关于java并发编程

//类receiveServer
import java.io.*;
import java.util.*;
import java.net.*;

public class recieveServer {
    final int RECEIVE_PORT = 9090; // 该服务器的端口号

    // receiveServer的构造器
    public recieveServer() {
        ServerSocket rServer = null; // ServerSocket的实例
        Socket request = null; // 用户请求的套接字
        Thread receiveThread = null;
        try {
            rServer = new ServerSocket(RECEIVE_PORT); // 初始化ServerSocket
            System.out.println("Welcome to the server!");
            System.out.println(new Date());
            System.out.println("The server is ready!");
            System.out.println("Port: " + RECEIVE_PORT);
            while (true) { // 等待用户请求
                request = rServer.accept(); // 接收客户机连接请求
                receiveThread = new serverThread(request); // 生成serverThread的实例
                receiveThread.start();// 启动serverThread线程
            }
        } catch (IOException e) {
            System.out.println(e.getMessage());
        }
    }

    public static void main(String args[]) {
        new recieveServer();
    } // end of main

} // end of class

//类serverThread
class serverThread extends Thread {

    Socket clientRequest; // 用户连接的通信套接字
    BufferedReader input; // 输入流
    PrintWriter output; // 输出流

    public serverThread(Socket s) { // serverThread的构造器
        this.clientRequest = s; // 接收receiveServer传来的套接字
        InputStreamReader reader;
        OutputStreamWriter writer;
        try { // 初始化输入、输出流
            reader = new InputStreamReader(clientRequest.getInputStream());
            writer = new OutputStreamWriter(clientRequest.getOutputStream());
            input = new BufferedReader(reader);
            output = new PrintWriter(writer, true);
        } catch (IOException e) {
            System.out.println(e.getMessage());
        }
        output.println("Welcome to the server!");
        // 客户机连接欢迎词
        output.println("Now is:" + new java.util.Date() + " " + "Port:"
                + clientRequest.getLocalPort());
        output.println("What can I do for you?");
    }

    public void run() { // 线程的执行方法
        String command = null; // 用户指令
        String str = null;
        boolean done = false;

        while (!done) {
            try {
                str = input.readLine(); // 接收客户机指令
            } catch (IOException e) {
                System.out.println(e.getMessage());
            }
            command = str.trim().toUpperCase();
            if (str == null || command.equals("QUIT")) // 命令quit结束本次连接
                done = true;
            else if (command.equals("HELP")) { // 命令help查询本服务器可接受的命令
                output.println("query");
                output.println("quit");
                output.println("help");
            } else if (command.startsWith("QUERY")) { // 命令query
                output.println("OK to query something!");
            }
            // else if …….. //在此可加入服务器的其他指令
            else if (!command.startsWith("HELP") && !command.startsWith("QUIT")
                    && !command.startsWith("QUERY")) {
                output.println("Command not Found!Please refer to the HELP!");
            }
        }// end of while
        try {
            clientRequest.close(); // 关闭套接字
        } catch (IOException e) {
            System.out.println(e.getMessage());
        }
        command = null;
    }// end of run
}