JavaIO和JavaNIO

2019-11-14 15:00:33

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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实现非阻塞服务

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

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。

这样会有什么问题呢？

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

那么NIO是如何帮助我们解决这种问题的呢？

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。

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);    }}