Java 网络编程 —— 异步通道和异步运算结果

从 JDK7 开始，引入了表示异步通道的 AsynchronousSockerChannel 类和 AsynchronousServerSocketChannel 类，这两个类的作用与 SocketChannel 类和 ServerSockelChannel 相似，区别在于异步通道的一些方法总是采用非阻塞模式，并且它们的非阻塞方法会立即返回一个 Future 对象，用来存放方法的异步运算结果

AsynchronousSocketChannel 类有以下非阻塞方法：

// 连接远程主机
Future<Void> connect(SocketAddress remote);
// 从通道中读入数据,存放到ByteBuffer中
// Future对象包含了实际从通道中读到的字节数
Future<Inleger> read(ByteBuffer dst);
// 把ByteBuffer的数据写入通道
// Future对象包含了实际写入通道的字节数
Future<Integer> write(ByteBuffer src);

AsynchronousServerSocketChannel 类有以下非阻塞方法：

// 接受客户连接请求
// Future对象包含连接建立成功后创建的AsynchronousSockelChannel对象
Future<AsynchronousSocketChannel> accept();

使用异步通道，可以使程序并行执行多个异步操作，例如：

SocketAddress socketAddress = ...;
AsynchronousSocketChannel client = AsynchronousSocketChannel.open();

//请求建立连接
Future<Void> connected = client.connect(socketAddress);
ByteBuffer byteBuffer = ByteBuffer.allocate(128);

//执行其他操作
//...

//等待连接完成
connected.get();

//读取数据
Future<Integer> future = client.read(byteBuffer);

//执行其他操作
//...

//等待从通道读取数据完成
future.get();

byteBuffer.flip();
WritableByteChannel out = Channels.newChannel(System.out);
out.write(byteBuffer);

下例的代码演示了异步通道的用法，它不断接收用户输入的域名并尝试建立连接，最后打印建立连接所花费的时间。如果程序无法连接到指定的主机，就打印相关错误信息。如果用户输入 bye，就结束程序

//表示连接一个主机的结果
class PingResult {

    InetSocketAddress address;
    long connectStart; //开始连接时的时间
    long connectFinish = 0; //连接成功时的时间
    String failure;
    Future<Void> connectResult; //连接操作的异步运算结果
    AsynchronousSocketChannel socketChannel;
    String host;
    final String ERROR = "连接失败";

    PingResult(String host) {
        try {
            this.host = host;
            address = new InetSocketAddress(InetAddress.getByName(host), 80);
        } catch (IOException x) {
            failure = ERROR;
        }
    }

    //打印连接一个主机的执行结果
    public void print() {
        String result;
        if (connectFinish != 0) {
            result = Long.toString(connectFinish - connectStart) + "ms";
        } else if (failure != null) {
			result = failure;
        } else {
            result = "Timed out";
        }
        System,out,println("ping "+ host + "的结果" + ":" + result);
    }

    public class PingClient {
        //存放所有PingResult结果的队列
        private LinkedList<PingResult> pingResults = new Linkedlist<PingResult>();
        boolean shutdown = false;
        ExecutorService executorService;

        public PingClient() throws IOException {
            executorService = Executors.newFixedThreadPool(4);
            executorService.execute(new Printer());
            receivePingAddress();
        }
    }

    public static void main(String args[]) throws IOException {
        new PingClient();
    }

    /*接收用户输入的主机地址，由线程池执行PingHandler任务 */
    public void receivePingAddress() {
        try {
            BufferedReader localReader = new BufferedReader(new InputStreamReader(System.in));
            String msg = null;
            //接收用户输入的主机地址
            while((msg = localReader.readLine()) != null) {
                if(msg.equals("bye")) {
                    shutdown = true;
                    executorService.shutdown();
                    break;
                }
                executorService.execute(new PingHandler(msg));
            }
        } catch(IOException e) {}
    }

    /* 尝试连接特定主机,生成一个PingResult对象,把它加入PingResults结果队列中 */
    public class PingHandler implements Runnable {
        String msg;
        public PingHandler(String msg) {
            this.msg = msg;
        }
        public void run() {
            if(!msg.equals("bye")) {
                PingResult pingResult = new PingResult(msg);
                AsynchronousSocketChannel socketChannel = null;
                try {
                    socketChannel = AsynchronousSocketChannel.open();
                    pingResult.connectStart = System.currentTimeMillis();
                    synchronized (pingResults) {
                        //向pingResults队列加入一个PingResult对象
                        pingResults.add(pingResult);
                        pingResults,notify();
                    }
                    Future<Void> connectResult = socketChannel.connect(pingResult.address);
                    pingResult.connectResult = connectResult;
                } catch (Exception x) {
                    if (socketChannel != null) {
                        try { socketChannel.close();} catch (IOException e) {)
                    }
                    pingResult.failure = pingResult.ERROR;
                }
            }
        }
    }

    /* 打印PingResults结果队列中已经执行完毕的任务的结果 */
    public class Printer implements Runnable {
        public void run() {
            PingResult pingResult = null;
            while(!shutdown) {
                synchronized (pingResults) {
                    while (!shutdown && pingResults.size() == 0 ) {
                        try {
                            pingResults.wait(100);
                        } catch(InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    if(shutdown && pingResults.size() == 0 ) break;
                    pingResult = pingResults.getFirst();

                    try {
                        if(pingResult.connectResult != null) {
                            pingResult.connectResult.get(500, TimeUnit,MILLISECONDS);
                        } catch(Exception e) {
                            pingResult.failure = pingResult.ERROR;
                        }
                    }

                    if(pingResult.connectResult != null && pingResult.connectResult.isDone()) {
                        pingResult.connectFinish = System.currentTimeMillis();
                    }

                    if(pingResult,connectResult != null && pingResult.connectResult.isDone() || || pingResult,failure != null) {
                        pingResult.print();
                        pingResults.removeFirst();
                        try {
                            pingResult.socketChannel.close();
                        } catch (IOException e) {}
                    }
                }
            }
        }
    }
}

PingClient 类定义了两个表示特定任务的内部类：

• PingHandler：负责通过异步通道去尝试连接客户端输入的主机地址，并且创建一个 PingResult 对象，它包含了连接操作的异步运算结果，再将其加入 PingResults 结果队列
• Printer：负责打印 PingResults 结果队列已经执行完毕的任务结果，打印完毕的 PingResult 对象会从队列中删除