-keys 集合中依次取出每個 SelectionKey 對象並從集合中刪除，，然後調用 isAcceptable()、isReadable() 和 isWritable() 方法判斷到底是哪種事件發生了，從而做出相應的處理

1. 處理接收連接就緒事件

if (key.isAcceptable()) {
    //獲得與SelectionKey關聯的ServerSocketChannel
    ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
    //獲得與客戶連接的SocketChannel
    SocketChannel socketChannel = (SocketChannel) ssc.accept();
    //把Socketchannel設置為非阻塞模式
    socketChannel.configureBlocking(false);
    //創建一個用於存放用戶發送來的數據的級沖區
    ByteBuffer buffer = ByteBuffer.allocate(1024);
    //Socketchannel向Selector註冊讀就緒事件和寫就緒事件
    socketChannel.register(selector, SelectionKey.OP_READ | SelectionKey.OP_WRITE, buffer);
}

2. 處理讀就緒事件

public void receive(SelectionKey key) throws IOException {
    //獲得與SelectionKey關聯的附件
    ByteBuffer buffer = (ByteBuffer) key.attachment();
    //獲得與SelectionKey關聯的Socketchannel
    SocketChannel socketChannel = (SocketChannel)key.channel();
    //創建一個ByteBuffer用於存放讀到的數據
    ByteBuffer readBuff = ByteBuffer.allocate(32);
    socketChannel.read(readBuff);
    readBuff.flip();
    //把buffer的極限設為容量
    buffer.limit(buffer.capacity());
    //把readBuff中的內容拷貝到buffer
    buffer.put(readBuff);
}

3. 處理寫就緒事件

public void send(SelectionKey key) throws IOException {
    //獲得與SelectionKey關聯的ByteBuffer
    ByteBuffer buffer = (ByteBuffer) key.attachment();
    //獲得與SelectionKey關聯的SocketChannel
    SocketChannel socketChannel = (SocketChannel) key.channel();
    buffer.flip();
    //按照GBK編碼把buffer中的位元組轉換為字元串
    String data = decode(buffer);
    //如果還沒有讀到一行數據就返回
    if(data.indexOf("\r\n") == -1)
        return;
    //截取一行數據
    String outputData = data.substring(0, data.indexOf("\n") + 1);
    //把輸出的字元串按照GBK編碼轉換為位元組，把它放在outputBuffer中
    ByteBuffer outputBuffer = encode("echo:" + outputData);
    //輸出outputBuffer的所有位元組
    while(outputBuffer,hasRemaining())
        socketChannel.write(outputBuffer);
    //把outputData字元審按照GBK編碼，轉換為位元組，把它放在ByteBuffer
    ByteBuffer temp = encode(outputData);
    //把buffer的位置設為temp的極限
    buffer.position(temp.limit()):
    //刪除buffer已經處理的數據
    buffer.compact();
    //如果已經輸出了字元串“bye\r\n”，就使SelectionKey失效，並關閉SocketChannel
    if(outputData.equals("bye\r\n")) {
        key.cancel();
        socketChannel.close();
    }
}

完整代碼如下：

public class EchoServer {
    
	private int port = 8000;
    private ServerSocketChannel serverSocketChannel = null;
    private Selector selector;
    private Charset charset = Charset.forName("GBK");

	public EchoServer() throws IOException {
        // 創建一個Selector對象
        selector = Selector.open();
        //創建一個ServerSocketChannel對象
        serverSocketChannel = ServerSocketChannel.open();
        //使得在同一個主機上關閉了伺服器程式，緊接著再啟動該伺服器程式時
        //可以順利綁定到相同的埠
        serverSocketChannel.socket().setReuseAddress(true);
        //使ServerSocketChannel工作於非阻塞模式
        serverSocketChannel.configureBlocking(false):
        //把伺服器進程與一個本地埠綁定
        serverSocketChannelsocket().bind(new InetSocketAddress(port));
    }
    
    public void service() throws IOException {
        serverSocketChannel.reqister(selector, SelectionKey.OP_ACCEPT);
        //第1層while迴圈
        while(selector.select() > 0) {
            //獲得Selector的selected-keys集合
            Set readyKeys = selector.selectedKeys();
            Iterator it = readyKeys.iterator();
            //第2層while迴圈
            while (it.hasNext()) {
                SelectionKey key = null;
                //處理SelectionKey
                try {
                    //取出一個SelectionKey
                    key = (SelectionKey) it.next();
                    //把 SelectionKey從Selector 的selected-key 集合中刪除
                    it.remove();
                    1f (key.isAcceptable()) {
                         //獲得與SelectionKey關聯的ServerSocketChannel
                        ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
                        //獲得與客戶連接的SocketChannel
                        SocketChannel socketChannel = (SocketChannel) ssc.accept();
                        //把Socketchannel設置為非阻塞模式
                        socketChannel.configureBlocking(false);
                        //創建一個用於存放用戶發送來的數據的級沖區
                        ByteBuffer buffer = ByteBuffer.allocate(1024);
                        //Socketchannel向Selector註冊讀就緒事件和寫就緒事件
                        socketChannel.register(selector, SelectionKey.OP_READ | SelectionKey.OP_WRITE, buffer);
                    }
                    if (key.isReadable()) { receive(key); }
                    if (key.isWritable()) { send(key); }
                } catch(IOException e) {
                    e.printStackTrace();
                    try {
                        if(key != null) {
                            //使這個SelectionKey失效
                            key.cancel();
                            //關閉與這個SelectionKey關聯的SocketChannel
                            key.channel().close();
                        }
                    } catch(Exception ex) { 
                        e.printStackTrace();
                    }
                }
            }
        }
    }
    
    public void receive(SelectionKey key) throws IOException {
        //獲得與SelectionKey關聯的附件
        ByteBuffer buffer = (ByteBuffer) key.attachment();
        //獲得與SelectionKey關聯的Socketchannel
        SocketChannel socketChannel = (SocketChannel)key.channel();
        //創建一個ByteBuffer用於存放讀到的數據
        ByteBuffer readBuff = ByteBuffer.allocate(32);
        socketChannel.read(readBuff);
        readBuff.flip();
        //把buffer的極限設為容量
        buffer.limit(buffer.capacity());
        //把readBuff中的內容拷貝到buffer
        buffer.put(readBuff);
    }
    
    public void send(SelectionKey key) throws IOException {
        //獲得與SelectionKey關聯的ByteBuffer
        ByteBuffer buffer = (ByteBuffer) key.attachment();
        //獲得與SelectionKey關聯的SocketChannel
        SocketChannel socketChannel = (SocketChannel) key.channel();
        buffer.flip();
        //按照GBK編碼把buffer中的位元組轉換為字元串
        String data = decode(buffer);
        //如果還沒有讀到一行數據就返回
        if(data.indexOf("\r\n") == -1)
            return;
        //截取一行數據
        String outputData = data.substring(0, data.indexOf("\n") + 1);
        //把輸出的字元串按照GBK編碼轉換為位元組，把它放在outputBuffer中
        ByteBuffer outputBuffer = encode("echo:" + outputData);
        //輸出outputBuffer的所有位元組
        while(outputBuffer,hasRemaining())
            socketChannel.write(outputBuffer);
        //把outputData字元審按照GBK編碼，轉換為位元組，把它放在ByteBuffer
        ByteBuffer temp = encode(outputData);
        //把buffer的位置設為temp的極限
        buffer.position(temp.limit()):
        //刪除buffer已經處理的數據
        buffer.compact();
        //如果已經輸出了字元串“bye\r\n”，就使SelectionKey失效，並關閉SocketChannel
        if(outputData.equals("bye\r\n")) {
            key.cancel();
            socketChannel.close();
        }
    }
    
    //解碼
    public String decode(ByteBuffer buffer) {
        CharBuffer charBuffer = charset.decode(buffer);
        return charBuffer.toStrinq();
    }
    
    //編碼
    public ByteBuffer encode(String str) {
        return charset.encode(str);
    }
    
    public static void main(String args[])throws Exception {
        EchoServer server = new EchoServer();
        server.service();
    }
}

阻塞模式與非阻塞模式混合使用

使用非阻塞模式時，ServerSocketChannel 以及 SocketChannel 都被設置為非阻塞模式，這使得接收連接、接收數據和發送數據的操作都採用非阻塞模式，EchoServer 採用一個線程同時完成這些操作

假如有許多客戶請求連接，可以把接收客戶連接的操作單獨由一個線程完成，把接收數據和發送數據的操作由另一個線程完成，這可以提高伺服器的併發性能

負責接收客戶連接的線程按照阻塞模式工作，如果收到客戶連接，就向 Selector 註冊讀就緒和寫就緒事件，否則進入阻塞狀態，直到接收到了客戶的連接。負責接收數據和發送數據的線程按照非阻塞模式工作，只有在讀就緒或寫就緒事件發生時，才執行相應的接收數據和發送數據操作

public class EchoServer {
    
	private int port = 8000;
    private ServerSocketChannel serverSocketChannel = null;
    private Selector selector = null;
    private Charset charset = Charset.forName("GBK");

	public EchoServer() throws IOException {
        selector = Selector.open();
        serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.socket().setReuseAddress(true);
        serverSocketChannelsocket().bind(new InetSocketAddress(port));
    }
    
    public void accept() {
        while(true) {
            try {
                SocketChannel socketChannel = serverSocketChannel.accept();
                socketChannel.configureBlocking(false);
                
                ByteBuffer buffer = ByteBuffer.allocate(1024);
                synchronized(gate) {
                    selector.wakeup();
                    socketChannel.register(selector, SelectionKey.OP_READ | SelectionKey.OP_WRITE, buffer);
                }
            } catch(IOException e) {
                e.printStackTrace();
            }
        }
    }
    
    private Object gate=new Object();
    
    public void service() throws IOException {
        while(true) {
            synchronized(gate){}
            int n = selector.select();
            if(n == 0) continue;
            Set readyKeys = selector.selectedKeys();
            Iterator it = readyKeys.iterator();
            while (it.hasNext()) {
                SelectionKey key = null;
                try {
    				it.remove();
                    if (key.isReadable()) {
                        receive(key);
                    }
                    if (key.isWritable()) {
                        send(key);
                    }
                } catch(IOException e) {
                    e.printStackTrace();
                    try {
                        if(key != null) {
                            key.cancel();
                            key.channel().close();
                        }
                    } catch(Exception ex) { e.printStackTrace(); }
                }
            }
        }
    }
    
    public void receive(SelectionKey key) throws IOException {
        ...
    }
    
    public void send(SelectionKey key) throws IOException {
        ...
    }
    
    public String decode(ByteBuffer buffer) {
        ...
    }
    
    public ByteBuffer encode(String str) {
        ...
    }
    
    public static void main(String args[])throws Exception {
        final EchoServer server = new EchoServer();
        Thread accept = new Thread() {
            public void run() {
                server.accept();
            }
        };
        accept.start();
		server.service();
    }
}

註意一點：主線程的 selector select() 方法和 Accept 線程的 register(...) 方法都會造成阻塞，因為他們都會操作 Selector 對象的共用資源 all-keys 集合，這有可能會導致死鎖

導致死鎖的具體情形是：Selector 中尚沒有任何註冊的事件，即 all-keys 集合為空，主線程執行 selector.select() 方法時將進入阻塞狀態，只有當 Accept 線程向 Selector 註冊了事件，並且該事件發生後，主線程才會從 selector.select() 方法返回。然而，由於主線程正在 selector.select() 方法中阻塞，這使得 Acccept 線程也在 register() 方法中阻塞。Accept 線程無法向 Selector 註冊事件，而主線程沒有任何事件可以監控，所以這兩個線程將永遠阻塞下去

為了避免對共用資源的競爭，同步機制使得一個線程執行 register() 時，不允許另一個線程同時執行 select() 方法，反之亦然