JavaIO和JavaNIO

BIO和NIO

BIO在之前的服务器处理模型中，在调用ServerSocket.accept()方法时，会一直阻塞到有客户端连接才会返回，每个客户端连接过来后，服务端都会accept一个新连接，接着启动一个线程去处理该客户端的请求。在这个新的线程中，也会在read()方法中阻塞，直到读取完数据，处理完成后销毁该处理线程。

这样会有什么问题呢？

当客户端并发访问增加后，服务端线程个数膨胀，频繁出现由于IO阻塞导致挂起的线程，系统性能将急剧下降，容易发生线程堆栈溢出、创建新线程失败等问题。

阻塞导致大量线程资源被浪费；阻塞可导致大量的上下文切换，很多切换其实是无意义的

Java自1.4以后，加入了新IO特性NIO，NIO带来了non-blocking特性。

那么NIO是如何帮助我们解决这种问题的呢（反应器设计模式）？

1). 由一个专门的线程来处理所有的 IO 事件，并负责分发。 
2). 事件驱动机制：事件到的时候触发，而不是同步的去监视事件。 
3). 线程通讯：线程之间通过 wait,notify 等方式通讯。保证每次上下文切换都是有意义的。减少无谓的线程切换。

服务端和客户端各自维护一个管理通道的对象，我们称之为selector，该对象能检测一个或多个通道 (channel) 上的事件。我们以服务端为例，如果服务端的selector上注册了读事件，某时刻客户端给服务端发送了一些数据，NIO的服务端会在selector中添加一个读事件。服务端的处理线程会轮询地访问selector，如果访问selector时发现有感兴趣的事件到达，则处理这些事件，如果没有感兴趣的事件到达，则处理线程会一直阻塞直到感兴趣的事件到达为止。

1、IO的例子

/* 字节IO */
    public void byteIO() throws FileNotFoundException, IOException {
        FileInputStream fin = new FileInputStream(new File(
                "D:\\test\\byteio_in.txt"));
        FileOutputStream fout = new FileOutputStream(new File(
                "D:\\test\\byteio_out.txt"));
        int c = -1;
        while ((c = fin.read()) != -1) {
            fout.write(c);
        }
        fin.close();
        fout.close();
    }

    /* 字符IO */
    public void charIO() throws FileNotFoundException, IOException {
        FileReader reader = new FileReader(new File("D:\\test\\chario_in.txt",
                ""));
        FileWriter writer = new FileWriter(new File("D:\\test\\chario_out.txt"));
        char[] charArr = new char[512];
        while (reader.read(charArr) != -1) {
            writer.write(charArr);
        }
        reader.close();
        writer.close();
    }

    /* bufferIO */
    public void bufferIO() throws FileNotFoundException, IOException {
        BufferedInputStream bufferReader = new BufferedInputStream(
                new FileInputStream("D:\\test\\bufferio_in.txt"));
        BufferedOutputStream bufferWriter = new BufferedOutputStream(
                new FileOutputStream("D:\\test\\bufferio_out.txt"));
        int c = -1;
        while ((c = bufferReader.read()) != -1) {
            bufferWriter.write(c);
        }
        bufferReader.close();
        bufferWriter.close();
    }

2、NIO的例子

/* NIO */
    public void NIO() throws FileNotFoundException, IOException {
        FileInputStream fin = new FileInputStream("D:\\test\\nio_in.txt");
        FileOutputStream fout = new FileOutputStream("D:\\test\\nio_out.txt");
        FileChannel finChannel = fin.getChannel();
        FileChannel foutChannel = fout.getChannel();
        ByteBuffer buffer = ByteBuffer.allocate(512);
        while (finChannel.read(buffer) != 1)//读到缓存
        {
            buffer.flip();//指针跳到缓存头
            foutChannel.write(buffer);
            buffer.clear();//重置缓冲区
        }
        fin.close();
        fout.close();
    }

3、NIO实现非阻塞Server服务

下面是一个NIO实现Server的例子

public class MultiPortEcho {
    private int ports[];
    private ByteBuffer echoBuffer = ByteBuffer.allocate(1024);

    public MultiPortEcho(int ports[]) throws IOException {
        this.ports = ports;
        go();
    }

    private void go() throws IOException {
        // Create a new selector
        Selector selector = Selector.open();

        // Open a listener on each port, and register each one with the selector
        for (int i = 0; i < ports.length; ++i) {
            ServerSocketChannel ssc = ServerSocketChannel.open();
            ssc.configureBlocking(false);
            ServerSocket ss = ssc.socket();
            InetSocketAddress address = new InetSocketAddress(ports[i]);
            ss.bind(address);
            SelectionKey key = ssc.register(selector, SelectionKey.OP_ACCEPT);
            System.out.println("Going to listen on " + ports[i]);
        }

        while (true) {
            int num = selector.select();
            Set selectedKeys = selector.selectedKeys();
            Iterator it = selectedKeys.iterator();

            while (it.hasNext()) {
                SelectionKey key = (SelectionKey) it.next();

                if ((key.readyOps() & SelectionKey.OP_ACCEPT) == SelectionKey.OP_ACCEPT) {
                    // Accept the new connection
                    ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
                    SocketChannel sc = ssc.accept();
                    sc.configureBlocking(false);

                    // Add the new connection to the selector
                    SelectionKey newKey = sc.register(selector,SelectionKey.OP_READ);
                    it.remove();

                    System.out.println("Got connection from " + sc);
                } else if ((key.readyOps() & SelectionKey.OP_READ) == SelectionKey.OP_READ) {
                    // Read the data
                    SocketChannel sc = (SocketChannel) key.channel();

                    // Echo data
                    int bytesEchoed = 0;
                    while (true) {
                        echoBuffer.clear();
                        int r = sc.read(echoBuffer);
                        if (r <= 0) {
                            break;
                        }
                        echoBuffer.flip();
                        sc.write(echoBuffer);
                        bytesEchoed += r;
                    }

                    System.out.println("Echoed " + bytesEchoed + " from " + sc);
                    it.remove();
                }

            }
        }
    }

    static public void main(String args[]) throws Exception {

        int ports[] = {1234,6765,7987};

        for (int i = 0; i < args.length; ++i) {
            ports[i] = Integer.parseInt(args[i]);
        }

        new MultiPortEcho(ports);
    }
}

参考资料:

http://www.ibm.com/developerworks/cn/education/java/j-nio/j-nio.html#ibm-pcon

http://weixiaolu.iteye.com/blog/1479656