伪异步IO编程

╰半夏微凉° 2022-05-12 01:04 149阅读 0赞

为了解决BIO（同步阻塞IO）带来的一个链路需要一个线程处理的问题，后来有人对它的线程模型进行了优化，后端通过一个线程池来处理多个客户端的接入，从而形成了一个客户端个数M：线程池最大数N的关系。M可以远大于N，通过线程池可以灵活的调整线程资源，设置线程的最大值，防止由于大量并发导致资源耗尽。  
  
服务端代码：

/** * @author j.tommy * Created by j.tommy on 2017/11/11. */
    public class TimeServer { 
        private ThreadFactory namedThreadFactory = new ThreadFactoryBuilder().setNameFormat("demo-pool-%d").build();
        private ExecutorService es = new ThreadPoolExecutor(1,100,0L, TimeUnit.MICROSECONDS,new LinkedBlockingQueue<Runnable>(1024),namedThreadFactory,new ThreadPoolExecutor.AbortPolicy());
        public static void main(String[] args) {
            TimeServer timeServer = new TimeServer();
            timeServer.start(8080);
        }
        private void start(int port) {
            ServerSocket ss = null;
            try {
                ss = new ServerSocket(port);
                System.out.println("TimeServer is running...");
                while (true) {
                    Socket socket = ss.accept();
                    es.submit(new TimeServerHandler(socket));
                }
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                if (ss != null) {
                    try {
                        ss.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    es.shutdownNow();
                }
            }
        }
        class TimeServerHandler implements Runnable {
            private Socket socket = null;
            public TimeServerHandler(Socket socket) {
                this.socket = socket;
            }
            @Override
            public void run() {
                BufferedReader br = null;
                PrintWriter pw = null;
                try {
                    br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                    pw = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()),true);
                    while (true) {
                        String input = br.readLine();
                        if (null == input) {
                            break;
                        }
                        System.out.println("接收到客户端请求：" + input);
                        if ("Server Time".equalsIgnoreCase(input)) {
                            pw.println((new Date()).toString());
                        }
                    }
                } catch (IOException e) {
                    System.out.println("ip:" + socket.getInetAddress().getHostAddress() + " I/O异常");
                }
                finally {
                    if (br != null) {
                        try {
                            br.close();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                    if (pw != null) {
                        pw.close();
                    }
                    if (socket != null) {
                        try {
                            socket.close();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        }
    }

客户端代码：

/** * @author j.tommy * Created by j.tommy on 2017/11/11. */
    public class TimeClient { 
        public static void main(String[] args) {
            TimeClient tc = new TimeClient();
            tc.connect("127.0.0.1",8080);
        }
        private void connect(String host,int port) {
            Socket socket = null;
            InputStream in = null;
            OutputStream out = null;
            try {
                socket = new Socket(host,port);
                in = socket.getInputStream();
                out = socket.getOutputStream();
                PrintWriter pw = new PrintWriter(new OutputStreamWriter(out),true);
                BufferedReader br = new BufferedReader(new InputStreamReader(in));
                System.out.println("请求服务器时间");
                pw.println("Server Time");
                String response = br.readLine();
                System.out.println("response:" + response);
            } catch (IOException e) {
                e.printStackTrace();
            }
            finally {
                if (socket != null) {
                    try {
                        socket.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
                if (in != null) {
                    try {
                        in.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
                if (out != null) {
                    try {
                        out.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }

伪异步IO通过线程池+任务队列来实现，当有新的客户端接入时，将客户端封装成一个新的Task，交给线程池处理。由于线程池和任务队列的大小都是可控制的，所以不论有多少个客户端访问，都不会导致资源耗尽和宕机。

伪异步IO的弊端：  
当对Socket的输入流进行读取时，它会一直阻塞，直到发生下面3种事件之一：  
1.有数据可读；  
2.可用数据已经读取完毕；  
3.发生空指针或IO异常。

这就意味着，当对方发送请求或应答消息缓慢、或者网络传输较慢时，读取输入流一方的线程将被长时间阻塞。在此期间，其他接入消息只能在任务队列中排队。

参考《Netty权威指南》