pop3clientdemo.java

这是用java编写pop3协议的客户端。

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


public class Pop3ClientDemo
{
	protected int port = 110;
	protected String hostname = "localhost";
	protected String username = "";
	protected String password = "";

	protected Socket socket;
	protected BufferedReader br;
	protected PrintWriter pw;

	// Constructs a new instance of the POP3 client
	public Pop3ClientDemo() throws Exception
	{
		try
		{
		 	// Get user input
			 getInput();

		 	// Get mail messages
			 displayEmails();
		}
		catch(Exception e)
		{
			System.err.println ("Error occured - details follow");
			e.printStackTrace();
			System.out.println(e.getMessage());
		}
	}

	// Returns TRUE if POP response indicates success, FALSE if failure
	protected boolean responseIsOk() throws Exception
	{
		String line = br.readLine();
		System.out.println("< "+line);
		return line.toUpperCase().startsWith("+OK");
	}

	// Reads a line from the POP server, and displays it to screen
	protected String readLine(boolean debug) throws Exception
	{
		String line = br.readLine();

		// Append a < character to indicate this is a server protocol response
		if (debug)
			System.out.println("< "+line);
		else
			System.out.println(line);
		return line;
	}

	// Writes a line to the POP server, and displays it to the screen
	protected void writeMsg(String msg) throws Exception
	{
		pw.println(msg);
		pw.flush();
		System.out.println("> "+msg);
	}

	// Close all writers, streams and sockets
	protected void closeConnection() throws Exception
	{
		pw.flush();
		pw.close();
		br.close();
		socket.close();
	}

	// Send the QUIT command, and close connection
	protected void sendQuit() throws Exception
	{
		System.out.println("Sending QUIT");
		writeMsg("QUIT");
		readLine(true);

		System.out.println("Closing Connection");
		closeConnection();
	}


	// Display emails in a message
	protected void displayEmails() throws Exception
	{
		BufferedReader userinput = new BufferedReader( new InputStreamReader (System.in) );

		System.out.println("Displaying mailbox with protocol commands and responses below");
		System.out.println("--------------------------------------------------------------");

		// Open a connection to POP3 server
		System.out.println("Opening Socket");
		socket = new Socket(this.hostname, this.port);

		br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
		pw = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()));

		// If response from server is not okay
		if(! responseIsOk())
		{
			socket.close();
			throw new Exception("Invalid POP3 Server");
		}
	
		// Login by sending USER and PASS commands
		System.out.println("Sending username");
		writeMsg("USER "+this.username);
		if(!responseIsOk())
		{
			sendQuit();
			throw new Exception("Invalid username");
		}

		System.out.println("Sending password");
		writeMsg("PASS "+this.password);
		if(!responseIsOk())
		{
			sendQuit();
			throw new Exception("Invalid password");
		}

		// Get mail count from server ....
		System.out.println("Checking mail");
		writeMsg("STAT");

		// ... and parse for number of messages
		String line = readLine(true);
		StringTokenizer tokens = new StringTokenizer(line," ");
		tokens.nextToken();
		int messages = Integer.parseInt(tokens.nextToken());
		int maxsize = Integer.parseInt(tokens.nextToken());

		if (messages == 0)
		{
			System.out.println ("There are no messages.");
			sendQuit();
			return;
		}
	
		System.out.println ("There are " + messages + " messages.");
		System.out.println("Press enter to continue.");
		userinput.readLine();
		
		for(int i = 1; i <= messages ; i++)
		{
			System.out.println("Retrieving message number "+i);
			writeMsg("RETR "+i);
			System.out.println("--------------------");
			line = readLine(false);
			while(line != null && !line.equals("."))
			{
				line = readLine(false);
			}
			System.out.println("--------------------");
			System.out.println("Press enter to continue. To stop, type Q then enter");
			String response = userinput.readLine();
			if (response.toUpperCase().startsWith("Q"))
			break;
		}
		sendQuit();
	}

	public static void main(String[] args) throws Exception
	{
		Pop3ClientDemo client = new Pop3ClientDemo();
	}

	// Read user input
	protected void getInput() throws Exception
	{
		String data=null;
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		System.out.print("Please enter POP3 server hostname:");
		data = br.readLine();
		if(data == null || data.equals(""))
			hostname="localhost";
		else
			hostname=data;

		System.out.print("Please enter mailbox username:");
		data = br.readLine();
		if(!(data == null || data.equals("")))
			username=data;

		System.out.print("Please enter mailbox password:");
		data = br.readLine();
		if(!(data == null || data.equals("")))
			password=data;
	}
}