谈谈TCP和UDP的一些简单应用

　　网络编程中最重要的就是SOCKET，它其实也就是监听端口的原理。和我们用手机发短信的原理应该是大致无二（我是这样理解的），而JAVA最出色的一点也就是“无痛苦连网”。

　　网络最基本的精神就是让两台机器连接起来，“被呼叫的一方”也就是服务器，而“找人的一方”则叫做客户机，所以说在连接中服务器、客户机也就是一个相对的概念了。而我们对机器的标识主要是通过IP地址和端口来区分的。

　　“传输控制协议”TCP和“用户数据报协议”是两种不同的协议，JAVA对这两种协议的支持基本是一致的，而它们本身最大的区别也就是发送的可靠性和速率，前者相比后者是可靠协议，后者当然是速度快得多了，下面我们分别用两个SOCKET下演示：

<table cellSpacing=0 cellPadding=0 width=600 bgColor=#ffffff border=0> <tr> <td>

eg1:

//Clients.java

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

public class Clients
{
public static void main(String[] args) throws Exception
{
InetAddress addr = InetAddress.getByName(null);
Socket socket = new Socket(addr,2000);
PrintWriter out =
new PrintWriter(
new BufferedWriter(
new OutputStreamWriter(
socket.getOutputStream())),true);
byte[] b = new byte[2048];
String msg = new String(b,0,System.in.read(b));
out.println(msg);
socket.close();
}
}

//Servers.java

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

public class Servers
{
public static void main(String[] args) throws Exception
{
ServerSocket s = new ServerSocket(2000);
try{
while(true){
Socket socket = s.accept();
try{
BufferedReader in =
new BufferedReader(
new InputStreamReader(
socket.getInputStream()));
StringBuffer sb = new StringBuffer();
int c;
while( (c = in.read()) != -1 ){
char ch = (char)c;
sb.append(ch);
}
System.out.println(sb.toString());

}catch(IOException e){
socket.close();
}finally{
socket.close();
}
}//while
}finally{
s.close();
}//try
}//main
}

此程式主要用Servers来进行无限监听，而Clients是客户机发送程式，他们的端口全采用2000。

eg2:

//UDPsend.java
import java.io.*;
import java.net.*;

/**
* This class sends the specified text or file as a datagram to the
* specified port of the specified host.
**/
public class UDPSend {
public static final String usage =
"Usage: java UDPSend ...\n" +
" or: java UDPSend -f ";

public static void main(String args[]) {
try {
// Check the number of arguments
if (args.length < 3)
throw new IllegalArgumentException("Wrong number of args");

// Parse the arguments
String host = args[0];
int port = Integer.parseInt(args[1]);

// Figure out the message to send.
// If the third argument is -f, then send the contents of the file
// specified as the fourth argument. Otherwise, concatenate the
// third and all remaining arguments and send that.
byte[] message;
if (args[2].equals("-f")) {
File f = new File(args[3]);
int len = (int)f.length(); // figure out how big the file is
message = new byte[len]; // create a buffer big enough
FileInputStream in = new FileInputStream(f);
int bytes_read = 0, n;
do { // loop until we've read it all
n = in.read(message, bytes_read, len-bytes_read);
bytes_read += n;
} while((bytes_read < len)&& (n != -1));
}
else { // Otherwise, just combine all the remaining arguments.
String msg = args[2];
for (int i = 3; i < args.length; i++) msg += " " + args[i];
message = msg.getBytes();
}

// Get the internet address of the specified host
InetAddress address = InetAddress.getByName(host);

// Initialize a datagram packet with data and address
DatagramPacket packet = new DatagramPacket(message, message.length,
address, port);

// Create a datagram socket, send the packet through it, close it.
DatagramSocket dsocket = new DatagramSocket();
dsocket.send(packet);
dsocket.close();
}
catch (Exception e) {
System.err.println(e);
System.err.println(usage);
}
}
}

//UDPreceive.java
import java.io.*;
import java.net.*;

/**
* This program waits to receive datagrams sent the specified port.
* When it receives one, it displays the sending host and prints the
* contents of the datagram as a string. Then it loops and waits again.
**/
public class UDPReceive {
public static final String usage = "Usage: java UDPReceive ";
public static void main(String args[]) {
try {
if (args.length != 1)
throw new IllegalArgumentException("Wrong number of args");

// Get the port from the command line
int port = Integer.parseInt(args[0]);

// Create a socket to listen on the port.
DatagramSocket dsocket = new DatagramSocket(port);

// Create a buffer to read datagrams into. If anyone sends us a
// packet containing more than will fit into this buffer, the
// excess will simply be discarded!
byte[] buffer = new byte[2048];

// Create a packet to receive data into the buffer
DatagramPacket packet = new DatagramPacket(buffer, buffer.length);

// Now loop forever, waiting to receive packets and printing them.
for(;;) {
// Wait to receive a datagram
dsocket.receive(packet);

// Convert the contents to a string, and display them
String msg = new String(buffer, 0, packet.getLength());
System.out.println(packet.getAddress().getHostName() +
": " + msg);

// Reset the length of the packet before reusing it.
// Prior to Java 1.1, we'd just create a new packet each time.
packet.setLength(buffer.length);
}
}
catch (Exception e) {
System.err.println(e);
System.err.println(usage);
}
}
}

</td></tr></table>

　　在UDP中主要的类是DatagramSocket()和DatagramPacket()，而在UDPreceive中，被接受的字节是受限制，这些感觉不是太好，既然buf是一个字节数组，我们实在是很奇怪为什么构建器自己不能调查出数组的长度呢？唯一能猜测的原因就是C风格的编程使然，那里的数组不能自己告诉我们它有多大。

　　而我们实际使用的过程中，当然不仅仅限于这些，其中要考虑有多台客户机来连接服务器，所以要考虑到线程Thread的使用，如果再加上SWING，就可以做一个类似于QQ的SOCKET功能了，这仅仅限于我在学习SOCKET时的一些领悟。供大家参考。