JavaIO和JavaNIO

2019-11-14 15:00:33

字體：大中小

供稿：網(wǎng)友

1、IO的例子

/* 字節(jié)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)//讀到緩存        {            buffer.flip();//指針跳到緩存頭            foutChannel.write(buffer);            buffer.clear();//重置緩沖區(qū)        }        fin.close();        fout.close();    }

3、NIO實現(xiàn)非阻塞服務(wù)

java自1.4以后，加入了新IO特性NIO，NIO帶來了non-blocking特性。

在之前的服務(wù)器處理模型中，在調(diào)用ServerSocket.accept()方法時，會一直阻塞到有客戶端連接才會返回，每個客戶端連接過來后，服務(wù)端都會accept一個新連接，接著啟動一個線程去處理該客戶端的請求。在這個新的線程中，也會在read()方法中阻塞，直到讀取完數(shù)據(jù)。

這樣會有什么問題呢？

阻塞導(dǎo)致大量線程資源被浪費；阻塞可導(dǎo)致大量的上下文切換，很多切換其實是無意義的

那么NIO是如何幫助我們解決這種問題的呢？

1). 由一個專門的線程來處理所有的 IO 事件，并負責(zé)分發(fā)。
2). 事件驅(qū)動機制：事件到的時候觸發(fā)，而不是同步的去監(jiān)視事件。
3). 線程通訊：線程之間通過 wait,notify 等方式通訊。保證每次上下文切換都是有意義的。減少無謂的線程切換。

服務(wù)端和客戶端各自維護一個管理通道的對象，我們稱之為selector，該對象能檢測一個或多個通道 (channel) 上的事件。我們以服務(wù)端為例，如果服務(wù)端的selector上注冊了讀事件，某時刻客戶端給服務(wù)端發(fā)送了一些數(shù)據(jù)，NIO的服務(wù)端會在selector中添加一個讀事件。服務(wù)端的處理線程會輪詢地訪問selector，如果訪問selector時發(fā)現(xiàn)有感興趣的事件到達，則處理這些事件，如果沒有感興趣的事件到達，則處理線程會一直阻塞直到感興趣的事件到達為止。

下面是一個NIO實現(xiàn)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);    }}