Java NIO 基础四 选择器

从最基础的层面上来看,选择器提供了问询通道是否就绪操作I/O的能力,选择器可以监控注册在上面的多个通道,通道注册时会返回选择键(记录通道与选择器之间的关联关系),选择器管理者这些注册的键、和就绪状态键的集合

SelectableChannel
所有继承SelectableChannel的通道都可以在选择器中注册,FileChannel没有继承这个类,所以无法使用选择器

选择键(SelectionKey) 选择键是选择器的重点内容,选择器就绪的通道通过返回选择键集合来通知

public abstract class SelectionKey {
    public static final int OP_READ
    public static final int OP_WRITE
    public static final int OP_CONNECT
    public static final int OP_ACCEPT
    public abstract SelectableChannel channel();
    public abstract Selector selector();
    public abstract void cancel();
    public abstract boolean isValid();
    public abstract int interestOps();
    public abstract void interestOps(int ops);
    public abstract int readyOps();
    public final boolean isReadable()
    public final boolean isWritable()
    public final boolean isConnectable()
    public final boolean isAcceptable()
    public final Object attach(Object ob)
    public final Object attachment()
}

选择键维护了通道和选择器之间的关联,可以通过选择键获取Channel或Selector,键对象表示一种特殊的关联关系,当这种关系需要终止时,可以调用cancel()方法取消,调用这个方法时,不会立即被取消,而是将这个键放到被取消的集合里,当Selector下次调用select()方法时会真正被清理掉。当通道关闭时,选择键会自动被取消,当选择器关闭时,所有键都会被清理掉。

一个选择器键包含有两个准备好的操作集合,包括感兴趣的事件集合instrest和就绪的操作集合ready,通过掩码保存

感兴趣的事件集合interestOps()
通常一个键的instrest注册时就已经确认,但是可以在注册后通过interestOps(newOps)传入一个新的ops来改变这个值

channel.register(this.selector, SelectionKey.OP_READ);

上面的代码注册的键interest包含read事件,可以在对通道IO异步处理时,改变这个ops来临时取消对read事件的关注,以防止重复处理未处理完的通道

就绪的操作集合readyOps()
通过这个方法返回就绪的操作,isReadable( ),isWritable( ),isConnectable( ), 和isAcceptable( )用来判断这些操作是否就绪,进行下一步的处理

下面列举两个示例来示范选择器的使用
单选择器单线程

public abstract class AbstractNioServer {
    protected final static String CHARSET = "utf-8";
 protected String ip;
 protected Integer port;
 protected Selector selector;
 public AbstractNioServer(String ip, Integer port) {
        this.ip = ip;
 this.port = port;
 }
    /**
 * 客户端连接请求
 *
 * @param key
 */
 protected abstract void accept(SelectionKey key) throws IOException;
 /**
 * 读取数据
 *
 * @param key
 */
 protected abstract void read(SelectionKey key) throws IOException;
 /**
 * 初始化服务器
 *
 * @throws IOException
 */ public void init() throws IOException {
        //设置服务器地址端口
 SocketAddress address = new InetSocketAddress(this.ip, this.port);
 //创建服务端通道
 ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
 //绑定服务器地址
 serverSocketChannel.bind(address);
 //设置为非阻塞模式
 serverSocketChannel.configureBlocking(false);
 //创建一个选择器
 this.selector = Selector.open();
 //将服务器通道注册到选择器中,ServerSocketChannel只支持accept事件注册,validOps返回16
 serverSocketChannel.register(this.selector, SelectionKey.OP_ACCEPT);
 }
    public void start() throws IOException {
        this.init();
 while (true) {
            int count = this.selector.select();
 if (count == 0) {
                //没有就绪的选择键
 continue;
 }
            Iterator iterator = this.selector.selectedKeys().iterator();
 while (iterator.hasNext()) {
                SelectionKey key = iterator.next();
 if (!key.isValid()) {
                    continue;
 }
                if (key.isAcceptable()) {
                    //连接请求
 accept(key);
 } else if (key.isReadable()) {
                    //修改键的感兴趣事件,防止被 select 重复调用,处理完事件后及时恢复
 key.interestOps(key.interestOps() & (~SelectionKey.OP_READ));
 //读取消息
 read(key);
 }
                iterator.remove();
 }
        }
    }
    /**
 * 恢复键的感兴趣事件
 * @param key
 */
 protected void resumeInterOpsRead(SelectionKey key) {
        //还原key的感兴趣事件
 key.interestOps(key.interestOps() | SelectionKey.OP_READ);
 //唤醒selector的select事件
 key.selector().wakeup();
 }
}
public class SingleNioServer extends AbstractNioServer {
    public static void main(String[] args) {
        SingleNioServer server = new SingleNioServer("127.0.0.1", 1008);
 try {
            server.start();
 } catch (IOException e) {
            e.printStackTrace();
 }
    }
    public SingleNioServer(String ip, Integer port) {
        super(ip, port);
 }
    @Override
 protected void accept(SelectionKey key) throws IOException {
        ServerSocketChannel serverChannel = (ServerSocketChannel) key.channel();
 //SocketChannel支持 read、write、connect 事件注册,validOps返回13=1+4+8
 SocketChannel channel = serverChannel.accept();
 if (channel == null) {
            return;
 }
        System.out.println("新的连接请求");
 channel.configureBlocking(false);
 //如果是阻塞通道进行注册,会抛出 IllegalBlockingModeException 异常
 channel.register(this.selector, SelectionKey.OP_READ);
 }
    @Override
 protected void read(SelectionKey key) throws IOException {
        SocketChannel channel = (SocketChannel) key.channel();
 try {
            ByteBuffer buffer = ByteBuffer.allocate(1024);
 int len = channel.read(buffer);
 buffer.flip();
 if (len > 0) {
                String str = Charset.forName(CHARSET).decode(buffer).toString();
 System.out.println("客户端消息:" + str);
 String msg = "消息已收到";
 byte[] sendData = msg.getBytes(CHARSET);
 ByteBuffer sendBuffer = ByteBuffer.wrap(sendData);
 channel.write(sendBuffer);
 super.resumeInterOpsRead(key);
 } else if (len == -1) {
                System.out.println("socket client close");
 key.cancel();
 channel.close();
 }
        } catch (IOException ex) {
            key.cancel();
 channel.close();
 }
    }
}

单选择器多线程

public class MulitpleNioServer extends AbstractNioServer {
    public static void main(String[] args) throws IOException {
        MulitpleNioServer server = new MulitpleNioServer("127.0.0.1", 1008);
 server.start();
 }
    /**
 * 线程池
 */
 private ExecutorService executorService = Executors.newFixedThreadPool(5);
 public MulitpleNioServer(String ip, Integer port) {
        super(ip, port);
 }
    @Override
 protected void accept(SelectionKey key) throws IOException {
        ServerSocketChannel serverChannel = (ServerSocketChannel) key.channel();
 //SocketChannel支持 read、write、connect 事件注册,validOps返回13=1+4+8
 SocketChannel channel = serverChannel.accept();
 System.out.println("新的连接请求");
 channel.configureBlocking(false);
 channel.register(this.selector, SelectionKey.OP_READ);
 }
    @Override
 protected void read(SelectionKey key) throws IOException {
        executorService.submit(new Runnable() {
            @Override
 public void run() {
                readData(key);
 }
        });
 }
    private void readData(SelectionKey key) {
        SocketChannel channel = (SocketChannel) key.channel();
 try {
            ByteBuffer buffer = ByteBuffer.allocate(1024);
 if (channel.isOpen()) {
                if (channel.isConnected()) {
                    int len = channel.read(buffer);
 buffer.flip();
 if (len > 0) {
                        String str = Charset.forName(CHARSET).decode(buffer).toString();
 System.out.println("客户端消息:" + str);
 String msg = "消息已收到";
 byte[] sendData = msg.getBytes(CHARSET);
 ByteBuffer sendBuffer = ByteBuffer.wrap(sendData);
 channel.write(sendBuffer);
 } else if (len == -1) {
                        System.out.println("socket client close1");
 key.cancel();
 channel.close();
 }
                    super.resumeInterOpsRead(key);
 }
            }
        } catch (IOException ex) {
            System.out.println("client is close2");
 key.cancel();
 try {
                channel.close();
 } catch (IOException e) {
                e.printStackTrace();
 }
        }
    }
}

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