Netty权威指南之伪异步I/O编程

为了解决同步阻塞I/O一个链路需要一个线程处理问题，对BIO模型做了优化——后端通过一个线程池处理多个客户端的请求接入，设置线程最大值，防止线程并发接入导致的线程耗尽。

当有新的客户端接入时，将客户端Socket封装成一个Task（该任务实现java.lang.Runnable接口）投递到后端的线程池中进行处理，JDK的线程池维护一个消息队列和N个活跃线程，对消息队列中的任务进行处理。由于线程池可以设置消息队列的大小和最大线程数，因此资源占用是可控的，无论多少客户端并发访问都不会导致资源耗尽和宕机。

伪异步I/O通信框架采用了线程池实现，避免了每次请求创建一个独立的线程造成的资源耗尽问题，底层通信依然采用BIO同步阻塞模型，无法从根本上解决问题。当对方发送请求或者应答消息比较缓慢（如数据大），或者网路传输较慢时（网速差）读取输入流一方的通信线程将被长时间阻塞，阻塞期间，其他接入消息只能在消息队列中排队。阻塞结束情况：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

package com.hjp.netty.pseudoasynio;


import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class TimeServerHandlerExecutePool {

    private ExecutorService executor;

    public TimeServerHandlerExecutePool(int maxPoolSize, int queueSize) {
        executor = new ThreadPoolExecutor(Runtime.getRuntime().availableProcessors(), maxPoolSize, 120L, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(queueSize));
    }

    public void execute(Runnable task) {
        executor.execute(task);
    }

}

package com.hjp.netty.pseudoasynio;

import sun.awt.windows.ThemeReader;

import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;

public class TimeServer {

    public static void main(String[] args) throws IOException {
        int port = 8080;
        if (args != null && args.length > 0) {
            try {
                port = Integer.valueOf(port);
            } catch (NumberFormatException e) {
                e.printStackTrace();
            }
        }
        ServerSocket server = null;
        try {
            server = new ServerSocket(port);
            System.out.println("The time server is start in port : " + port);
            Socket socket = null;
            TimeServerHandlerExecutePool singleExecutor = new TimeServerHandlerExecutePool(50, 10000);
            while (true) {
                socket = server.accept();
                singleExecutor.execute(new TimeServerHandler(socket));
            }
        } finally {
            if (server != null) {
                System.out.println("The time server close");
                server.close();
                server = null;
            }
        }
    }

}

package com.hjp.netty.pseudoasynio;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Date;

public class TimeServerHandler implements Runnable {

    private Socket socket;

    public TimeServerHandler(Socket socket) {
        this.socket = socket;
    }

    @Override
    public void run() {
        BufferedReader in = null;
        PrintWriter out = null;
        try {
            in = new BufferedReader(new InputStreamReader(this.socket.getInputStream()));
            out = new PrintWriter(this.socket.getOutputStream(), true);
            String currentTime = null;
            String body = null;
            while (true) {
                body = in.readLine();
                if (body == null) {
                    break;
                }
                System.out.println("The time server receive order : " + body);
                currentTime = "QUERY TIME ORDER".equalsIgnoreCase(body) ? new Date(System.currentTimeMillis()).toString() : "BAD ORDER";
                out.println(currentTime);
            }
        } catch (Exception e) {
            if (in != null) {
                try {
                    in.close();
                } catch (IOException e1) {
                    e1.printStackTrace();
                }
            }
            if (out != null) {
                out.close();
                out = null;
            }
            if (this.socket != null) {
                try {
                    this.socket.close();
                } catch (IOException e1) {
                    e1.printStackTrace();
                }
                this.socket = null;
            }
        }
    }
}

客户端代码和BIO客户端代码一样