两个程序间通信（一）：Java Socket

File server and client

GENERAL USAGE
—————————————

• Start server.java first by typing “java server” in a terminal window.

• Start the client.java by typing “java client ‘host-name’ ‘file-name’” in a terminal
  window where host-name is the host to use and file-name is the required file.

• 这个是单线程处理，接受连接请求和处理连接是基本的业务逻辑。

• 多线程和线程池的解决方案

Server：

//*****************************************************************
// server.java 
//
// Allows clients to connect and request files.  If the file
// exists it sends the file to the client.  
//*****************************************************************

import java.lang.*;
import java.io.*;
import java.net.*;
import java.util.Scanner;

public class server 
{
    private final static int PORT = 12345;
    private final static int QUEUE_SIZE = 10;
    private final static int BUF_SIZE = 4096;
    
    public static void main(String[] args)
    {
        try {
            // Create the server socket
            ServerSocket sSocket = new ServerSocket(PORT, QUEUE_SIZE);
            
            // Socket is set up and will wait for connections
            while (true) {  //不用true会好一些
                // Listen for a connection to be made to this socket and accept it
                Socket cSocket = sSocket.accept();
                byte[] byteArray = new byte[BUF_SIZE];
                
                // Get the name of the file from the client
                Scanner scn = new Scanner(cSocket.getInputStream());
                String fileName = scn.next();
                
                // Send the contents of the file
                BufferedInputStream bis = new 
            BufferedInputStream(new FileInputStream(fileName));
                OutputStream outStream = cSocket.getOutputStream();
                while(bis.available() > 0) {
                    bis.read(byteArray, 0, byteArray.length);
                    outStream.write(byteArray, 0, byteArray.length);
                }
                            
                // Close
                bis.close();
                cSocket.close();
            }
        }
        catch(EOFException eofe) {
            eofe.printStackTrace();
            System.exit(1);
        }
        catch(FileNotFoundException fnfe) {
            fnfe.printStackTrace();
            System.exit(1);
        }
        catch(IllegalArgumentException iae) {
            iae.printStackTrace();
            System.out.println("Bind failed");
            System.exit(1);
        }
        catch(IOException ioe) {
            ioe.printStackTrace();
            System.out.println("Could not complete request");
            System.exit(1);
        }   
    }
}

Client：

//*****************************************************************
// client.java 
//
// Connects to the server and sends a request for a file by 
// the file name.  Prints the file contents to standard output.  
//*****************************************************************

import java.lang.*;
import java.io.*;
import java.net.*;

public class client 
{
    private final static int PORT = 12345;
    private final static int BUF_SIZE = 4096;
    
    public static void main(String[] args)
    {       
        // Set up socket using host name and port number
        try {
            // Get host name and file name from command line arguments
            String host = args[0];
            String fileName = args[1];
            
            Socket s = new Socket(host, PORT);
            byte[] byteArray = new byte[BUF_SIZE];
            // Send filename to the server
            PrintWriter pw = new PrintWriter(s.getOutputStream(), true);
            pw.println (fileName);
            
            // Get the file from the server and print to command line
            DataInputStream fromServer = new DataInputStream(s.getInputStream());
            
            while(fromServer.read(byteArray) > 0) {
                System.out.println(new String(byteArray, "UTF-8"));
            }
            
            fromServer.close();
            s.close();
        }
        catch(IndexOutOfBoundsException iobe) {
            System.out.println("Usage: client host-name file-name");
            System.exit(1);
        }
        catch(UnknownHostException unhe) {
            unhe.printStackTrace();
            System.out.println("Unknown Host, Socket");
            System.exit(1);
        }
        catch(IOException ioe) {
            ioe.printStackTrace();
            System.exit(1);
        }
    }
}

注意：

Server进入Socket cSocket = sSocket.accept();的时候，判断语句内容其实不是很重要，有的地方写成 if ( !serverSocket.close( )){ 假设我们一定要进入accept()，那含义是一样的。ServerSocket的accept()本身是个当前线程阻塞方法（一个线程在执行过程中暂停，以等待某个条件的触发，或者说是等待所有资源到位），那么，当它只有接受一个客户端的链接时，才会往下执行，在此之前将一直等待，无限原地循环，如果直接关闭ServerSocket，那么会报socket异常，因为：

public Socket accept() throws IOException {  
       if (isClosed())  
           throw new SocketException("Socket is closed");    //here
       if (!isBound())  
           throw new SocketException("Socket is not bound yet");  
       Socket s = new Socket((SocketImpl) null);  
       implAccept(s);  
       return s;  
   } 

解决方案，可以创建一个新的socket链接，并且改变flag值：

public void stopThread(){  
        this.flag = false;  
        try {  
            new Socket("localhost",50001);  
        } catch (UnknownHostException e) {  
            e.printStackTrace();  
        } catch (IOException e) {  
            e.printStackTrace();  
        }  
    }  

网上看到的另外一个版本：
Server：

package socket;
 
import java.io.*;
import java.net.*;
 
public class TcpServer {
    public static void main(String[] args) throws Exception {
        ServerSocket server = new ServerSocket(9091);
        try {
            Socket client = server.accept();
            try {
                BufferedReader input =
       new BufferedReader(new InputStreamReader(client.getInputStream()));
                boolean flag = true;
                int count = 1;
 
                while (flag) {
                    System.out.println(客户端要开始说话了，这是第 + count + 次！);
                    count++;
                     
                    String line = input.readLine();
                    System.out.println(客户端说： + line);
                     
                    if (line.equals(exit)) {
                        flag = false;
                        System.out.println(客户端不想玩了！);
                    } else {
                        System.out.println(客户端说：  + line);
                    }
 
                }
            } finally {
                client.close();
            }
             
        } finally {
            server.close();
        }
    }
}

Client:

package socket;
 
import java.io.*;
import java.net.*;
import java.util.Scanner;
 
public class TcpClient {
    public static void main(String[] args) throws Exception {
        Socket client = new Socket(127.0.0.1, 9091);
        try {
            PrintWriter output =
                    new PrintWriter(client.getOutputStream(), true);
            Scanner cin = new Scanner(System.in);
            String words;
 
            while (cin.hasNext()) {
                words = cin.nextLine();
 
                output.println(words);
 
                System.out.println(写出了数据:  + words);
            }
 
            cin.close();
        } finally {
            client.close();
        }
    }
}

这是一个socket套接字通信的图：