JAVA中的BIO、NIO、EPOLL

前几天听了清华周老师讲的nio的课，感觉讲得很清楚，所以记录一下，一方面自己理解一次，加深印象，另一方面，防止后面忘记，时长回顾；以下是自己的理解内容，表达能力有限，仅供个人参考：

一、BIO

BIO：同其名字，阻塞IO，accept、read、connect、write操作都会阻塞；

                            

 

 BIO代码示例：

public class NIOExample {

    final static int PORT = 8088;

    final static ThreadPoolExecutor executor =
            new ThreadPoolExecutor(3,3,3L, TimeUnit.SECONDS,new LinkedBlockingQueue<Runnable>(100));


    public static void main(String[] args) {
       final ConcurrentLinkedQueue<SocketChannel> clients = new ConcurrentLinkedQueue();
        try {
            ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
            serverSocketChannel.bind(new InetSocketAddress(PORT));
            //配置非阻塞
　　　　　　　serverSocketChannel.configureBlocking(false);
            InetSocketAddress local = (InetSocketAddress)serverSocketChannel.getLocalAddress();
            System.out.println(local.getAddress());
            //一个线程负责处理有效连接上的数据
　　　　　　new Thread(new Runnable(){
                public void run() {
                    connectionHandle(clients);
                }
            }).start();
            //一个线程负责接收连接
　　　　　　while(true){
                //非阻塞，没有客户端连接进来，返回-1，BIO会一直阻塞
　　　　　　　　SocketChannel client = serverSocketChannel.accept();
                if(client != null){
                    client.configureBlocking(false);
                    clients.add(client);
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static void connectionHandle(ConcurrentLinkedQueue<SocketChannel> clients) {
        while(true){
            final SocketChannel client = clients.poll();
            final ByteBuffer byteBuffer = ByteBuffer.allocate(512);
            if(client != null){

                executor.execute(new Runnable() {
                    public void run() {
                        try {
                            InetSocketAddress remoteAddress = (InetSocketAddress)client.getRemoteAddress();
                            System.out.println("收到消息，对方端口号："+remoteAddress.getPort());
                            client.read(byteBuffer);
                            byteBuffer.flip();
                            System.out.println("收到消息:"+new String(byteBuffer.array()));
                        } catch (IOException e) {
                            e.printStackTrace();
                        }

                    }
                });
            }
        }
    }
}

三、epoll

在系统内核中select、poll、epoll都是多路复用器，java中对多路复用器的封装是在Selector中；

     

selector代码示例：

public class SelectorExample {

    public static void main(String[] args) throws IOException {
        Selector selector = Selector.open();
        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        //selector多路复用器必须是非阻塞的
        serverSocketChannel.configureBlocking(false);
        serverSocketChannel.bind(new InetSocketAddress("127.0.0.1",8088));
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

        Set<SelectionKey> selectionKeys = selector.selectedKeys();

        Iterator<SelectionKey> iterator = selectionKeys.iterator();
        ByteBuffer readBuff = ByteBuffer.allocate(1024);
        ByteBuffer writeBuff = ByteBuffer.allocate(1024);
        while(iterator.hasNext()){
            try {
                SelectionKey selectionKey = iterator.next();
                if(selectionKey.isAcceptable()){
                    //接收到连接。做什么事
                    // 创建新的连接，并且把连接注册到selector上，而且，
                    // 声明这个channel只对读操作感兴趣。
                    SocketChannel socketChannel = serverSocketChannel.accept();
                    socketChannel.configureBlocking(false);
                    socketChannel.register(selector, SelectionKey.OP_READ);
                }else if(selectionKey.isReadable()){
                    //客户端有数据发送过来，读数据
                    SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                    readBuff.clear();
                    socketChannel.read(readBuff);

                    readBuff.flip();
                    System.out.println("received : " + new String(readBuff.array()));
                    selectionKey.interestOps(SelectionKey.OP_WRITE);
                }else if(selectionKey.isWritable()){
                    writeBuff.rewind();
                    SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                    socketChannel.write(writeBuff);
                    selectionKey.interestOps(SelectionKey.OP_READ);
                }
            } finally {
                iterator.remove();
            }
        }

    }
}

 

epoll的总结：

1.Server向selector注册事件

2.selector调用系统内核，系统内核调用epoll_create方法为绑定的端口分配一块内存

3.当用channel连接进来时，会记录channel的元数据信息，调用内核方法epoll_ctl方法，将信道元数据保存到内存中，

4.当信道有数据传送过来时，系统内核将传送的数据保存到channel对应的queue中

5.selector调用系统内核epoll_wait方法，对queue中存放的数据进行读取